Sexual Selection and Body Size in Male Red-Winged Blackbirds

Sexual Selection and Body Size in Male Red-Winged BlackbirdsAuthor(s): William A. SearcySource: Evolution, Vol. 33, No. 2 (Jun., 1979), pp. 649-661Published by: Society for the Study of EvolutionStable URL: http://www.jstor.org/stable/2407788 .

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Evolution, 33(2), 1979, pp. 649-661

SEXUAL SELECTION AND BODY SIZE IN MALE RED-WINGED BLACKBIRDS

WILLIAM A. SEARCY'

Department of Zoology, University of Washington, Seattle, Washington 98195

Received March 16, 1978. Revised September 18, 1978

In most species of birds adult males are larger than adult females. The two major hypotheses proposed to explain this pat- tern of sexual size dimorphism are appro- priately termed the sexual selection and competition avoidance hypotheses. The sexual selection hypothesis states that large size gives males an advantage in in- trasexual competition for females or in epigamic displays, activities not engaged in by females. Males will thus be larger than females, and the difference should be greater among polygynous and promiscuous species where there is greater vari- ability in male reproductive success and thus stronger sexual selection (Darwin, 1871; Huxley, 1938; Fisher, 1958). The sexual selection hypothesis, then, explains the relationship between pronounced sexual dimorphism in size and nonmonogamous mating systems in birds (Darwin, 1871; Selander, 1958, 1972; Amadon, 1959; Orians, 1961), a relationship particularly evident among the Icteridae (Selan- der, 1958).

The competition avoidance hypothesis states that sexual size dimorphism is favored because it reduces competition between males and females of the same species (Selander and Giller, 1963; Selan- der, 1966, 1972; Johnston and Selander, 1973). If size affects the range of resources used (e.g., larger individuals eat larger food items), then those males differing most in size from females would overlap in resource utilization with the fewest individuals and thus have the most resources available. This hypothesis may be especially applicable to monogamous, ter-

1 Present address: Rockefeller University Field Research Center, Tyrrel Road, Millbrook, New York 12545.

ritorial species in which sexual selection is weak and the male and female in a pair may often be each other's worst competi- tors. The competition avoidance hypothesis does not explain the trend toward increased dimorphism in nonmonogamous species, which implies that sexual selection must be at least one of the causes of dimorphism in families, such as the Icter- idae, exhibiting this trend. The two hypotheses are not mutually exclusive, and competition avoidance could explain the small degree of size dimorphism in monogamous Icterids while sexual selection accounts for the increased dimorphism in nonmonogamous species.

Adult male red-winged blackbirds (Age- laius phoeniceus) are about 20% larger than females in wing length. Male redwings should be strongly affected by sexual selection since there is a great range of variation in mating success among male redwings. Female redwings in Washing- ton populations choose mates largely on the basis of the quality of their territories (Searcy, 1979) so the outcome of inter- male competition for territories should have a strong effect on the direction of sexual selection. The ability to win aggressive encounters should be of advantage in competition for a territory, and large males should have an advantage in aggressive encounters. Thus large males should presumably have an advantage in competition for territories, which provides a clear mechanism by which sexual selection could favor large size in male redwings.

If increased size dimorphism in nonmonogamous Icterids is caused because large size in males is favored by sexual selection, what selective forces act to limit male size? Selander (1965) thought that in

649



650 WILLIAM A. SEARCY

the promiscuous great-tailed grackle (Quiscalus mexicanus), large size made males less efficient at foraging than females and thus exposed them to a greater risk of starvation in winter. He also thought that the conspicuous size of males made them more vulnerable to predation. Using estimates of sex ratios in roosts, Se- lander was able to show that males suffer greater mortality than females in winter. Thus it seems probable that size in male great-tailed grackles is set by a balance between sexual selection favoring increased size in the breeding season and selection for survival ability favoring de- creased size. Note, however, that it has not been shown that large males actually do better reproductively than small males, nor has it been shown that small males survive with greater probability than large though the latter is a reasonable inference from available data.

The present study is an attempt to discover the selective forces determining male body size in a population of red- winged blackbirds. I started by investi- gating whether Selander's hypothesis about size in male grackles could be applied to these redwings. Redwings possess the advantage for such a study that male reproductive success is proportional to harem size (Orians, 1972; Holm, 1973) which can be measured relatively easily. Another advantage of redwings is the site fidelity of territorial males (Nero, 1956; Searcy, 1979), which allows reliable estimates of survivorship.

