Ecological Research

(2003)

18,

73–80

Blackwell Science, LtdOxford, UKERE

Ecological Research0912-38142003 Ecological Society of Japan

181January 2003

534Unit size and territory quality in

Malurus

K. Chan and J. D. Augusteyn10.1046/j.0912-3814.2002.00534.x

Original Article7380BEES SGML

*Author to whom correspondence should beaddressed. Present address: Faculty of Science, Univer-sity of the Sunshine Coast, Maroochydore DC, Queen-sland 4558, Australia. Email: kchan@usc.edu.au

†

Present address: Natural Resource Management,Queensland Parks and Wildlife Service, CentralRegion, Rockhampton, Queensland 4701, Australia.

Received 22 November 2001. Accepted 26 June 2002.

Relationship between bird-unit size and territory quality in three species of fairy-wrens (

Malurus

spp.) with overlapping territories

K

EN

C

HAN

*

AND

J

OHN

D.

A

UGUSTEYN

†

Central Queensland University, Rockhampton, Queensland 4702, Australia

The relationship between unit size and territory quality and size was examined in variegated, superband red-backed fairy-wrens in an area in which the birds’ territories overlapped extensively.Variegated fairy-wrens had the least number of paired birds and the largest mean unit size. Morethan half of the superb and red-backed fairy-wrens occurred in pairs. Territory size did not differbetween the species. A positive correlation between unit size and territory size was found only inthe variegated fairy-wren. Stepwise regression analyses with backward elimination on 10 habitatvariables showed that unit size was associated with different territory-quality measures in the threefairy-wren species. It is suggested that species that are more inclined to form pairs or small groupsduring the breeding season are also more sensitive to ecological factors and prefer not to form largegroups in a resource-limited environment.

Key words:

brigalow; fairy-wren;

Malurus

; territory quality; unit size.

INTRODUCTION

Fairy-wrens (Genus:

Malurus

) belong to an assem-blage of ground-dwelling bird species whosemembers breed either in pairs or in groups. In thebreeding season, group members all help to raisethe young (termed ‘cooperative breeding’) and thenumber of helpers from each breeding unit mayvary from one to six or more (Rowley 1965;Pruett-Jones & Lewis 1990; Rowley & Russell1995; Russell & Rowley 2000; Higgins

et al

.2001). Helpers may benefit breeders by assistingwith feeding nestlings, thereby easing the load onthe breeders (Rowley 1965; Russell & Rowley1988; Dunn & Cockburn 1996). Such cooperationmay allow breeders to produce more clutches ormore successful fledglings each breeding season

(Rowley 1965; Ligon

et al

. 1991). It might beexpected that units containing large number ofhelpers would occupy large territories because ofimproved access to resources (Nias & Ford 1992;Brooker & Rowley 1995). The larger the territory,the greater the number of neighboring territories,and the greater the opportunity to access neigh-boring females (Brooker

et al

. 1990; Brooker &Rowley 1995). Thus, for males, the chances ofextra-pair copulations with neighboring femalesshould increase (Tidemann 1986; Russell & Row-ley 1988; Dunn & Cockburn 1996).

Habitat and, in particular, habitat or territoryquality has often been reported to influence coop-erative breeding (e.g. Rowley & Brooker 1987;Tidemann 1990; Ligon

et al

. 1991; Nias & Ford1992; Brooker & Rowley 1995). In the superbfairy-wren,

Malurus cyaneus

, group size has beenfound to vary with territory size and habitat qual-ity (Nias 1984; Nias & Ford 1992). Birds raisedon high-quality territories may prefer to remainand help parents feed the young rather than moveto vacancies on low-quality territories because, byremaining and queuing for a favorable social posi-tion, there is a chance that the birds may inherittheir natal territory. In the interim, they may gainaccess to critical resources and mates. Such benefits

74 K. Chan and J. D. Augusteyn

may outweigh the disadvantages associated withassisting the raising of young that may not begenetically related. Birds raised on low-qualityterritories have little to gain by remaining in thepoor-quality habitat and will improve their fitnessby dispersing at an early age to breed indepen-dently. Furthermore, studies (e.g. Stacey & Ligon1987) have found that birds raised on low-qualityterritories are more likely to change territorieseach year than are individuals occupying high-quality territories. Therefore, the presence of help-ers, which indicates breeding success in previousyears (Russell & Rowley 2000), can be expected tobe greater in high-quality territories.

