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Where Have All theWhitethroats Gone?Published online: 23 Jun 2009.

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Where Have All theWhitethroats Gone?

by Derek Winstanley, Robert Spencer and Kenneth WilliamsonThe 'crash' of the W hitethroat population in 1969 was one of themore startling revelations of the Trust's Common Birds Census;but more alarming is the bird's failure to make any sort ofrecovery. Having investigated the possible causes of the decline,the authors are convinced that the Whitethroat is the victim of arecent climatic deterioration in its winter quarters, the drought-stricken Sahel Zone of West Africa.

UNTIL 1968 the Whitethroat Sylvia communis was an abundant andwidely distributed summer visitor in suitable habitats throughout Britainand Ireland; in 1969, however, the 'annual index' obtained from the Com-mon Birds Census of the British Trust for Ornithology showed that it haddecreased dramatically, by about 77% of the previous year's level (Batten1971 and Figure 1). Sudden declines in bird population levels are re-corded from time to time, severe winters in Europe being a common causeamong resident species, but the fecundity of most passerines enables themto make good these losses within three or four years (see Batten 1969,1971, for Wren Troglodytes troglodytes and others). Since its 1969 col-lapse the Whitethroat has given no sign of recovery, and remains at avery low level (Batten 1973).

The Common Birds Census was launched in 1962 in response to arequest by the Nature Conservancy for a programme capable of monitor-ing annual fluctuations in bird population levels, and the 'annual index'expresses the population as a percentage of that recorded in 1966, bywhich time it was felt that most resident species would have made asubstantial recovery from the reductions imposed by the severe wintersof 1962 and 1963. The data for most species, including the summervisitors, are derived from censuses made on some 300 plots in widelyscattered localities, and are therefore free from local bias. Furthermore,the 1969 Whitethroat crash was noted in other parts of western Europe,indicating an absolute decrease in total population rather than a re-dis-tribution within the breeding range. Quoting Vauk, Berthold (1973)says that at the bird observatory on Helgoland the spring of 1969 broughtthe lowest ever Whitethroat catch of 77 birds, as against a highest catchof 614 in 1968—a drop of 87%. He adds that migrating Whitethroats atRadolfzell on Lake Constance, southwest Germany, drawn probablyfrom between 5°-15° E., showed a 59% drop in autumn 1969 comparedwith the previous year, and he considers that the decline may well haveextended through western and central Europe as far as 15 0 E.

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Information available from the BTO Bird Ringing and Nest RecordSchemes shows that the Whitethroat enjoyed a successful breeding seasonin 1968, and good numbers were recorded on autumn migration (TablesI and II). The intensity of the ringing effort was much the same from1967-72.

TABLE 1. VVHITETHROAT RINGING TOTALS

1967 1968 1969 1970 1971Juv. /Adult 8,097 10,205 1,797 3,634 3,047Pullus 751 800 100 130 79

TOTAL 8,842 11,005 1,897 3,764 3,126

TABLE II. WHITETHROAT NEST RECORD CARD TOTALS

1967 1968 1969 1970 1971 1972256 326 45 68 68 53

130

110

90

30

1962 64 66 68 70 72

imerm. Farmland muumuu Wood land

Figure 1. Common Birds Census Whitethroat 'annual index', 1962-72

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The decline, therefore, was not due to factors operating within Britainor probably elsewhere in western Europe, and can be accounted for onlyby exceptional mortality during the 1968 migration to Africa, or on thewintering grounds on the southern fringe of the Sahara, or during thereturn migration in spring 1969. Furthermore, whatever the nature ofthe factors causing the decline, it is clear that they must have remainedoperative since 1969 to keep the population level depressed.