METHODS

Fieldwork for this study was done at the Turnbull National Wildlife Refuge in eastern Washington State. The refuge is located in a region of ponderosa pine (Pi- nus ponderosa) parkland interspersed with shallow lakes. Red-winged blackbirds nest in emergent vegetation (mainly cattails [Typha latifolia] and bulrushes [Scir- pus sp.]) along the margins of the lakes. Fieldwork was conducted late March through June in 1974, 1975, and 1976, and May 6-9, 1977.

I estimated body size by wing length,

generally considered the best measure of size in birds (Hamilton, 1961; James, 1970). Wing length correlates closely with fat-free weight at least in some birds (Con- nell et al., 1961), and, unlike weight, wing length does not fluctuate greatly either daily or seasonally. Wing length was measured in 1974 as the chord of the wing, but in 1975 and 1976 the wing was flat- tened when measured because this pro- cedure gave a more repeatable value.

Males were captured using decoy traps (Smith, 1972) and mist nets. Captured males were measured, banded with unique combinations of colored leg bands, and released. Weights were measured with a 100 g spring scale. Weights of territorial males declined by 0.09 g per day in 1974 (Pearson's r = -.327, P < .05) and by 0.11 gperdayin 1975 (r = -.371, P < .05), but there was no significant decline in 1976. I will refer to the actual weight at the time of measurement as "weight" and to the weight of a bird corrected back to the day the first individual was captured, using the assumption that each individual lost weight at the average rate, as the "corrected weight."

Time budgets of territorial males were estimated once in 1976 (March 22-April 5) and at three different stages of the 1975 season (March 25-April 11, May 7-May 24, and June 4-June 8). Estimates were made by visiting territories at 15 min in- tervals through eight h periods in the middle of the day and noting the owners' activities.

Harem sizes were calculated as the number of females needed to account for the nesting activity on a territory. It was assumed that a female would not renest sooner than seven days after her first clutch was destroyed or 20 days (including the time spent feeding fledglings) after her last young fledged if her first nest was suc- cessful. Nests started late in the season were attributed to a female who had nest- ed earlier on the same territory whenever a long enough period intervened between the disappearance of all eggs or young from the first nest and the appearance of eggs in the second. Thus my estimate of



SELECTION IN RED-WINGED BLACKBIRDS 65 1

TABLE 1. Correlations of harem size with wing length. r, = Spearman rank correlation coefficient.

N r, P

1974 42 -.210 >.05 1975 34 .075 >.05 1976 41 .319 <.05

the number of females settling on a territory is a minimal one because there are undoubtedly cases where the owner of the first nest failed to renest on the territory and a new female moved in to start the second nest. However, I am not able to correct for these occurences because the females were not marked.

RESULTS

Tests of Selander's hypothesis.-One way to test Selander's hypothesis is to compare survival rates of males and females. If large size carries a price in de- creased survival, then male redwings should have lower survival rates than females; however, this is a weak test since males and females differ in other charac- teristics besides size which could affect survival. Haigh (1969) sexed 58 redwing nestlings found dead in the nest: 39 were males and 19 were females. If the primary sex ratio (at hatching) was 1 to 1, then these data indicate that males are more likely to die as nestlings than are females (X2 = 6.90, P < .01). However, Haigh's best estimate of the primary sex ratio, based on the sexes of fledglings in broods where none of the nestlings died, was 44 males to 33 females, or 1.33 to 1. If this estimate of the primary sex ratio is used, then Haigh's data do not indicate that male nestlings are significantly more likely to die (X2 = 2.45, P > .10). Fankhauser (1971), using banding records of 400 females and 953 males, found nearly iden- tical mortality for the two sexes as adults: 57.9% annual mortality for males and 56.7% for females.

Stronger tests of Selander's hypothesis can be made by comparing survival rates and mating success of large and small

males. Selander's hypothesis predicts that large males should have larger harems than small males but lower survival rates. The first prediction was tested by calcu- lating Spearman rank correlation coeffi- cients between male size and harem size for each of the three seasons (Table 1). Only in 1976 was a significant positive correlation found.

If small males have a higher survival rate than large males, then the mean wing length of the males surviving from one year to the next should be lower than the mean wing length of the nonsurvivors. This prediction was tested by dividing the territorial males which were measured in a year into those that were seen the next year and those that were not and comparing the mean wing lengths of the two groups. The 17 males that survived from 1974 to 1975 had an average wing length of 127.9 mm while the 25 nonsurvivors had an average of 128.8 mm. The difference is not significant (t = 1.02, P > .10 by a one-tailed test). For the 1976-1977 interval, the mean wing length of the 20 survivors (131.0) was again smaller than the mean wing length of the 21 nonsurvivors (131.6) but not significantly so (t = .709, P > .20 by a one-tailed test). How- ever, for the 1975-1976 interval, the 25 survivors had a larger mean wing length (130.8) than the 10 nonsurvivors (130.2) though again the difference was not significant (t = .625, P > .50 by a two- tailed test).