In the drier parts of Central Queensland, threespecies of fairy-wrens (variegated

Malurus lamberti

,superb and red-backed

Malurus melanocephalus

)occur sympatrically. The superb fairy-wren is themost studied of the three species. Little is knownabout the variegated fairy-wren and publishedwork on the red-backed fairy-wren is almost non-existent (Higgins

et al

. 2001). Being congeners,the three species may be expected to exhibit asimilar diet and social organization. All three spe-cies maintain group and pair units during thebreeding season (Rowley & Russell 1997). Mosthabitat studies relating to the superb fairy-wrenare limited to a single-species response (e.g. Row-ley 1965; Nias 1984; Ligon

et al

. 1991; Nias &Ford 1992; but see Tidemann 1990). The presentstudy was carried out in an area where the superbfairy-wrens’ territories overlapped extensively withthose of variegated and red-backed fairy-wrens.Territorial overlap between these species suggestsselection of essentially the same broad habitat.This situation provides a great opportunity toinvestigate the relative response of coexisting,cooperative congeners to a resource-limiting envi-ronment. The main aim of this study was to deter-mine if the three sympatric species of fairy-wrensdiffer in their relationship between unit size andterritory quality.

METHODS

Study area

The three coexisting species of fairy-wrens werestudied from June 1997 to May 1998 at Taunton

National Park (Scientific) in Central Queensland,170 km west of Rockhampton (Fig. 1). The11 626 ha reserve was established to enhance theconservation of the endangered bridled nailtailwallaby,

Onychogalea fraenata

. The area may bedescribed as subtropical and semiarid, with annualrainfall averaging

<

400 mm. The vegetation istypical of the Brigalow Belt BiogeographicRegion, containing dense

Acacia

communitiesdominated by brigalow

Acacia harpophylla

andopen grassy eucalypt woodlands dominated by

Eucalyptus cambageana

and/or

Eucalyptus populnea

intermixed with shrubs such as

Eremophila mitch-ellii, Terminalia oblongata

and whitewood

Atalayahemiglauca

. Various parts of the reserve are beingmanipulated to improve food and shelter for thewallaby. There are cleared areas that contain arange of grass species including

Paspalidiumcaespitosum

,

Paspalidium constrictum

,

Enteropogonacicularis

,

Sporobolus

sp. and

Chloris divaricata

, che-nopods such as

Salsola kali

,

Einadia nutans

,

Sclero-laena bicornis

,

Trianthema triquetra

and

Enchylaenatomentosa,

and other forbs such as

Abutilon

sp. and

Sida

sp.Although fairy-wrens occupy a variety of vege-

tation communities at Taunton, they were partic-ularly common in regrowth areas that had beenrecently disturbed. Five parts (sites) of the reservein which the three species of fairy-wrens co-occurred were selected. Site 1 was the southern-most site and was located approximately 3 kmfrom Site 2. Sites 3–5, which were positioned close

Fig. 1.

Map location of Taunton National Park(Scientific).

115

115

130

130 145

145

15

30

15

30

RockhamptonTaunton National Park (scientific) X

Brisbane

AUSTRALIA

Unit size and territory quality in

Malurus

75

to one another (and where it was later discoveredthat some overlapping of territories occurred),were located between Sites 1 and 2 at a distanceof approximately 2 km and 1 km, respectively.Because the sites were selected on the basis of co-occurrence, the size of each site was determined bythe overall spread of territories by the three fairy-wren species.

Bird capture and observations

A total of 171 fairy-wrens were caught using mistnets. The birds were weighed and color-bandedwith unique combinations of color before beingreleased. Color-banding allowed visual tracking ofmovement by individuals. Reference points wereestablished to enable accurate mapping of territo-ries. These were wooden poles spaced at 100 mintervals along transects. The position of thesepoints was recorded using a geographic position-ing system, which was used in conjunction with abase station located at Central Queensland Univer-sity, Rockhampton, to obtain submeter accuracy.Territory boundaries were determined by observ-ing the movement of known groups of birds andby noting any territorial disputes that occurredbetween neighboring groups. These boundarylimits were plotted on a map marked with recog-nizable on-ground landmarks such as woodenpoles and isolated trees. The perimeter of a terri-tory was later checked by noting where the birdsturned back when they were slowly herded in onedirection.