METEOROLOGICAL BACKGROUND TO THE MIGRATIONThere is evidence that the movement of birds is most widespread andintense during anticyclonic weather, and is much reduced or inhibitedduring the passing of low pressure disturbances (Williamson 1969). Dis-orientation in overcast weather, and strong head or cross winds, causingbirds to alight in inhospitable habitats, are factors which could lead tomortality during migration. But as the autumn migration of the White-throat extends over at least three months, and the spring passage at leasttwo (Davis 1967), any natural phenomenon such as gales or torrentialrain would be of too short duration to decimate the west European popu-lation. An analysis of the daily weather reports for western Europe andnorthwest Africa for the periods July-October 1968 and March-May 1969did not reveal any such widespread and persistent conditions. Neitherwere there persistently strong head or cross winds, though some dayswere unfavourable. The weather on many days in autumn 1968, andagain in spring 1969, appeared to be entirely favourable for migration—that is, days of anticyclonic weather with light winds, clear skies and goodvisibility, or of northerly tail winds in autumn and southerly tail winds inspring. From a meteorological viewpoint there appears to be no goodreason why Whitethroats should not have migrated successfully fromwestern Europe to West Africa in 1968 and back again in spring 1969.

POSSIBLE EFFECTS OF TOXIC RESIDUESBerthold (1973) suggests that the use of persistent insecticides in north-west African countries through which the Whitethroats pass on migrationcould have brought about the decline; but these would have had to beselective since a number of other passerine species which migrate acrossthe region do not appear to have been affected, or have shown only aslight decrease. Moreover, since the Whitethroat population is not re-covering, recontamination would have to be occurring annually. Theonly evidence he offered in support of this view is a statement that White-throats returning to Sweden in the spring of 1971 had concentrations oftoxic insecticides and polychlorinated biphenyls larger than had beenfound in Whitethroats emigrating in 1970 (Persson 1972). The concentra-tions of DDT and PCBs were in fact from two to three times higher inthe returning birds, but despite this Persson's study population near Lund(which had declined from 44 breeding pairs in 1968 to 26 pairs in 1969,increasing to 32 pairs in 1970) showed a 68% breeding success, which is

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of a similar order to that observed in other species. She shows, moreover,that degradation of the DDT residues in the birds' tissues occurs duringthe breeding season, so that the initial high concentration does not ad-versely affect their fecundity (Persson 1974).

It is unlikely that the Swedish birds have the same migration routes andwinter quarters as populations from Britain, France, Denmark and WestGermany, since of all European ringing recoveries plotted by Zink (1973),70% of Whitethroats ringed between 00 and 10 0 E. migrate in autumn ona heading between southwest and south-southwest, while 75% of thoseringed between 10° and 20° E. head between south-southwest and south-southeast. Since Berthold wrote, Persson (1974) has produced evidencethat spring migrants sampled at the Capri Bird Station, Italy, in 1972, had

Figure 2. Normal (1931-60) May-October rainfall (mm.) to the south of the Saharain West Africa. The stations used in the analysis, and in Figures 4-6, are shown

by dots.

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significantly lower levels of organochlorine residues than had a sample of21 Whitethroats taken on arrival at Ottenby Bird Station, on the island ofOland, in spring 1971. Her conclusion is that the birds must have becomecontaminated during their passage across Europe, and not in Africa.

Nevertheless, the possibility that Whitethroats could become con-taminated whilst in their African winter quarters needs to be explored.Enquiries made by us show that insecticides were used in locust controloperations in northwest Africa in 1968 and 1969, but their application wasextremely limited. Most of the spraying in Morocco was confined to theSous Valley, to protect the citrus groves over an area of some 600 km 2 ; ittook place in late November and December (when the great bulk of theWhitethroat passage must have gone), and used Dichlorovos (DDVP),which has a high locust toxicity but negligible perisistence. Farther south,rather smaller regions in the Adrar of Mauritania and the Adrar desIforas of Mali were affected by insecticides between August 1968 andMay 1969, most of this control employing low dosage rates of dieldrin.From August 1970 to May 1971 a total of some 1,300 km2 was treatedwith dieldrin during locust control in Mali, Niger and Mauritania, butthis is small compared with the massive dosages that are sprayed overlarge areas of Europe every year. Also, in west and northwest Africa, afar greater amount of insecticides—and particularly the highly persistentdieldrin—was applied over a much wider area during locust control workin the late 1950's and early 1960's, apparently without any adverse effecton the Whitethroat population (Figure 1).