Similar data were gathered by D. M. Johnson and associates (pers. comm.). These authors, working at a winter roost in Houston, Texas, took wing measure- ments of live redwings captured in traps and of redwings found dead at the roost. The average wing length of 24 live adult males was 127.9 versus an average of 126.7 for 279 dead adult males. The difference is in the opposite direction from that predicted by Selander's hypothesis but is not significant (P > .20). The average wing length of 60 live first year males was 122.2 versus an average of 122.0 for 154 dead first year males. Again, the difference is in the opposite direction




from that predicted but is not significant (P > .20).

To summarize, the prediction that mating success would increase with size was upheld only one year in three. No support was found for the prediction that adult females would have higher survival than adult males, nor for the prediction that small males would have higher survival than large males. The evidence on differential mortality among the sexes as nestlings was somewhat ambiguous.

A model of male size in redwings.-I have argued that males are larger than females in redwings because of sexual selection, and yet males do not seem to pay a price for large size in increased mortality. What, then, acts to limit size in male redwings? To answer this question, it is con- venient first to construct a model of optimal size in the absence of sexual selection. If sexual selection were not affecting size, optimal size would be that size where survival and parental investment are maxi- mized. Survival should be influenced both by predation and energy balance, but I will neglect predation because the direction of the effect of predation is not known, that is, it is not known whether predation selects for larger or for smaller size in redwings. Since there is evidence that large male redwings are not more likely to die than small ones, predation must not select strongly against large size anyway. The amount of parental investment which can be made also depends on energy balance.

Energy balance is most favorable at the size at which an animal has the most energy left over after its maintenance requirements have been met to expend on parental care, energy storage, etc. Both the efficiency of foraging and the cost of maintenance will change with body size. Let us define the profit of foraging (in units of energy/time) as a function f of size (s) and the cost of maintenance (also in energy/time) as a function g of size. Then the optimal size in terms of energy balance is that size where f(s) - g(s) is maximal. This is not quite correct because a small animal can do more with a given

amount of energy than can a large one. Let us suppose that the most crucial activity of an animal has a cost, again depen- dent on size, of h(s), in units of energy per unit activity. Then the optimal size

is that size where f(s) ( ) is maximal. h (s) The units of this expression are

energy/time - energy/time unit activity energy/unit activity time

An imaginary example may clarify this formulation. Suppose that it is of great importance to the reproductive success of a male bird to be able vigorously to chase rivals from his territory. Suppose further that on average he must spend half his daylight hours in territory defense, and during the rest of the day he is able to forage. Maintenance metabolism costs him y kcal/h, and while foraging he makes a profit of x kcal/h. If chases cost him z kcal/m, then for a 24 h period with 12 h of daylight, he will be able (without de- pleting energy reserves) to afford

(6x - 24y) kcal/day _ (6x - 24y) z kcal/meter z

meters/day of chases. Since I am using maintenance metabo-

lism to mean the rate of energy expenditure in the absence of any activity or energy storage, g(s) in a thermal neutral environment is equivalent to basal metabolic rate (BMR). The shape of the BMR versus size curve is well known for many taxa; for passerines, BMR in kcal/h is 4.78 W .726 where W is weight in kg (Aschoff and Pohl, 1970; see Fig. 1). At temperatures below thermal neutrality, maintenance metabolism is equivalent to stan- dard metabolic rate (SMR), which has a different relationship to size at every temperature. However, the general shape of the SMR versus size curve at any temperature below thermal neutrality should be similar to the BMR curve, that is, the SMR curve will rise as size increases but with ever decreasing slope (Kendeigh, 1969).



SELECTION IN RED-WINGED BLACKBIRDS 653

9 (s

z

small SIZE large

FIG. 1. Hypothetical curves relating size to energy intake and maintenance energy costs. f(s) = energy profit per time foraging for individuals of size s. g(s) = energy cost of maintenance for individuals of size s.

BMR is a very general expression for h(s) since the costs of many activities are approximately constant multiples of BMR (Hemmingsen, 1960; Berger and Hart, 1972; King, 1974). For example, Berger and Hart (1972) found that flight in birds cost 45.5 W 73 kcal/h, which is equivalent to 9.3 x BMR (King, 1974). Hemmingsen (1960) found that the maximal 'output of energy/time in activity is approximately 20 x BMR for animals of a wide range of

f(s) - g(s) . sizes. Thus i() is a general expres-

sion for the amount of activity which can be afforded by an animal over a given period of time with a given fraction of that period devoted to foraging. Maxi- mizing this expression also maximizes the length of time an individual can survive at temperatures within the thermal neutral zone on the profit of a given period of foraging.