In total, 200 fairy-wrens from 40 breeding units(21 red-backed, 9 superb and 10 variegated) werestudied. The size of a unit, which is defined as thenumber of adults that intermingle on a daily basisduring the breeding season, was determined ini-tially by the movement behavior of color-bandedindividuals. Unbanded individuals that mixedpersistently with a known unit containing bandedbirds were considered members of that unit.Observations to determine group size were madeevery 2 weeks from June 1997 to April 1998; dur-ing this period, breeding commenced in Augustand finished in May for all three fairy-wren species.The minimum unit size was a single pair of breed-ing birds. All other bird units consisted of onebreeding pair accompanied by one or more auxil-iary helpers.

Habitat variables

A series of transects was established in the five siteswhere the fairy-wren units were being studied.These transects crisscrossed to ensure all overlap-ping territories were covered. Habitat samplingwas carried out at 100-m intervals along eachtransect. Because of crisscrossing transects andvariation in territory size between bird groups, theintervals (and therefore number) between the mea-surements were not fixed. Care was taken to ensurethat each territory had a minimum of five measure-ments, which was considered sufficient for repre-senting the habitat features characterizing thesemiarid environment found in the reserve. Themedian number of measurement points per terri-tory was eight.

The selection of habitat variables was based onresults obtained from previous researchers (Nias1984; Tidemann 1990; Nias & Ford 1992;Brooker & Rowley 1995) who had identifiedimportant parameters for describing fairy-wrenhabitats. The habitat variables measured along thetransect in this study were percentage of groundcover (GCOV), height of ground cover (HTGC),percentage projected foliage of brigalow shrubs(FOLB), percentage projected foliage of shrubsother than brigalow (FOLS), density of brigalowtrees (BDEN), density of trees/shrubs other thanbrigalow (TDEN), average height of brigalow(HTBR) and arthropod biomass across a range ofplants in spring (SPIN) and summer (SUIN),including spring and summer combined (TOIN).Trees were defined as woody plants

>

1 m in heightthat did not branch at or near the ground. Shrubswere defined as woody plants that were

<

3 m talland with the trunk branched from or near theground. Arthropods were collected from four dom-inant shrub species (

A. harpophylla, A. hemiglauca,E. mitchellii

and

T. oblongata

) and the lowerbranches of two tree species (

A. harpophylla

and

A. hemiglauca

) using the shake and clip method, inwhich a large 1 m

¥

1 m plastic bag was placedover a branch, the branch shaken, clipped andsprayed with a household insecticide. Arthropodspecimens were then sorted and dried in an ovenfor 48 h at 55°C before being weighed. The vege-tation samples were sun dried until a constantweight was reached. A summary of the measuredhabitat variables is presented in Table 1.

76 K. Chan and J. D. Augusteyn

Data analysis

Data were tested for normal distribution andequality of variance prior to parametric analysis.Means of body mass, territory size and unit sizewere compared between interspecifics using one-way

ANOVAS

. Spearman rank correlation was usedto determine the relationship between unit sizeand territory size. Unit size for each species wasregressed in a stepwise fashion (backward method)against the habitat variables. Statistical tests wereperformed using SPSS 10.0 (SPSS 1999) computersoftware.

RESULTS

Body mass, territory size and unit size

Body mass differed between the three species offairy-wrens (

F

=

26.58, d.f.

=

2,139,

P

<

0.001),

with the red-backed fairy-wren being significantlysmaller than the variegated and superb fairy-wrens(Table 2).

Aggressive interactions between bird units wereuncommon and the intrusion of neighbors wasoften tolerated. However, bird groups did use arepertoire of calls during the breeding season indefense of their territories. At dawn the males andoccasionally some females would sing in duet froma prominent perch. Neighboring units would thenrespond by calling back. This behavior, togetherwith movement of banded birds, revealed anextensive overlap of territories between speciesand, to a lesser extent, within species in the fivesites (Fig. 2).