As an explanation of the crash in the Whitethroat population, exposureto toxic chemicals whether during migration or on the wintering groundis clearly unsatisfactory. It is more probable that the factors responsiblefor the decline have been operating over a very large area, and, in view ofthe lack of any recovery, over a period of successive years; and the mostlikely area in which the mortality is occurring is the birds' winter quarterson the southern fringe of the Sahara Desert, in what is generally calledthe Sahel Zone.

ECOLOGICAL CONDITIONS IN THE WINTER QUARTERSEcological conditions south of the Sahara, between 10° and 20° N., aredetermined much more by the alternation of the dry and wet seasons thanby temperature. Rain falls in a limited season from May or June untilOctober, the rest of the year being practically rainless. The rains spreadnorthwards during summer and retreat southwards during autumn, withthe result that the mean seasonal rainfall decreases from 1,200 mm. ormore at 10° N. to about 50 mm. only at 20° N. There is a strongly markedzonal arrangement of the isohyets, as seen in Figure 2.

The Palaearctic migrants arrive in this region at the end of the wetseason, when the vegetation is green and relatively plentiful; but asMoreau (1966) has pointed out, they are faced with an ecosystem that isdeteriorating during most of their stay. Some passerines (for example theWillow Warbler Phylloscopus trochilus, and the flycatchers Muscicapa

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striata and Ficedula hypoleuca) stay in the northern semi-arid zone forseveral weeks and then move farther south (Morel 1973), but the White-throat apparently remains in the Sahel Zone between about 12°48° N.Here the feeding conditions after six months without rain must still begood enough, under normal circumstances, for migrants to accumulatesufficient fat deposits to provide for their return crossing of the SaharaDesert in spring. The vegetation in this zone is mainly grass steppe withAcacia, Balanites, Zizyphus and other trees and scrub, many with ever-green or semi-evergreen foliage, a number producing berries and otherfruits which are eaten by passerines. All this must harbour a considerablearthropod fauna. Conditions during the dry season are less poor thanwould appear superficially, with trees at every stage of leaf, flower andfruit production, and however much of a paradox it may seem, ecologicalconditions must normally be favourable for the survival of the White-throats and their congeners. The abrupt increase in resources "cannot befully exploited by the resident birds alone, whose populations are adjustedto the period when food is scarcest", and it is precisely because of theabsence of potential competitors that such vast numbers of immigrantPalaearctic birds are able to winter there successfully (Morel 1973).

Figure 3. Mean percentage of normal (1931-60) May-October rainfall at Nouak-chott, Atar (Mauritania), Gao, Tessalit (Mali), Agades (Niger), and Khartoum

(Sudan).

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WHERE HAVE THE WHITETHROATS GONE?

1960 61 62 63 64 65 66 67 68 69 70 71 72

Figure 4. Mean percentage of normal (1931-60) May-October rainfall for 60 stationsin Mauritania, Senágal, Gambia, Guinea, Upper Volta, Dahomey, Togo, MaliChad, Nigeria and Central African Republic, between 10° and 20° N. See Figure 2

for location map.

Rainfall must obviously be of great importance in maintaining theecological balance, and in particular in determining the nature andamount of the food supply. The biomass available to the migrants on theirarrival and during subsequent months will depend greatly on the totalprecipitation during the wet season, and especially on the amount whichfalls towards the end of the rainy period.

RAINFALL TRENDS TO THE SOUTH OF THE SAHARAIn the following discussion of rainfall trends to the south of the Sahara,we take as 'normal' the average May-October values over the 30-years'period 1931-1960. Figure 3 shows the mean percentage of normal at sixstations—Nouakchott, Atar (Mauritania), Gao, Tessalit (Mali), Agades(Niger) and Khartoum (Sudan)—from 1941-1972. The important featureis that there are not random variations from year to year but ratherdefinite trends. Rainfall was 15% to 40% above normal throughout the1950's, but has been below normal in eight out of the last ten years, withincreasingly low values being recorded.