The shape of f(s) is unknown, but we can make certain assumptions about it. It is logical to assume that for a given type of animal feeding on a given type of prey (e.g., a blackbird feeding on emerging insects), f(s) will go to zero below some size (e.g., as the blackbird becomes too small

X- f(s) CT)g(s)

z

, g(s) g(s)

Sopt

small SIZE large FIG. 2. Optimum size where selection maximizes

the difference between energy intake per time and maintenance energy costs per time scaled to maintenance energy costs. f(s) = energy profit per time foraging for individuals of size s and g(s) = maintenance energy costs for individuals of size s. S0,t is the size at which f(s)Ig(s) - g(s)lg(s) is maximal. Sex- ual selection shifts actual size above S,,t, thus decreasing f(s)/g(s) - g(s)lg(s).

to handle and swallow insects). Similarly, f(s) must decrease above a certain size (e.g., as the blackbird becomes too large to walk among cattails). The exact shape of the curve between these two sizes has little effect on the argument, as long as the curve is unimodal. I will assume the curve has the shape graphed in Figure 1.

Probable shapes of typical f(s)lg(s) and g(s)lg(s) curves are graphed in Figure 2. The optimal size energetically is that size where the difference between these two curves is greatest, that is, at the peak of f(s)lg(s). I will call this energetically optimal size So,,t.

If sexual selection has moved the size of male redwings above S,pt, then the larger

an individual is, the smaller fs) -

g(s) is g(s) '

for that individual. Thus counterselection against large size could be due to large males being more susceptible to starvation, but as mortality does not increase with size in male redwings, starvation seems to be unimportant as a selection pressure on size. Large individuals should be able to perform less activity on the




energy gained in a certain period of foraging. Large males should thus perform less parental care than small males, but male redwings in Washington populations contribute little parental care, and their contribution has little effect on the success of their broods (Searcy, 1979). Large males should also not be able to perform as much territory defense behavior as small males. This suggests the hypothesis that size is limited in male redwings because males are able to perform less and less territorial defense behavior as their size increases. This disadvantage should increase exponentially as the amount of territorial behavior goes to zero and so should eventually cancel out the advantage of large males in aggressive encounters. Further, males will be able to perform less epigamic display behavior as size increases, which may also lower pairing success. Weatherhead and Robertson (1977) have found that the vigor and duration of epigamic display does affect male redwing pairing success, at least in Ontario populations, and they believe that the amount of epigamic display performed is energy limited. Thus because of increasing energy limitation of territory defense and epigamic behavior as size increases, sexual selection may not continually favor larger body size but, instead, may favor a stable body size somewhat above SPt.

Evidence for the hypothesis.-The hypothesis that large males suffer an ener- getic disadvantage which tends to cancel out their advantage in aggressive encounters during competition for territory leads to the prediction that mating success in male redwings will not increase with size. As we have already seen, this prediction was borne out in two out of three years of this study. Only in 1976 was there a positive correlation between harem size and male wing length (Table 1).

The hypothesis assumes that some males, at least, have difficulty getting enough energy to perform sufficient epigamic or territorial defense behavior. The impressive amount of energy invested by males in territory defense and display be-

havior makes this assumption credible. Male redwings in eastern Washington set up territories in early February and defend them two to three months before any females start to nest. During much of Feb- ruary and March, snow may cover the ground and lakes may be frozen so that presumably many food sources are un- available. Many males continue to defend territories into July, thus defending for a total of five months. Daily time investment rises from a brief period at morning and evening to an average of more than 70% of daylight hours in May. Ridding territories of intruding males may involve many minutes of energy-costly aerial chase. At peak display times, males display hundreds of times an hour, and though individual displays may cost little, the cost added over many months must be substantial. How much of this display behavior is directed towards warding off rivals and how much towards attracting females is not known; most displays probably serve both functions simulta- neously. For this reason, I will lump territory defense and epigamic behavior to- gether as "territorial behavior."

The fact, mentioned earlier, that in two years the weights of territorial males declined in April and early May is evidence that males are under energy stress. The decline was 0.09 g per day in 1974 (P < .05) and 0. 1 1 g per day in 1975 (P < .05). Males in 1976 were estimated to have lost 0.06 g per day, but the loss rate was not significantly greater than 0 (r =-.188, P > .05).