It was not possible to accurately determine theboundary of two of the territories of the red-backed fairy-wren units. After omitting these twoterritories from the analysis, the mean territorysize for three species of fairy-wrens was found to

Table 1

Descriptive statistics for the 10 habitat variables measured within the territories of variegated, superb andred-backed fairy-wrens

VariableVariegated Superb Red-backed

Mean

±

SD Range Mean

±

SD Range Mean

±

SD Range

GCOV (%) 24.2

±

6.2 20.0 30.1

±

6.6 15.0 28.7

±

7.6 26.0HTGC (cm) 22.6

±

6.9 20.4 26.2

±

6.6 20.1 25.9

±

7.3 25.2FOLS (%) 21.5

±

13.2 39.9 14.7

±

9.1 27.4 15.7

±

8.4 33.4FOLB (%) 9.1

±

10.2 30.6 9.6

±

8.8 29.8 10.6

±

10.0 28.5TDEN (trees per 10 m

2

) 0.38

±

0.28 0.93 0.31

±

0.15 0.44 0.35

±

0.19 0.67BDEN (trees per 10 m

2

) 1.27

±

1.16 3.90 0.69

±

0.28 1.03 0.76

±

0.58 2.88HTBR (m) 3.78

±

1.00 3.27 3.78

±

1.00 3.28 3.52

±

1.38 6.00SPIN (g 100 g

-

1

dry veg) 0.020

±

0.020 0.060 0.014

±

0.015 0.005 0.020

±

0.021 0.073SUIN (g 100 g

-

1

dry veg) 0.439

±

0.359 1.035 0.512

±

0.635 2.010 0.498

±

0.559 2.010TOIN (g 100 g

-

1

dry veg) 0.461 ± 0.364 1.076 0.526 ± 0.633 2.021 0.517 ± 0.554 2.021

(GCOV), Percentage of ground cover; (HTGC), height of ground cover; (FOLB), percentage projected foliage of brigalowshrubs; (FOLS), percentage projected foliage of shrubs other than brigalow; (BDEN), density of brigalow trees; (TDEN), densityof trees/shrubs other than brigalow; (HTBR), average height of brigalow; (SPIN), arthropod biomass across a range of plants inspring; (SUIN), arthropod biomass across a range of plants in summer; (TOIN), arthropod biomass across a range of plants inspring and summer combined.

Table 2 Mean ± SD (n) body mass, unit size and territory size in variegated, superb and red-backed fairy-wrens

Variegated Superb Red-backed P-value

Body mass (g) 8.0 ± 0.9 (63) 8.4 ± 0.5 (14) 7.2 ± 0.7 (65) <0.001Unit size 6.7 ± 2.4 (10) 3.1 ± 1.4 (9) 3.0 ± 1.7 (21) <0.001Territory size (ha) 7.3 ± 4.5 (10) 8.6 ± 3.7 (9) 5.4 ± 2.6 (19) >0.05

Unit size and territory quality in Malurus 77

range from 5.4 ha to 8.6 ha (Table 2). These differ-ences were not significant (F = 2.902, d.f. = 2,35,P > 0.05). Territory size increased with unit sizefor variegated fairy-wrens (rs = 0.697, P < 0.05),but not for superb (rs = 0.492, P > 0.05) and red-backed fairy-wrens (rs = 0.427, P > 0.05, Fig. 3).

Only one of the 10 (10.0%) variegated fairy-wren units studied consisted of a single pair ofindividuals. In contrast, 55.6% of the superb fairy-wren units and 66.7% of the red-backed fairy-wren units studied consisted of a single pair. Thelargest unit size for variegated, superb and red-backed fairy-wrens was 10, five and eight, respec-tively. Mean unit size was largest in the variegatedfairy-wren (F = 14.47, d.f. = 2,37, P < 0.001,

Table 2). Unit size remained stable throughout thebreeding season. ‘Divorce’ was not observed exceptfor one pair of red-backed fairy-wrens. Furgling orthe use of colored ornaments was common, partic-ularly in red-backed fairy-wrens. Often this behav-ior involved males from neighboring territories inpursuit of a female building a nest.

