Yellow Wagtail MotaciJua fjlava migration from West Africa to Europe: pointers towards a consewation strategy for migrants on passage

B R I A N WOOD Ecology G Conservation Unit, Department of Biology, University College London. Cower Street, London WC I E 6BT, U K

The biology of one passerine migrant, the Yellow Wagtail Motacillu //uvu, is examined in order to identify factors which need to be considered when attempting to conserve migrants on passage. The distribution of Yellow Wagtails in West Africa, prior to northward (spring) migration, is affected by food supplies and by different racial and sexual responses to environmental factors. Consequently, each breeding population relies on a wide range of sites, but those occupied towards the end of the dry season and ones used as stepping-off points for migration may be particularly vital.

Energetic considerations point towards a non-stop trans-Saharan flight being used rather than a series of short flights with resting stops in the desert. However, adverse condi- tions during migration could make stop-overs unavoidable. The most direct routes between wintering and breeding areas are preferred, and the initial desert crossing is accomplished at a faster pace than later stages of the journey.

Yellow Wagtails emigrate from West Africa in a sequence related to the onset of spring on their breeding sites, rather than in response to rainfall in Africa: moult, fattening and emigration are probably endogenously controlled. However, competition from African resident birds may also be an important factor affecting the timing of migration.

Races breeding in southern Europe are most likely to be adversely affected by degrada- tion of the Sahel. Empirical evidence suggests that conditions on the wintering grounds are more likely to regulate population size than conditions experienced on migration, but more detailed studies are needed in Africa before sound conservation strategies can be designed.

Although they may escape the viscissitudes of the Euro- pean winter and its associated food shortages, migrant birds nevertheless face considerable hardships whilst overwintering in Africa. W e know that many are almost constantly on the move during t h e months of the northern winter (Curry-Lindahl 1981) and could thus be considered to be 'on passage' for the greater part of their lives. Probably the principal reason why many migrants con- stantly change their distribution whilst in Africa is that they need to respond to changing environmental circum- stances. Outside the breeding season they are not tied to a fixed site and so, can move according to the availability of resources. Those that make t h e best moves gain the most resources and so maximize their chances of surviving to breed again in Europe the following summer.

Natural environmental changes have occurred throughout the time during which the current patterns of bird migration have been evolving (Moreau 1972). The migrant species that we presently observe are those which have been best able to adjust their strategies to deal with such changes. Whilst natural changes will con- tinue to occur in the future, the environments used by

migrants are being altered much more rapidly by the acti- vities of man. Our concern for the inability of migrants to cope with such rapid alterations leads us to attempt to suggest ameliorations so as to conserve migrant species.

One strategy for the conservation of bird migrants would be to attempt to minimize induced environmental changes. This would be likely to have limited success, since most changes are made in the belief that they will benefit mankind (Timberlake 1987). Alternatively, we could attempt to identify those changes which are most detri- mental and design ways of minimizing their impact. Unfor- tunately, most changes harm some species and benefit others, so it is impossible to draw up a realistic balance- sheet.

It is easy to identify changes to the environment which are certainly harmful to migrants. Any reduction in produc- tivity will reduce a site's capacity to support birds and other wildlife. By far the greatest of such changes is the degradation of the sahel zone immediately to the south of the Sahara, and the associated spread of desertification. Alterations in the climate and vegetation of this region are known to parallel declines in migrant bird species (e.g.

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Winstanley ef al. 1974, Kanyamibwa et al. 1990). They may be a result of recent mis-use of the sahel (Sinclair & Fryxell 1985). a consequence of global climatic change (Pearce 1991 t, or a combination of both.

Regrettably however, we simply do not have sufficient data to show how mortality rates are altered by most major environmental changes. Without direct evidence, a catalogue of changes to the environment which are known to be detrimental to bird migrants can do little more than fuel speculation as to t h e causes of declines in bird numbers. In a few cases, sufficient data have been collected to enable predictions to be made (e.g. Kan- yamibwa et a/. 1990, Baillie & Peach 1992).

Rather than attempt a general overview, an alternative approach is adopted here. By seeking to understand better the ways in which migrant birds exploit the environment during their journeys, it may be possible to suggest certain sites or circumstances critical to their survival. I f these can be identified, it could be possible to direct conservation efforts more effectively.