The assumption that males curtail their territorial behavior for want of sufficient energy implies that if males had more energy available, they would increase territorial behavior. This prediction was tested by an experiment in which males were provided food on their territories and their display rates compared to those of a control group of unfed males. Experimental males were fed a mixture of cracked corn and commercial dog food on elevated trays on their territories. Control and experimental males held territories on the same lake, and were chosen to yield a re-




TABLE 2. Results of feeding experiment. Averages for the measures of territorial behavior are given for the experimental group (N = 7) and control group (N = 7). Feeding was begun between observation rounds 1 and 2. * indicates a significant difference between the experimental and control averages at the .05 level according to a one-tailed Mann-Whitney U test.

Song Flight Minutes Song number displays on rate

Round 1 experimentals 33.4 3.7 20.0 1.67 controls 34.4 3.4 19.5 1.75

Round 2 experimentals 45.3 2.1 18.8 2.37 controls 40.0 3.7 20.0 2.00

Round 3 experimentals 38.6* 3.3* 20.0* 1.93 controls 23.3 1.0 14.4 1.69

Round 4 experimentals 35.0 3.9* 19.8 1.79 controls 22.0 1.3 14.9 1.33

Round 5 experimentals 33.4* 3.0* 19.4 1.71* controls 21.9 0.6 17.6 1.13

Round 6 experimentals 35.0 3.9* 19.9 1.76 controls 23.4 1.0 17.2 1.20

peating series of an experimental male, a male which was neither fed nor observed (a blank male), a control male, and a blank male, down the lake shore. Displays were recorded for 20 min periods during the first two h after sunrise. One round of observations was made on the seven experimental and seven control males before feeding began, and five rounds during feeding.

The results of the experiment are presented in Table 2. The numbers of songs and flight displays are those given during a 20 min period, the number of minutes are those out of the 20 min observation period that the male spent on his territory, and song rate is the number of songs given per minute on the territory.

There was very little difference in display rates between the experimentals and controls before feeding began. Feeding was begun between rounds 1 and 2, but

there was still no significant difference in round 2, perhaps because many of the experimentals seemed not to have discov- ered their food trays. In rounds 3, 4, 5, and 6, the averages for the experimentals are higher than for the controls for each measure of territorial behavior, and half the differences are significant. There is good evidence that fed males spent more time on their territories (round 3), and that they sang more often when they were on territory (round 5). The most consistently significant difference between the experimentals and controls was in flight displays, and it is perhaps significant that flight displays are energetically more ex- pensive than songs.

A possible objection to this experiment is that the positive results occurred not because the food provided energy but because the food attracted more intruding males who had to be chased away or more females to be courted. I calculated the average numbers of males and females chased during the observation periods in each round of observation. The only significant difference between the experimental and control averages occurred in the number of males chased in round 1, before feeding began. Thus this objection can be dismissed.

I have provided evidence that males are under energy stress and that the amount of energy available to males limits their territorial activity. Is there evidence that energy stress is greater for large males than for small ones? Such evidence could consist either of a demonstration that large males use up their energy reserves more quickly than small males, or of a demonstration that large males limit their territorial behavior more than small males. I investigated both possibilities.

I determined whether large males had proportionately smaller energy reserves than small males at the time of weighing by determining whether large males were further below the expected weights for their sizes. Expected weights were found using data from J. E. Mosimann and F. C. James (unpub. ms.) on geographic variation in wing length and weight in




Florida populations of redwings. A power function was fitted to the average weights and wing lengths for 12 samples from different localities (Fig. 3). The equation for the curve is w = 0.000381 (L2503), where w is weight in grams and L is wing length in mm (r = 0.919, P < .01). Al- though Florida redwings are substantially smaller than Washington redwings, the curve fits the average corrected weights and wing lengths from my Washington samples impressively well. If we assume that the redwing populations studied by Mosimann and James did not differ in any systematic way in the average percentage of weight made up by stored fat, then this curve can be used to predict, for given wing lengths, weights with a constant percentage of the weight as stored fat. We can thus use the percentage deviation of the weight of individual males from the weight predicted by this curve as a measure of the relative sizes of their energy reserves.

I divided the territorial males into those above average and below average in wing lengths and compared the percentage de- viations of their actual weights from their expected weights. For all three years, large males were further below their expected weights than were small males. In 1974, 22 below average size males were on the average 3.1% above their expected weights and 20 above average size males were 4.6% below their expected weights, the difference being significant at the .001 level by a one-tailed Mann-Whitney U test. In 1975, 19 below average size males averaged 1.7% below their expected weights versus 6.9% below for 16 above average size males (P < .01). In 1976, 20 below average size males averaged 4.6% below expected versus 8.3% below for 20 above average size males (P < .01). Thus there is evidence that large males have proportionately smaller energy reserves than small males after two to three months of holding territory.