In order to study the variations in greater detail, the seasonal May-October rainfall was calculated from monthly totals for 60 stations inMauritania, Senegal, Gambia, Mali, Upper Volta, Niger, Chad, Nigeria,Dahomey, Togo and the Central African Republic, between 10° and 20 0

N., from 1960 to 1972. The locations are shown in Figure 2. The seasonalmean percentages of normal for the 60 stations are shown graphically inFigure 4. Similar trends are observed as for the six sample stations com-bined in Figure 3.

The geographical distribution of rainfall in 1964, a year in whichFigure 4 shows there was about normal precipitation throughout theregion as a whole, is depicted in Figure 5(a). It is evident that there weresome areas where it was above, and others where it was below, normal.

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WHERE HAVE THE WIETETHROATS GONE?

These are the sort of deviation patterns to be expected in a 'normal' year.During the wet 1950's there were extensive areas with above normal rain-fall, and relatively limited areas where precipitation was low.

Figure 5(b) shows the rainfall deviations in 1968, a year in whichFigure 4 shows that precipitation was 25% below normal throughout theregion as a whole, and which Figure 3 shows had the lowest rainfall since1949. In 1968 there were very few areas with rainfall approaching, orgreater than, normal. In Mauritania, Senëgal and northern Mali, the pre-cipitation was as much as 70% below normal, and the isohyets in centraland southern Mauritania were up to 400 km. farther south than usual.

Rainfall increased in 1969 but was nevertheless 10% below normal. In1970 it was 23% below normal, and the values for 1971 and 1972 were36% and 37% below normal respectively. Figure 5(c) shows that thedeviations in 1971 were greatest along the southern fringe of the Sahara,where stations with a normal seasonal fall of 100-150 mm. received only4-6 mm The isohyets in Mauritania and northern Chad were generally150-200 km., and locally up to 400 km., farther south than usual.

In Figure 6 we see that the percentage of normal seasonal rainfall, intwo-degree latitudinal bands, decreased with advancing latitude duringthe 1960's and early 1970's. The decrease in the latitude band 10°-12° N.has been only 5%, whereas in the band 16°-18° N. precipitation has been45% below normal. The decrease has continued since 1958 and at the sixsample stations used for Figure 3 it has amounted to about 70% ofnormal since that date.

DISCUSSIONThere can be little doubt that the most likely cause of the rapid decreasein the number of Whitethroats in western and possibly central Europe ishigh mortality in their winter quarters between about 12°-18° N. in WestAfrica, where the continuing drought conditions have seriously upset thedelicate ecological balance. The seasonal rainfall decreased by some 22%of normal throughout this zone from 1967 to 1968, to a level 25% belownormal, and had continued to decline to a level 37% below normal by1972. Present indications are that 1973 showed no improvement.

The Common Birds Census, besides measuring a continuing decline inthe British Whitetbroat population, has also detected decreases since 1968in other species which winter in the affected areas of West Africa; theseare the Sedge Warbler Acrocephalus schoenobaenus, Redstart Phoeni-curus phoenicurus (declining, however, since 1965), Spotted Flycatcher,Garden Warbler Sylvia born and Yellow Wagtail Motacilla flava flavis-sinza. Moreover, the view that climatic factors are responsible is strength-ened by the fact that some species, notably the Whitethroat, Garden andSedge Warblers and Redstart, showed a slight upsurge in numbers in1970, corresponding with the better rainfall of 1969.

It is probable that the high mortality has occurred as a result of reducedavailability of food, water and shelter, a situation which clearly could havebeen exacerbated by the demands on a deteriorating environment made by

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14°- 16° NI 22222 I 12°- 14°NAirAir 10°- 12° N

'mom 16° - 18° N

1960 1962 1964 1966 1968 1970 1972

BIRD STUDY

Figure 6. 3-year running-means of the mean percentage of normal (1931-60) May-October rainfall in 2 degree latitude bands between the Atlantic coast of West

Africa and 20° E.