Because individual species differ in their requirements, ideally a wide range of species and their migration strate- gies should be reviewed. At present, the lack of suffi- ciently detailed information for enough species prevents even this approach. Instead, I will review in detail the ecology of one migrant species the Yellow Wagtail Motacilla /lava. This should at least indicate if this approach is a fruitful one to follow.

M E T H O D S

In this review of migration between the Palaearctic and Africa, Moreau ( 1972) came to the conclusion that insecti- vorous passerines are the most numerous bird migrants. In selecting an insectivorous passerine migrant for detailed analysis I have sought to identify one which will yield the greatest possible information. The Yellow Wagtail affords several advantages. It is relatively easy to sex using plumage characteristics, even in non-breeding plumage (Svensson 1970, Wood 1976). Numerous races have been described (Grant & Mackworth-Praed 1952, Vaurie 1957, Sammalisto 1961) in which the pattern of head plumage (at least in males) is diagnostic. It roosts in large congregations and consequently very many have been trapped and ringed.

In Africa, topography has a strong influence on rainfall and vegetation patterns, but these are relatively simple in West Africa, where there is a single dry season each year. The rainy season begiris progressively later towards the north, is shorter and produces less rainfall. Thus, migrant birds wintering at the latitude of lbadan (7"20") will experience rainfall during March, but ones wintering at Kano (12'03'N) would have to stay until the end of May before the wet season begins.

To design a strategy for the conservation of long- distance bird migrants, information is needed about

several biological parameters. Briefly, these are:

( 1 ) Migration routes and knowledge of the location and magnitude of any concentrations of birds that occur regularly during migration.

(2) Habitats used during every stage of the journey. 13) The strategies used when crossing inhospitable

regions, such as seas and deserts. (4 ) Specific locations used as departure and arrival

points on either side of inhospitable regions. (5) The places at which birds fatten in preparation for

migration and the foods that they eat. Sites at which other energy-demanding processes take place (e.g. moult) may also be important.

(6) Timing of migrations: these may differ between races and sexes of the same species. The seasonality of the environments used and weather conditions encoun- tered by migrants may also affect timing.

I will address each of these factors as they might affect Yellow Wagtails on their northward migration from West Africa to Europe.

RESULTS

Foods and foraging techniques

Throughout the year Yellow Wagtails are predominantly insectivorous and forage on the ground and by making short sallies to catch flying insects. Although they may perch in trees and shrubs, they do not apparently normally feed there. They prefer to forage in grassland and along water margins, where insects may be abundant, but also associate closely with herbivorous mammals which disturb insects a s they move.

Analysis of the stomach contents of birds collected in central Nigeria shows that Coleoptera, Hemiptera. Hymen- optera and Orthoptera are important foods (Wood 1976). Although there is one authenticated case of Yellow Wagtails taking berries in the period immediately prior to northward migration (Fry et al. 1972) the habit does not seem to be regular in this species. In this respect it is at a disadvantage to many warblers and other migrant species, which preferentially consume fruits and often do so voraciously at times of migration. Since arthropod biomass is directly related to the steady decline in soil moisture availability during the course of the dry season (Gillon G Gillon 1974, Wood 1976, 19781, Yellow Wagtails may face particularly difficult circumstances immediately before migration. Late in the dry season they spend over 75% of daylight time actively foraging (Wood 1976).

Migration routes

Many Yellow Wagtails have been ringed in Nigeria and recoveries show that most migrate along great circle routes to breeding ranges in the Baltic states (Wood 1982).

BRIAN WOOD IB IS 134 SUPPC. I 68

Concentrations of recoveries in Italy and Malta are probably due mainly to trapping and shooting. In Italy their distribution is predominantly coastal, but otherwise no obvious concentration of birds or selection of specific flyways is apparent.

Distribution prior to migration

The distribution of birds just before migration is particu- larly important since it will affect their opportunities for feeding and the total distance they may cover on the journey. At least three factors appear to determine distributions.