I tested the prediction that large males would perform less territorial behavior using two different measures: display rates and percentage of the day spent guarding

80

75

70

E 65

w 60

55

50

45

105 110 115 120 125 130

WING LENGTH (mm)

FIG. 3. Relationship of weight and wing length in male redwings. x = mean from one of the Florida localities (data from Mosimann and James, unpub.). o = mean from one of the three years of this study. Curve is fitted to Flordia data; with equation, w = .000381(L2503), where w = weight in grams and L = wing length in millimeters.

the territory. Table 3 shows the correlations between wing length and three measures of display rates defined earlier: song number, flight displays, and song rate. The correlations tend to be negative in 1974 and 1975 (10 of 12 are negative), and two are significant at the .01 level. For the display rates observed in 1976, all three correlations are positive but not significant. Table 3 also shows the correlations between display rates and condition of the males, measured as percentage deviation of their corrected weights from their expected weights. For 1974 and 1975, most of the correlations are positive (10 of 12) and in four cases they are significant at the .05 level. The correlations for 1976 are negative though not significant.

Table 4 shows the correlations between wing length and estimates of the percentage of the day males spent guarding their territories. No estimates were made in 1974. Estimates were made during three




TABLE 3. Correlations of display rates with body size and condition. Condition is measured as percentage deviation of the measured weightfrom that predicted by the curve in Fig. 3. r, = Spearman rank correlation coefficient. a = probability from two-tailed test; all other probabilities are from one-tailed test.

Song number Flight displays Song rate Song number Flight displays Song rate vs. vs. vs. vs vs. vs.

wing length wing length wing length condition condition condition

1974 rs -.115 -.079 -.172 .133 .128 .156 5/8-5/29 N 42 42 42 42 42 42

P .235 .311 .138 .107 .116 .073

1975 rs -.559 -.012 -.626 .498 -.091 .509 3/25-4/11 N 20 20 20 20 20 20

P .006 .480 .002 .013 .704a .011

4/12-5/6 rs -.183 .102 -.164 .380 .048 .373 N 34 34 34 34 34 34 P .151 .566a .177 .014 .394 .015

5/26-6/3 rs -.144 .229 -.210 .215 -.073 .237 N 28 28 28 28 28 28 P .232 .242a .142 .136 .714a .118

1976 rs .223 .156 .223 -.097 -.285 -.097 3/22-4/8 N 31 31 31 31 31 31

P .228a .404a .228a .604a .122a .604a

periods in 1975, and for two of these periods there was a negative correlation between wing length and percentage of time spent guarding the territory, but the correlations are not significant. In 1976, the single set of estimates correlated positively but not significantly with wing length. Table 4 also shows the correlations between the percentage of the day spent guarding the territory and condition. In 1975 there was a positive correlation in two of three cases, and one correlation is significant. In 1976 there was a negative but not significant correlation.

Thus there is evidence, albeit weak, that small males performed more territorial activity than did large males in 1974 and 1975, the years in which body size failed to correlate with harem size. There is also evidence to support the hypothesis that the condition of males in terms of energy stores affects the level of territorial activity. The fact that large males performed as much territorial behavior in 1976 as did small males may explain why body size correlated positively with harem size that year as large males would have lost their disadvantage in territorial activity while retaining their advantage in aggressive encounters.

Why were large males able to perform as much territorial behavior as small males in 1976 and not in 1974 or 1975? The obvious hypothesis is that environmental conditions were somehow better in 1976 than in 1974 or 1975 so that either energy intake was up or maintenance costs were down, or both, and a sufficient sur- plus of energy was left for territorial behavior even for large males. This is a difficult hypothesis to support because energy balance depends on a complex blend of weather conditions and food availability, and when during the season the crucial periods are is unknown. Weather records are available but information on food availability is not. Table 5 shows average monthly temperatures for February through May and the number of days in each month with more than a trace of snow cover for the nearest weather station, the Spokane Airport, about 20 miles from the study sites. Snow cover probably lessens food availability. 1975 was clearly the coldest year and also had substantially more days of snow cover than either 1974 or 1976. On the other hand, 1974 had no more snow cover than 1976, and three of four months were warmer in 1974 than in 1976. As noted earlier, average weight of




TABLE 4. Correlations of percentage of time spent guarding the territory with body size and condition (measured as percentage deviation of measured weight from that predicted by the curve in Fig. 3). r. = Spearman rank correlation coefficient. a =

probability from two-tailed test; other probabilities are from one-tailed test.