nomad herdsmen and their stock, whose numbers greatly increased duringthe congenial 1950s. The most critical period for the birds may well bethe few weeks towards the end of the dry season when food is scarcest andthe birds must lay down pre-migratory fats to supply energy for the returncrossing of the Sahara Desert. (They eat Maeru and Salvadora berries andfrequently forage for insects on the ground, according to Moreau (1972).)If, because of the unfavourable conditions now found in the Sahel Zone,the Whitethroats attempted to shift their winter quarters farther southinto the less seriously affected regions, they would probably be broughtinto acute competition with resident African birds (which are more num-erous there) and Palaearctic wintering species, which presumably are much

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better adapted to exploit the dwindling local resources. Species such asthe Willow Warbler and flycatchers, which can and do withdraw south-wards some time after their autumn arrival in the Sahel Zone, may thenreplace native species which would otherwise be competing with them;in any event, they would be less adversely affected by the smaller rainfalldecreases south of about 12° N. (Figure 6), and would show less abruptdeclines.

The probable cause of this continuous decline in rainfall along thesouthern fringe of the Sahara is a decrease in the intensity and northwardextent of the ascending branch of the tropical Hadley cell circulation,associated with a general weakening of the global atmospheric circulation(Winstanley 1973 (a) ). This means that not only do the moist southwestwinds not penetrate as far inland as in, say, the 1950's, but also that theweaker upcurrents (or stronger downcurrents) inhibit the rain-makingprocesses. In higher latitudes, the amplitude of the troughs in the wester-lies has increased during the past decade, resulting in higher rainfall overthe Mediterranean Basin and along the northern fringe of the SaharaDesert. The effect of these changes in the global atmospheric circulationhas been a southward shift of the main precipitation belts in the northernhemisphere, amounting to 1 . -14- . of latitude over the past 15 years. Thesechanges have resulted in the southward shift of the Sahara, with increas-ing rainfall to the north and severe droughts to the south from Mauritaniaacross the continent to Ethiopia (Winstanley 1973 (a) and (b)). Recentdroughts in similar latitudes in northern South America, southern Arabiaand India are possibly part of this global pattern of climatic change

The present study has shown that superimposed on the long-term cli-matic changes which take place over thousands of years there are rela-tively short-term changes and even dramatic fluctuations from one yearto the next. Species can adapt to the more gradual changes, but the drama-tic short-term ones can alter population levels drastically, though some-times only temporarily. This suggests that it is the rate of climatic changeas much as the magnitude that is of vital importance in the populationdynamics of some species.

In order to understand the Earth as a planetary ecosystem, it is neces-sary to monitor climate and to identify and study the tele-connectionsexisting between climatic and other ecological changes in different regions.In face of the continuing growth of planned and inadvertent modificationof the environment by man, it is important to know the nature and scaleof the repercussions throughout the system of natural changes in regionalecological conditions, for only then can we assess man's impact. If White-throats have decreased dramatically as a result of a climatic change con-comitant with the weakening global atmospheric circulation, then theremust surely also be significant changes in numbers and distribution ofother birds, mammals and insects. We know that along the southernfringe of the Sahara the current drought has seriously disrupted man's

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highly functionally adapted way of life, nomadic pastoralism, and is caus-ing death to cattle, sheep, goats, camels and even man himself (Dalby andHarrison Church 1973).

It has been suggested that rainfall along the southern fringe of theSahara is currently at a low point of an irregular fluctuation and that itcan be expected to increase towards 1980, but that this may be super-imposed on the long downward trend of a 200-years' cycle (Winstanley1973 (b)). It will be interesting to see whether the Whitethroat populationcan recover its pre-1968 situation, or whether it will stabilise at a lowerlevel.

SUMMARYA dramatic decrease in the Whitethroat Sylvia communis population of Britain,by 77% of the previous year's level as measured by the Common Birds Census, tookplace in 1969. A similar decline was noted in other parts of western Europe (Ber-thold 1973). The population has shown no signs of recovery.

Evidence from the BTO Bird Ringing and Nest Record Schemes shows thatproductivity was high in 1968, so the losses must have occurred either during themigration periods (autumn 1968, spring 1969) or in winter quarters in West Africa.

Examination of the weather records indicates that no adverse meteorologicalfactors were involved during the migration seasons. A possibility that the lossesmight be due to exposure to toxic residues resulting from locust control operationsin northwest Africa, in autumn 1968 and since, is also examined, but it is con-sidered that the scale of application of pesticides was too fragmentary to causesuch huge losses and keep the population level depressed.