(a\ Lrap-frog migration patfem. Yellow Wagtails show a pattern of migration in which the northernmost breeders winter furthest south and thus 'leap-frog' over more southerly breeders, which occupy the north of the winter ranges (Wood 1975, 19761.

(b) Re-distribution during winter months. Counts of Yellow Wagtails occupying a large roost near Vom in central Nigeria revealed that this population was declining progressively at a rate of 3 I '% per month [Wood 1978). The loss of birds was probably a reflection of increas- ingly difficult feeding conditions as the dry season progressed. Foraging success (measured as the number of arthropods taken by a foraging Yellow Wagtail per 100 pacest aiso declined progressively throughout the dry season at about 25% per month whilst, at the same time. arthropod biomass in foraging areas fell at 57% per month.

However, despite the continuing decline in available food. the disappearance of birds from this roost was unlikely to be due to starvation-induced mortality. Analysis of seven years' recapture data for Yellow Wagtails using the roost a t Vom shows an apparent annual mortality rate (actual mortality plus dispersal of birds from the roost) of 63.5% IFig, It. This is similar to the 688 mortality rate estimated by Ashford ( 1970) for another roost of Yellow Wagtails at Kano, further north in Nigeria. It seems highly likely that most of the reduction in numbers is caused by birds moving south to more favourable regions as the dry season progresses, and there is a little ringing evidence to support this Wood 1978).

(cl Sex ratios. The sexes of Yellow Wagtails are not equally distributed throughout their winter range; males predominate in the north and females in the south (Fig 2). There are insufficient data to examine any changes in sex ratio which may also occur at indi- vidual sites during the course of the winter. although such changes may be expected as the number of birds at northern sites declines.

Fattening and departure locations

Most trans-Saharan migrants show massive weight

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Figure I . The rate of recapture of Yellow wagtails in successive seasons at a roost near Vom. central Nigeria. The regression line is: log Y = -0.635X + 3.075; r z = 0.899. This indicates an apparent mean annual mortality (actual mortality plus loss of birds from this roost to elsewhere) of 63.5%.

increases in the period immediately prior to their depar- ture from wintering areas (Fry el at. 1970). These increases reflect the accumulation of fat (Ward 1963) and changes in other body constituents (Fogden 1972, Fry el al. 1972, Baggot 1975). In Nigeria, and elsewhere, these weight increases are mostly accomplished a t the locations occupied by migrants immediately before they set out on the northward journey [Curry-Lindahl 1963, Ward 1964. F r y et a/ . 1972). Thus, most birds fatten well to the south of the Sahara.

There is little direct evidence to demonstrate that birds proceed gradually northwards to the desert's edge, but a considerable body of circumstantial evidence is available. During April, when Yellow Wagtails have begun their migration to the Palaearctic, an influx of southerly

YELLOW W A G T A I L M I G R A T I O N 69 I992

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Figure 2. Sex-ratios of Yellow Wagtails in Nigeria (with samde sizes). Least-sauares

t 1 r I I regression line is: 7 8 9 10 II 12 log arcsin Y = 0.975 + 0.0484 X Ir' = 93.8%).

Latitude North

wintering males M.1. thunbergi occurs in central Nigeria (Wood 1976). Differential timing of occurrence of each race was also observed at Lake Chad (Fry et a/. 1972). There are also numerous reported occurrences in sub-Saharan regions during spring of palaearctic migrant species which do not overwinter so far north. The maiority of these birds carry substantial fat deposits (Smith I966a. pers. obs.). Thus, it seems likely that many migrant species undertake the initial part of their migration in gradual stages, having already fattened in their winter quarters further south.

Timing of northward migration

Some authors have postulated that environmental cues in Africa may be used to determine the onset of spring migration (Owen 1969, Dingle & Khamala 1972). The

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progressive northward movement of the rain belt asso- ciated with the inter-tropical convergence could both provide such a cue and also substantially enhance the availability of arthropods, so providing a means of achieving pre-migratory fattening.