With wing With length condition

1975

March 25-April 11 r, -.279 .414 N 26 26 P .084 .018

May 7-May 24 r, .122 -.226 N 33 33 P .498a .208a

June 4-June 8 rs -.216 .143 N 26 26 P .145 .243

1976

March 22-April 5 rs .189 -.222 N 29 29 P .328a .248a

territorial males declined significantly in 1974 and 1975 but not in 1976, but the difference in rate of decline is not significant (Analysis of Covariance, F = 0.393, P > .05). The average weight of males was actually lower in 1976 than either 1974 or 1975, but again the differences are not significant (1974 vs. 1976, t = 0.27, P > .05; 1975 vs. 1976, t = 1.06, P > .05). There is no clear evidence, then, that 1976 had the best environmental conditions of the three years.

Selection for survival ability. -Does selection for survival ability have any effect on size? I have already shown that survival does not exercise any directional selection on size, at least in adult males, but it is still possible that survival has a stabilizing effect on size. To investigate this possibility, I divided the redwings captured each year into categories of the one- fourth with the shortest wing lengths, the one-fourth with the longest wing lengths, and the one-half in the middle, and calculated the percentage survival of each group (Table 6). In each year the middle group had the highest survival, but even if all three years are pooled and the small

TABLE 5. Weather data, collected by the National Oceanic and Atmospheric Administration Station at the Spokane Airport.

Number of Average days with

temperature more than a (degrees trace of

centigrade) snow cover

1974 February 1.9 2 March 3.6 0 April 8.0 0 May 10.1 0

1975 February -4.1 28 March 1.1 4 April 5.4 2 May 11.5 0

1976 February 0.1 2 March 1.7 0 April 7.3 0 May 12.5 0

and large groups combined, the difference in survival of the middle group versus the combined large and small group is not significant (X2 = 2.18, P > .10). Obviously, more data on survival versus wing length are needed before it can be decided whether survival selection has a stabilizing effect on size.

DISCUSSION

The view of body size in male redwings which has been developed here is that size, in the absence of sexual selection, is that which optimizes an individual's energy balance. Sexual selection has caused the evolution of a large average size because large males have an advantage in aggressive encounters which leads to increased success in inter-male competition for territories and thus to increased mating success. However, large males suffer a disadvantage in competition for territories in that they cannot, because of energy limitation, perform as much territory defense behavior as small males. Large males are also unable to perform sufficient epigamic behavior and thus may attract fewer females than smaller males with equally good territories. Thus at some size, there is no further selection for size increase, and size is stabilized by the action of sexual selection alone.




TABLE 6. Percentage survival of body size categories. Categories: the quarter of the males measured that year with the largest wing lengths, the quarter with the smallest wing lengths, and the half in the middle. N = number of males in the size category.

Largest Median Smallest

Percentage returning 36% 48% 30% in 1975 from 1974 N = 11 N = 21 N = 10

Percentage returning 67% 82% 56% in 1976 from 1975 N = 9 N= 17 N=9

Percentage returning 40% 52% 50% in 1977 from 1976 N = 10 N = 21 N = 10

Total 47% 59% 45% N = 30 N = 59 N = 29

I have provided evidence that the amount of territorial behavior performed is energy limited and that large males are more severely limited both in energy stores and in the amount of territorial behavior performed than are small males. Thus it seems that large males do have a disadvantage which would tend to negate their presumable advantage in aggressive encounters during competition for territories. However, it is not clear whether in the long run the advantage and disadvantage exactly cancel each other so that large males do not experience any net sexual selection advantage. Large males may experience a slight mating advantage, perhaps only expressed in occasional good years, which is balanced by a slight survival disadvantage, perhaps experi- enced in the nestling stage. Alternatively, the observed mating advantage of large males in 1976 might be balanced by a mating disadvantage in particularly bad years while survival has no effect on size.

If there is stabilizing selection on size due to medium-sized individuals surviving better than extreme-sized individuals, this would be somewhat destructive to the view of body size just presented. If my model of optimal size is correct, large males could possibly be at a disadvantage in survival relative to medium-sized males, but small males, if anything, should be at an advantage. One way to

reconcile stabilizing selection for survival with my model is to postulate differentf(s) curves for winter and spring. This is actually possible since redwings feed largely on insects in the spring and on grain in the winter. Larger size might be advan- tageous to males feeding on grain in winter flocks because it might be possible for large males to monopolize the best feeding patches. Thus the optimal size energetically could be larger in winter than in spring so that medium-sized males could be above the optimum in spring but near it in winter, when stabilizing selection could occur. This is entirely speculative, of course; there is not even firm evidence that selection for survival has a stabilizing effect on size.