Habitat and unit size association

Stepwise regression showed that the ‘best’ model(Table 3) for describing the relationship betweenunit size of variegated fairy-wrens and habitatquality was foliage cover of brigalow and summerarthropod biomass (r2 = 0.817, F = 13.357,d.f. = 2,6, P < 0.01). In areas occupied by superbfairy-wrens, the height of ground cover and foliagecover of non-brigalow shrubs associated negativelywith bird-unit size (r2 = 0.837, F = 15.410,d.f. = 2,6, P < 0.01). For the red-backed fairy-wren, the ‘best model’ describing unit-size changewith habitat comprised ground cover and densityof brigalow (r2 = 0.416, F = 6.424, d.f. = 2,18,P < 0.01).

DISCUSSION

At Taunton National Park variegated, superb andred-backed fairy-wrens occurred sympatrically and

Fig. 2. Map showing the five sites at TauntonNational Park (Scientific) where overlapping territoriesof the three species of fairy-wrens were studied. Dis-tance between the sites is not to scale.

Fig. 3. Relationship between territory size and unitsize in variegated (�), superb (�), and red-backed (�)fairy-wrens.

1210864200

5

10

15

20

Unit size

Ter

rito

ry s

ize

(ha)

78 K. Chan and J. D. Augusteyn

occupied overlapping territories during the breed-ing season. Pair- and group-breeding units werefound in all three species. The social organizationof the superb fairy-wren is well known and,although the mean unit size at Taunton wasslightly larger than that found in other studies(Rowley 1965; Higgins et al. 2001), the maxi-mum unit size (5) is not the largest that has beenrecorded (up to nine individuals have beenreported: Pruett-Jones & Lewis 1990; Higginset al. 2001). Social organization of the variegatedfairy-wren is less well known, but units containing10–12 birds have been reported (Higgins et al.2001). The current study is consistent with thesereports; variegated fairy-wrens tend to live inlarger groups than the other two species and breed-ing units rarely contained single pairs of birds.Very little is known about the social organizationof the red-backed fairy-wren. In this study thelargest red-backed fairy-wren unit contained eightadult birds, but there was a clear predominance inpaired behavior in the species during the breedingperiod, although groups of more than 20 wereobserved outside the breeding season.

Size of territories for all three species was gen-erally larger at Taunton than previously recorded(Higgins et al. 2001). This may reflect limitedresources at Taunton. Brigalow shrubsA. harpophylla may be considered equivalent to‘brambles’ or ‘wandoo trees’ in other areas favoredby fairy-wrens (Nias 1984; Ligon et al. 1991; Nias& Ford 1992; Brooker & Rowley 1995). Largeareas existed at Taunton that contained few or nobrigalow trees or shrubs, and comparatively fewfairy-wren units were found in those habitats. Itappears that brigalow shrubs offer good cover frompredators and inclement weather.

For the variegated fairy-wren, summer arthro-pod biomass and decreasing brigalow foliage coverbest explained the increase in unit size. Arthropodbiomass was highest on A. hemiglauca, which growsamong mature brigalow trees. Therefore, varie-gated fairy-wrens may not actually avoid brigalow;rather, it may be an artefact of the increased bio-mass of A. hemiglauca in tall brigalow. Tall briga-low trees (rather than dense brigalow shrubs) allowfor other woody species, such as A. hemiglauca, togrow because of reduced competition at the shrublayer. This does not, however, preclude the likeli-hood that the birds use tall brigalow trees for theirdaily activities. In summer arthropod biomass isexpected to increase with area size, and birds inlarge unit sizes will need to occupy larger territo-ries to support extra group members. This is con-sistent with several studies on cooperativelybreeding birds that have shown that unit size var-ied positively with habitat quality (Brown & Balda1977; Nias 1984; Komdeur 1992; Nias & Ford1992; Langen & Vehrencamp 1998). These resultssuggest that occupation of good habitats confersan advantage to cooperative breeding. Althoughreproductive success was not measured in the cur-rent study, it has been found to correlate with thequality of the occupied habitat in other fairy-wrenspecies (e.g. M. splendens, Rowley & Russell 1990).Breeding success may not necessarily be restrictedto the dominant male in the group because extra-pair mating will also increase for subordinatemales in high-quality territories. Increased inci-dence of mating enhances the probability ofgenetic contribution to the next generation. Basedon the high incidence of furgling behavior in thisstudy, the level of promiscuity might be expectedto be as high as that demonstrated by Mulder et al.