However, timing of fattening of Yellow Wagtails in Nigeria is both latitude-specific and highly synchronized between years at any one latitude (Fig. 3). irrespective of the exact time of arrival of rainfall. Moreover, it is those birds wintering in the northern parts of the country which fatten and migrate first, followed sequentially by birds that have wintered further south (Fig. 4). The northern- most wintering birds do not experience any rainfall what- soever before they migrate, whereas those in the south do not leave for Europe until some weeks after the first rains of the wet season. The migration pattern is thus the exact

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Figure 3. Pre-migratory fattening of Yellow Wagtails at Vom and Kano in Nigeria. Mean body masses k 2 s.e. are shown for all samples > 10 birds. Years are indicated as follows: 1964 (stars: data from Smith & Ebbutt 1965); I973 (triangles); 1974 (circles); 1975 (squares). Open symbols =

Kano, solid symbols = Vom. Trend lines fitted by eye.

B R I A N WOOD I B I S 134 SUPPL I 70

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Figure 4. Timing of fattening of Yellow Wagtails in Nigeria Mean body masses are shown for Kano and Vom, combining data for several seasons, and for M./flavci at Malamfatori during spring 1970 (Fry d a1 lY721. f.at values for individual birds a t lbadan are from Ward IYb6 Trend lines fitted by eye.

opposite of that which would be expected if rainfall was the cue initiating departure.

Because the racial distribution of birds in winter follows a leap-frog pattern, initiation of spring migration by any particular group of birds fits quite closely with the time at which their breeding ranges will become available during t h e palaearctic spring. The pre-migratory moult of different races of Yellow Wagtail also fits this pattern (Wood 1976). Moult, fattening and migration may all be

timed by internal circannual rhythms, adjusted to events experienced the previous autumn, as has been demon- strated to occur in Blackcaps Sylvia borin [Berthold 1984).

The process of fattening before migration takes place progressively and there is variation between individuals in the timing and extent of fattening. This is especially noticeable between the sexes, with females showing increases in body-mass substantially later than males (Fig. 5).

Strategies used to cross deserts and seas

It has been conventionally assumed that migrants normally attempt to cross inhospitable areas such as deserts and seas in non-stop flights (Moreau 19721. Recent observations from desert oases suggest that this may not necessarily always apply (Biebach et al. 1986, Bair- lein 1987) and an alternative short-hop strategy has been proposed. It is possible to make some estimates of the consequences of these alternative strategies for a migrant such as the Yellow Wagtail.

An estimate has been made previously of the migra- tory range, flight speed and progressive decline in body- mass for Yellow Wagtails crossing the Sahara from Nigeria to north Africa during t h e pre-nuptial migration (Wood 1982). Yellow Wagtails would need to fly non-stop for about 70h, using up the whole of their lipid reserves in the process, in order to make such a desert crossing. Unless t h e y dehydrated before setting out, thereby reducing their overall mass whilst retaining the same lipid mass, a non-stop crossing could only just be attained in still air.

This model is presented in Figure 6, with some minor alterations and the inclusion of additional data. In this case, birds are assumed to depart on a trans-Saharan crossing from the latitude of Kano (12"N) but all other parameters remain the same as previously (Wood 1982). Whilst calculated patterns of decline in body-mass and distance travelled are based on Pennycuick's ( 1969) formulae, which some authors consider to be unduly opti- mistic (e.g. Bairlein 1987). there is nevertheless an excel- lent agreement between predicted patterns and actual body masses reported from the central Sahara and the Fezzan. The body masses of Yellow Wagtails at dawn in central Nigeria are somewhat lower than predicted body masses at the end of the migratory flight (Fig. 6). This is almost certainly due to a slight overnight dehydration. I f some dehydration of trans-Saharan migrants was incor- porated into this model, the agreement with observed body masses would be even better. It should also be noted that, although Colomb Bechar and Defilia lie considerably to the west of the route likely to be taken by birds emigrating from Nigeria, body masses recorded there in spring (Smith 1966b. Ash 1969) fit the predicted pattern remarkably closely.