The cause seems likely, therefore, to be in the wintering area, the arid steppe onthe southern fringe of the Sahara, between 12 0 -18° N. in West Africa. Ecologicalconditions in this zone are described, and an intensive study is made of the rainfallregime over the period 1941-72. During the 1950's rainfall was substantially abovenormal, but since 1968 has been well below normal, resulting in severe drought.This appears to be a link in a chain of climatic changes brought about by a generalweakening of the global atmospheric circulation.

It is considered that this rapid and continuing climatic deterioration is responsiblefor an unusually heavy mortality among wintering Whitethroats, and is probablyaffecting other Palaearctic migrants less severely. The climatic change is thoughtto be long-term and the Whitethroat population may well stabilise at a relativelylow level compared with pre-1968.

APPENDIXThe magnitude and even the sign of precipitation changes associated with changesin the global atmospheric circulation vary with latitude. The relatively small de-crease in precipitation at about 10° N. is of equal significance with the much largerdecrease between about 12°-18° N. This suggests that as the global circulationweakens, there is not a uniform decrease in vertical velocities throughout the ascend-ing branch of the Hadley cell, but rather that the largest decrease occurs towardsthe poleward limit of the ascending branch. One would perhaps expect acorresponding decrease in downcurrents (or increase in upcurrents) towards thepoleward limit of the descending branch of the Hadley cell in the southern hemis-phere, with a consequent increase in precipitation in those latitudes in June-October.Conversely, the pattern should reverse with the change of seasons. In the periodNovember-March a decrease in the strength and poleward extent of the ascendingbranch of the Hadley cell would be associated with a decrease in rainfall at about20° S., which is -borne out by recent droughts in Botswana and Rhodesia. In thenorthern hemisphere, a decrease in downcurrents towards the poleward limit of the

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descending branch of the Hadley cell would lead to an increase in rainfall at about200 N., of which recent high winter rainfall in Mauritania, along the Red Seacoastal plains and in Arabia is evidence (Winstanley 1973(a) ). This zone of highNovember-March rainfall merges into the zone of increased rainfall at about 30° N.which is associated with the above mentioned expansion of the circumpolar vortexof westerly winds.

From an analysis of patterns of rainfall distribution associated with changes bothin the strength of the general atmospheric circulation and in the latitude of itscomponent parts, it is possible to postulate the following physical linkage betweenmid-latitude and tropical circulations. The latitudinal temperature gradient deter-mines the degree of baroclinic eddy activity, which in turn controls the flux ofangular momentum polewards. As the latter decreases the Hadley cell circulationmust also decrease to provide a momentum balance in the upper troposphere. Newell(1973) also propounded this theory in explaining the climate of the GalapagosIslands at 20,000 BP. This was the time of the last glacial maximum when strongzonal westerlies were displaced 10°-1 5° latitude towards the equator, and rainfallwas presumably high over the Mediterranean Basin and along the northern fringeof the Sahara (Lamb 1968). According to the above theory the Hadley cell mustalso have been stronger to provide a momentum balance in the atmosphere. Con-sequently, rainfall over the southern Sahara, and also over the Kalahari, musthave been higher as a result of increased upcurrents and poleward extension of theascending branch of the Hadley cell circulation. This would explain the coincidencebetween glacial maxima in mid-latitudes and pluvials in the sub-tropical deserts(see e.g. Moreau 1966).

According to this theory, there is no necessity for a constant relationship be-tween rainfall patterns to the north and to the south of the Sahara. With a strongglobal circulation there must always be an increase in the strength and polewardextent of the Hadley cell, giving higher rainfall along the equatorial fringes of theSahara and Kalahari Deserts. The latitude of the axis of the strongest zonal wester-lies, however, will be determined by the latitude of the strongest temperaturegradient and so may be displaced towards either high or low latitudes, with respec-tively low or high rainfall to the north of the Sahara. With weak zonal westerliesthe Hadley cell must also decrease, leading to a narrowing of the tropical rainfallbelt; but increased meridionality in the circumpolar westerlies will then give higherrainfall to the north of the Sahara. This is the situation we have at the present time.