One selection pressure which has not been considered is predation. Predation was ignored because it is not known whether large or small size would be favored under predation pressure. Plausible arguments can be made for both alterna- tives. Large males may be more conspicuous to predators and may be more exposed to predation because they must feed longer both to grow and to sustain them- selves. On the other hand, large size may make males difficult for certain predators to capture and handle. The evidence shows that there is no strong directional selection due to mortality so predation cannot be exerting a strong directional pressure on size unless it is nearly can- celled out by some opposing pressure.

Although I started by discussing sexual size dimorphism, I have largely neglected female size. Females should have an f(s) curve similar to that of males as the diets of the two sexes are similar, at least in the spring (Orians, 1979). The g(s) curve should also be similar to that of males. The strongest selection on females probably is to maximize the number of young they can feed (Lack, 1954, 1966). Thus females should have that size which maxi-

mizesf(s) , ) , where h'(s) measures the h'(s)

energy cost of feeding an average offspring for a female of size s, assuming that




this energy cost depends on the sizes of the offspring at independence and that the size of offspring is correlated with the size of the mother. h'(s) may have a similar shape to g(s) because a large fraction, perhaps the majority, of the energy provided to the young is used by them in maintenance metabolism rather than growth (Ricklefs, 1974). Thus the size of female redwings may be near the energetically optimum size for both sexes. Sexual selection selects for a larger size in males thus creating sexual size dimorphism.

The hypothesis that energy limitation of territory defense and/or epigamic behavior limits male size can be applied to other species. Males of nonmonogamous species characteristically devote more time and energy to display than males of monogamous species (Selander, 1972), and thus energy limitation of display is more likely to affect nonmonogamous species. For example, leking male grouse, for a period of months in late winter-early spring, spend at least several hours each morning and evening in display, and some display all night (Wiley, 1973). Besides display- ing, males must expend energy on bound- ary clashes and copulation attempts. Kruijt et al. (1972) noted that a common cause of loss of territory in black grouse (Lyrurus tetrix) is apparent ill health of the male, which might very likely be brought on by energy depletion. Collias and Collias (1967) found in the polygynous village weaverbird (Textor cucullatus) that males must build and display four nests for every one accepted by a female, and that building one nest requires 30 to 40 km of flight, obviously a great expenditure of energy. It is possible that in these species, as in redwings, large males experience a more severe energy limitation of territory defense or epigamic behavior than small males, and thus that sexual selection may not continually favor larger male size but instead may favor a stable size somewhat above female size.

SUMMARY

Male red-winged blackbirds are about 20% larger than females. Selander (1965)

proposed that size in male Icterids is set by a balance between sexual selection favoring large size and survival selection favoring small size. The trend towards greater size dimorphism in nonmonogamous Icterid species indicates that large size in males has evolved because of sexual selection. However, in a population of redwings in Washington State, large males had a higher pairing success than small males only one year in three, and there was no evidence of counterselection against large size due to higher survival of small males.

The hypothesis is proposed that counterselection against large size is due to large males having less energy left after maintenance requirements are met to expend on territory defense and epigamic behavior. Thus large males suffer an energy disadvantage in competition for mates which tends to balance their presumable advantage in aggressive encounters. Evidence for the hypothesis is: (1) males lose weight in the spring, indicating they are stressed for energy; (2) males provided with food on their territories dis- played more than a control group, indicating that territorial behavior is energy limited; (3) small males have larger energy reserves proportional to their size than do large males; and (4) large males perform less territorial behavior than small males. Thus size in male redwings may be stabilized by the action of sexual selection alone.

ACKNOWLEDGMENTS

I am very grateful to Gordon Orians for his help in planning this study and for his advice on preparation of the manuscript. I am also grateful to Sievert Rohwer, Thomas Schoener, W. James Erckmann, Steven Fretwell, Daniel Johnson, and two anonymous reviewers for helpful criti- cisms of the manuscript. Frances James and Daniel Johnson kindly allowed me to cite unpublished data. Margaret Searcy was of immense help with fieldwork and with preparation of the figures. I thank Jon M. Malcolm, Donald White, and the staff of the Turnbull National Wildlife




Refuge for their cooperation with the study. Financial support was provided by the National Science Foundation, grant BMS75-14937.

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Sexual Selection and Body Size in Male Red-Winged Blackbirds