A similar model can be constructed for a Yellow Wagtail making a desert crossing that includes daily stops

I992 YELLOW WAGTAIL MIGRATION 71

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Figure 6. The predicated pattern of decline in body-mass with distance travelled for Yellow Wagtails emigrating from Kano (latitude ITNI and crossing the Sahara in non-stop flight. Body masses prior to departure and 95% ranges t k 2 s.d.) were calculated by the method used by Wood ( 1982). Maximum recorded body-mass of each sex is also indicated. Birds are assumed to maintain a constant water index of 203 throughout. T h e recorded mean body masses and ranges of birds caught at locations in the northern Sahara and the Mediterranean basin are shown for comparison with predicted values. Also shown are mean body masses and 95% ranges (plus minimum recorded body masses) of birds at dawn in central Nigeria (Vom) during mid-winter. Lines showing the predicted pattern of body mass terminate a t the point at which all fat reserves would have been consumed.

B R I A N WOOD I B I S 134 SUPPL. I 72

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Latjtude North

Figure 7. The predicted pattern of change in body mass and distance travelled by a male Yellow Wagtail crossing the Sahara from latitude 12". using a 'stop-over' strategy (solid linet. It is assumed that this bird sets out a t dusk and stops from midday each day for a period of 6h, during which it does not feed. A normal water index of 203 is maintained throughout. Also shown are predicted body masses I iz 2 s.d., with maximum value) of males a t latitude I2'N, body masses at dawn in central Nigeria during mid-winter I k 2 s.d., with minimum value1 and body masses (plus ranges) of birds captured a t oases and in the Mediterranean basin. (Original data from Hoggar and Colomb Bechar (Smith I966bt did not discriminate between sexes. An adjustment has been made assuming that body masses are normally distributed with males having masses 6% > females).

(Fig. 71. I f i t sets out at dusk (1800hrst and rests in the desert for 6h from noon each day, this model predicts that it would only reach latitude 26-30" by the time its fat reserves were exhausted. Calculations are based on the assumption that such migrants do not feed whilst resting and have a resting metabolic rate equivalent to that of sleeping birds. I t does not include an allowance for energy required to climb to cruising altitude each evening.

Reception areas north of the Sahara and onward travel

The coastal regions of north Africa are at their most verdant during spring and i t seems likely that migrants which reach this far north will have comparatively little difficulty in finding adequate food. Yellow Wagtails reaching oases in the northern Sahara in spring appear to be able to find food readily (Erard & Larigauderie 1972). Despite these observations, their body masses continue to decline as they progress northwards, only rising substantially once the desert is behind them (Fig. 61. Unfortunately, there appear to be no detailed studies from the north African coast in spring, that would indicate if birds re-fatten there and how quickly they regain higher body masses. However, migrants caught in spring near the north coast of Tunisia and on Majorca mostly have body masses that are several grams higher than mid-winter values (Wood 1991). By the time they reach Malta. the mean body-mass of Yellow Wagtails is 18.6g and i t remains at this level at least as far as the north coast of the Mediterranean (Wood 1982). This i s over 3 g heavier than during mid-winter in central Nigeria.

An examination of the latitude of recoveries of Yellow Wagtails which have overwintered in Nigeria (Fig. 8 ) suggests that most birds reach the Mediterranean basin soon after their departure from winter quarters. Indeed, there are two recoveries which support the idea of rapid initial passage (Table 1 ). However, thereafter the remaining journey to breeding sites appears to be accom- plished at a much more leisurely pace. Dement'ev et al. (1954) recorded the mean date of first arriving Yellow Wagtails in Lithuania (55"N) as 20 April. The first birds reach southern Finland (60"N) by I May and northern Finland (65"N) b y 15 May. However, at the Danish island observatory of Christians0 (55"19'Nl peak passage of Yellow Wagtails occurs as late as 22 May ILyngs eta / . 19901. This is at least 30 days after birds belonging to Scandin- avian breeding populations fatten in West Africa.

DISCUSSION

African distribution prior to migration

In West Africa, and probably elsewhere, the distribution of Yellow Wagtails prior to the northward migration to breeding areas in the Palaearctic is highly complex. A combination of the effects of the 'leap-frog' pattern of racial distribution, latitudinal variation in sex ratio, and the decline in the number of birds in northerly wintering populations during the course of the dry season, will mean that Yellow Wagtails which form part of the same local breeding population in Europe are highly unlikely to b e found wintering together in West Africa. Consequently, the preservation of specific sites in West Africa would be

I992 YELLOW WAGTAIL M I G R A T I O N 73

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Figures 8. Latitudinal pattern of recoveries of Yellow Wagtails during the course of the year. Birds recovered within a year of ringing are shown by solid symbols, others by open symbols. The Nigerian site of ringing or recovery is indicated as follows: lbadan (triangles). Vom (circles), Kano (squares). Malamfatori (inverted triangles). The latitudes and maximum duration of residence of wagtails at Nigerian sites are also shown, also rapid recovery of two trans-Saharan migrants Iby lines joining ringing and recovery sites). Shading indicates birds that were probably on breeding sites.