Palaeoclimatological evidence which suggests a varying relationship betweenrainfall in different latitudinal bands from one epoch to another should thus notnecessarily be regarded as spurious. The postulated linkage between tropical andextra-tropical circulations offers an explanation as to why the Sahara Desert shouldat times be narrowed or widened, and at other times be shifted north or south.Moreau (1966) discusses the significance of such climatic changes in the evolutionof African and Palaearctic bird faunas.

REFERENCESBATTEN, L. A. 1969. Bird population changes on farmland and in woodland for the years 1967-68.

Bird Study, 16:163-168.BATTEN, L. A. 1971. Bird population changes on farmland and in woodland for the years 1968-69.

Bird Study, 18: 1-8.BATTEN, L. A. 1973. Bird population changes on farmland and in woodland for the years 1971-72.

Bird Study, 20:303-307.DAVIS, P. E. 1967. Migration seasons of the Sylvia warblers at British bird observatories. Bird Study,

14:65-95.MOREAU, R. E. 1966. The Bird Faunas of Africa and its Islands. London.MOREAU, R. E. 1972, The Palaearctic-African Bird Migration Systems. London.NEWELL, R. E. 1973, Nature, 245:91-92.BERTHOLD, P. 1973. On the marked decline of the Whitethroat Sylvia communis and other species of

songbird in western Europe. (Original in German). J. Orn., 114:348-360.PERSSON, B. 1972. DDT content of Whitethroats lower after summer stay in Sweden. Amino, 1:34-35.PERSSON, B. 1974. Degradation and seasonal variation of DDT in Whitethroats Sylvia communts.

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MOREL, G. 1973. The Sahel Zone as an environment for Palaearctic migrants. Ibis, 115:413-417.WINSTANLEY, D. 1973(a) Nature, 243:464-465.WINSTANLEY, D. 1973(b). Nature, 245:190-194.LAMB, H. H. 1968. The Changing Climate, London.WILLIAMSON, K. 1969. Weather systems and bird movements. Q. I. Roy. Met. Soc., 95 : 414-423.ZINK, G. 1973. Der Zug Europaischer Singvogel I. Volgelwarte Radolfzell.DALRY, D. and a. 3. HARRISON CHURCH (Eds.) (1973). Drought in Africa. Centre for African Studies,

School of Oriental and African Studies, University of London.Information on the use of pesticides in Africa was taken from the following publications: Reportof the Ann-Locust Research Centre for 1968, MOD, London (1969); Desert Locust SituationSummaries of the Centre for Overseas Pest Research; Perret, T. Locust control in Morocco—recentand future developments. Proc. Int. Study Conf. on Current and Future Problems of Acridology,London 6-15 July 1970; MacCuaig, R. D., Insecticide Index, UNDP, Desert Locust Project, F.A.O.,Rome 1966. Miss Louise Bennett of COPR also kindly supplied information on insecticides usedagainst Desert Locusts in West Africa between August 1968 and May 1969.

Sources of rainfall and other meteorological data are: Daily Weather Reports of the Meteoro-logical Office, Bracknell; Daily Weather Reports of the Algerian Meteorological Service, Algiers;WMO Tech. Note No. 117 (1971); World Weather Records, Weather Bureau, U.S. Dept. Comm.,Washington D.C. 1959; ESSA., U.S., Dept. Comm. Washington D.C. 1967; Monthly Climatic Datafor the World, Weather Bureau, Dept. Comm., subsequently Environment Data Service, ESSA,now NOAA, U.S. Dept. Comm., Washington D.C. 1961-1972.

Derek Winstanley, 139 Trinity Road, Tooting, London SW17 7HJ. (NowEcological Systems Research Division, Environment Canada, OttawaKIA OH3.)

Robert Spencer, Ringing and Migration Section, BTO, Beech Grove,Tring HP23 5NR, Hertfordshire.

Kenneth Williamson, Populations Section, BTO, Beech Grove, TringHP 23 5NR, Hertfordshire.

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