Table 3 Standardized coefficients for the best models derived from stepwise regression analysis for predicting unitsize in variegated, superb and red-backed fairy-wrens

Variegated Superb Red-backed

Intercept 6.260 Intercept 10.320 Intercept -2.282FOLB -0.613 HTGC -1.089 GCOV 0.645SUIN 0.609 FOLS -0.588 BDEN 0.467

(GCOV), Percentage of ground cover; (HTGC), height of ground cover; (FOLB), percentage projected foliage of brigalowshrubs; (FOLS), percentage projected foliage of shrubs other than brigalow; (BDEN), density of brigalow trees; (SUIN), arthropodbiomass across a range of plants in summer.

Unit size and territory quality in Malurus 79

(1994), who found that 95% of broods in a popu-lation of superb fairy-wrens contained nestlingssired by males from outside the territory of thebird unit responsible for raising the young. A largeterritory with a large number of helpers is benefi-cial to all members of the unit because of theincreased opportunity for extra-pair copulations. Ahigh quality territory would allow inexperiencedbirds to delay their natal dispersal. By remainingin a territory and helping to raise another birds’young, the inexperienced birds’ reproductiveopportunity is likely to be improved rather thanrestrained.

Unit size of superb and red-backed fairy-wrensresponded differently to the habitat variables.Decreasing ground cover height and non-brigalowshrubs emerged as the most important determi-nants of unit size variation in superb fairy-wrens,whereas ground cover and brigalow density relatedpositively to unit size in red-backed fairy-wrens.Therefore, unit size associated with different terri-tory-quality measures in the three fairy-wren spe-cies. Unlike the variegated fairy-wren, however,unit size in superb and red-backed fairy-wrens didnot vary with territory size. It is possible that thepredominance of paired birds in these two speciesmay have affected the statistical results. However,it was clear that some of the single pairs occupiedmuch larger territories than those units containingmore than two members. Not only did superb andred-backed fairy-wrens have on average smallerunit sizes than those of variegated fairy-wrens, buttheir maximum unit size was also smaller.

Differences in unit size between the fairy-wrenspecies at Taunton cannot be attributed to habitatdifferences as the birds experienced similar habitatconditions because their territories overlappedextensively. Biological or genetic factors may actto limit the tendency to form groups in superb andred-backed fairy-wrens, at least compared to thevariegated fairy-wren. Ecological factors such asfood supply and competition may constrain themaximum territory and grouped unit size ofsuperb and red-backed fairy-wrens. Variegatedfairy-wrens are known to be tolerant of congeners(e.g. Tidemann 1990; Tibbetts & Pruett-Jones1999; Higgins et al. 2001). This may suggest thateither their social system is less affected by thepresence of competitors or that they are good com-petitors. The same may not apply to superb and

red-backed fairy-wrens. The predominance ofpaired units and their reluctance to form largegroups may indicate that the presence of congenerscould act to limit the number of birds in a terri-torial unit that can defend resources against otherfairy wrens. The red-backed fairy-wren may bemost affected by this because of its small body size.In general, species that are more inclined to formpairs or small groups during the breeding seasonmay be more sensitive to ecological factors whenforming large groups in a resource-limited envi-ronment, as large groups would require large,high-quality territories. Tidemann’s (1990) studyfound a relationship between fairy-wren unit sizeand insect numbers in all years except 1982, whichwas a drought year. In the spring–summer of1997/1998, rainfall at Taunton National Park wasonly 77% that of the corresponding period in1996/1997 and 83% of the rainfall in 1995/1996.The drier conditions could have affected insectsupplies and subsequent bird responses. Such envi-ronmental factors may have further negated anytendency to form large groups in the superb andred-backed fairy-wrens. A longer study would berequired to further explore this idea.

ACKNOWLEDGEMENTS

We thank the Queensland Parks and Wildlife Ser-vice for permission to work in Taunton NationalPark (Scientific) and its staff who assisted in vari-ous aspects of the work. We thank M. Coates andS. McKillup for comments on aspects of the study,and R. Melzer and anonymous reviewers for com-ments to our manuscript. We also thank A. Gould,J. Gray, B. Heinrich, R. Long, L. Pullin and D.Vavryn who volunteered their time to assist onfield trips.

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