Ring number Ringing/recovery details Lapse

BE 45668 27/03/68 Kano I2"03'N 08"25'E 05/04/68 Malta 35"52'N 14"30'E 9 days

15/04/68 Xania, Crete 35"31'N 24"OI'E 8 days two Yellow Wagtails that made a rapid transit HR 36874 07/04/68 Malamfatori I3"33'N I3"23'E Table 1 . Ringing and recovery details of

of the Sahara desert

unlikely to be an effective means of conserving particular breeding populations of migrants.

Food resources in West Africa during the northern winter follow a fairly predictable seasonal pattern but vary locally in less predictable ways. The distribution of migrant birds is probably constantly adjusting to take account of these changes. Even male Yellow Wagtails that defend individual feeding territories in central Nigeria, which they may occupy for entire dry seasons, and re- occupy in subsequent winters (wood 1976). show explor- atory behaviour that may enable them to assess food supplies outside their territory and thus provide an 'insur- ance' against unpredictable fluctuations. Their communal

roosting may provide a means for sharing knowledge about food resources which may alter from day-to-day (Ward & Zahavi 1973, DeGroot 1980).

Food must be particularly hard to obtain towards the end of the dry season. It is something of a paradox that the more northerly wintering birds can fatten and migrate before the first rainfall of the year, despite a progressive 57% decline in arthropod biomass per month. Towards the end of the dry season, competition for food may be some- what reduced because few birds remain in the northern parts of the wintering range. However, those that remain feed more rapidly than would otherwise be expected: their choice of prey is catholic, but they may become less

B R I A N WOOD LBIS 1 3 4 S U P P L . I 74

selective as prey biomass declines, or try harder to obtain the few prey that are available (Wood 1978).

Presumably, the advantage to be gained by staying as far north as possible in West Africa is that Yellow Wagtails that do so are the first to fatten and migrate back to Europe. They may thus have first choice of breeding sites and any associated advantages (such as choice of matel. Early northward migration will be most important for the most southerly European breeding populations, because of the earlier arrival of spring in southern Europe.

The latitudinal variation in the sex ratio of Yellow Wagtails in West Africa follows a pattern that has also been reported for other species in Europe and North America (Ketterson & Nolan 1979, Perdeck & Clason 1983. Owen & Dix 19871, in which dominance apparently plays a role (Ketterson 1979, Creenberg 1985). Dominance mediated access to food could also be an important factor determining why females fatten later than males at each latitude.

In those areas immediately south of the desert, from which birds take-off on their trans-Saharan flight, compe- tition for resources may be particularly important. The observed pattern of fattening and migration means that, after the northernmost wintering populations have departed, these areas become vacant for use by birds which may have wintered and fattened further south. Without the prior departure of northerly wintering birds, the more southern ones may be obliged to set out on a trans-Saharan flight from the latitude at which they had overwintered. The addition of the extra distance they would then be required to fly could be critical in deter- mining how successful they were in reaching breeding areas in Europe.

Trans-Saharan journeys

Yellow wagtails appear to migrate along great circle routes, which form the shortest distance between wintering and breeding sites. The evidence currently avail- able points to them using non-stop flight to cross the Sahara. Using less direct routes or a stop-over strategy to cross the desert would require greater resources and would only be likely to be used if they could be certain of finding additional food and suitable resting places on route or if weather conditions prevented the use of the non-stop strategy.

Rate of northward travel In southern Nigeria, pre-migratory fattening occurs about 25 days later than in the north (Fig. 4). However, over 50 days elapse between t h e arrival of birds at southern breeding grounds in Italy and northern breeding sites in Scandinavia. The relatively slow progress northwards through Europe follows a rapid migration from West Africa to the Mediterranean Basin. Why do birds that will travel

on to Scandinavia journey north to the Mediterranean so soon after the southernmost breeders? If they stayed longer in West Africa they could use better feeding oppor- tunities created by the arrival of the wet season, before fattening and departing northwards.

Perhaps competition from African resident species encourages migrants to depart as soon as possible (Sinclair 1978). This could also be the reason why some Yellow Wagtails remain throughout the winter in the dry northern parts of West Africa and emigrate before the arrival of the wet season, rather than move further south, to where rainfall would enable more food to be found. This situation requires much greater scrutiny, as it could be the key to our understanding of the migration systems, and our ability to conserve migrants on passage.

C O N C L U S I O N S

Detailed examination of only one species cannot hope to portray the full range of strategies used by trans-Saharan bird migrants. However, it does indicate some of t h e con- siderations which must be taken into account if successful strategies are to be devised for their conservation.

Several particularly important conclusions can be drawn from this review, and additional questions can be identified.

Protection of a wide range of sites will be necessary for use by migrant birds in Africa, since individual birds use many different places in the course of the winter. A 'leap-frog' migration pattern produces partial segregation in Africa of populations from different breeding latitudes, but this is complicated by inter- sexual differences in the use of sites and by re- distribution in response to changing environmental parameters. The type of distribution pattern shown by Yellow Wagtails will mean that males of races breeding in southern Europe will be most likely to be harmed by environmental degradation in the sahel zone, immediately south of the Sahara. Conversely, females of the northernmost breeding populations will be affected most by adverse changes in the south of the wintering range. As food supplies decline during the time they are in West Africa, many Yellow Wagtails move further south. They may initially remain as far north as possible to avoid competition with African resident species, but much more data are needed to confirm this supposi- tion. W e cannot tell if birds would remain to over- winter in the northern Sahel if conditions there were made more favourable to their survival. The amount of fat carried by a Yellow Wagtail about to set out on migration across the Sahara desert from West Africa is just enough to carry it across in contin-

1992 YELLOW WAGTAIL MIGRATION 75

uous flight in still air. This fat store is accumulated at the latitude inhabited towards the end of the winter sojourn in West Africa but, in order to maximize the chances of a successful desert crossing, birds move to the southern edge of the desert before embark- ation on a non-stop flight. Consequently, both these sites are vital for successful migration.

If the effective width of the Sahara was increased as a consequence of mis-use of the land on its northern and southern margins, bird migrants could be expected to suffer higher mortality rates. However, we do not know if the fat deposits of migrants are the maximum they can carry or if additional deposits could be accumulated in the event of unavoidably longer journeys.

Finally, empirical evidence suggests that environmental conditions on the wintering grounds of some species may have more of an effect on overall mortality rates than conditions encountered on migration. During periods of severe drought in the Sahel, species such as the White- throat Sylvia communis, which winters in the northern Sahel, suffered drastic population declines (Marchant et al. 1990). At the same time, more southerly wintering species, which nevertheless must have passed through or overflown the northern Sahel on migration, were hardly affected at all. They too would be expected to decline in abundance on their European breeding grounds if northern Sahelian sites were vital to their migration strategies.

It is clear that many more data are needed from Africa if we are to succeed in saving long distance bird migrants. But additional data are also required for breeding popul- ations in Europe. Changes in the Sahel will most strongly affect migrants breeding in the south of Europe, but the majority of long-term population studies have been made in central and northern Europe (e.g. Baillie & Peach 1992).

Much of the original work on which this paper is based was supported by the Natural Environment Research Council, Aberdeen University, the Skene Bequest and the British Ornithol- ogists' Union. I am grateful for the use of facilities in the Depart- ment of Zoology, Aberdeen University, the Department of Biological Sciences, Ahmadu Bello University, and the Department of Biology, University College London.

Dr J.R. Lang, I.R. Best, Dr W.P. Taylor and R. Sharland kindly allowed me access to unpublished data. Numerous friends in Nigeria provided help in many ways during field-work. Professor C.H. Fry provided guidance and R.E. Moreau provided inspiration throughout my early studies.

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