Temperate migrants and resident bird species in Afro‐tropical savannahs show similar levels of ecological generalism

The specificity of an animal's habitat requirements will determine its ability to deal with anthropogenic climate and habitat change. Migratory birds are thought to be particularly vulnerable to such change, but theory predicts that they should be largely generalists. This prediction was tested with the aim of assessing whether migratory Palaearctic‐breeding birds wintering in the savannah biome of Africa are more or less generalist in their habitat use compared with taxonomically and ecologically similar Afro‐tropical resident species. The degree of specialization of these species groups to certain habitat characteristics was assessed and compared by calculating the relative occurrence of the species along habitat gradients, where wide occurrence indicates generalism and narrow occurrence indicates specialism. Palaearctic migrants as a group could not clearly be distinguished as generalists relative to Afro‐tropical residents with respect to habitat attributes. The only indication of greater flexibility in Palaearctic migrants was a significant tendency to use habitats over a wider latitudinal range. The results suggest that migrants are generalists, but not necessarily more generalist than taxonomically similar resident species that also occur over a wide range of habitat types within the savannah biome. The availability of specific habitat requirements on the wintering grounds in Africa is therefore unlikely to be a primary limiting factor for many Afro‐Palaearctic migratory bird species.     

Perspectives on Palaearctic and Nearctic bird migration; comparisons and overview of life-history and ecology of migrant passerines

In this paper we compare ecological attributes of tropical migrant passerines from the Nearctic and western Palaearctic, focusing particularly on habitat association patterns during both breeding and wintering seasons. Three regions were compared: Europe, western and eastern North America. Breeding bird census data from 32 studies (each including at least four stages of forest succession) were used to assess the association patterns of breeding habitats among tropical migrants. For each species we calculated an index of habitat diversity and habitat preference.
Tropical migrants preferred earlier successional stages than other birds in Europe. The opposite was true in eastern North America. In eastern North America, tropical migrants tended to be associated with a smaller range of serai stages than other passerine species. In their winter quarters, Palaearctic migrants live primarily in open habitats, such as savannas, whereas eastern Nearctic migrants make more frequent use of evergreen forests. Migrants from western North America show the greatest match between breeding and wintering habitats.
We relate the results to the taxonomy and probable history of contemporary avifaunas and vegetation formations of the Old and New World. Taxonomically, tropical migrants from different parts of the Holarctic are less closely related to each other than residents and short-distance migrants. Tropical and temperate avifaunas are more closely related to each other in the New World than in the Old World. Conservation implications of the between-continent differences are briefly discussed.     

FACTORS AFFECTING THE FOOD SUPPLY AND BREEDING SEASON OF RESIDENT BIRDS AND MOVEMENTS OF PALAEARCTIC MIGRANTS IN A TROPICAL AFRICAN SAVANNAH

The fact that Palaearctic migrants arrive in the northern tropical savannah of Africa during the dry season suggests potential competition for food with African species. Moreover, in the southern tropical savannah African species breed during the rainy seasons, when Palaearctic migrants are present. In the equatorial area of Serengeti, East Africa, an index of the food supply for insectivorous birds was obtained from 3 years of light-trap measurements and sweep net samples. Adults of Lepidoptera, Coleoptera, Orthoptera and Isoptera are sparse in the dry season but become locally abundant after the first rainstorms that mark its close. They are apparently blown by converging winds ahead of the inter-tropical front and settle to lay eggs where rain has fallen. These early storms therefore produce localized superabundances of food. In the ensuing rainy seasons insect abundance remains high. African insectivorous birds breed during the wet period, reaching a peak two months after the insect increase. It is suggested that this lag is due to the need to recover body condition, build up reserves for eggs, develop gonads and wait for vegetation and insect larvae to develop. In the samples available, breeding records of above-ground nesters peaked in the first rains, while ground-nesters peaked in the second (main) rains. Predators bred towards the end of the rains, when there is an abundance of fledglings and small mammals. Thus the food supply could act as the ultimate factor determining the timing of the breeding season in this area. Palaearctic migrants arrive in the Serengeti 4–10 weeks ahead of the main rain front. However, most species are only found where rain has fallen recently. When conditions dry up they move on to other wet areas. Thus they overlap with African species only where there is a superabundance of insects. When the rains become widespread Palaearctic migrants disperse into their usual habitats, and therefore appear not to compete for available resources with closely related species of African birds. The situation in West Africa, where residents and migrants overlap throughout the dry season, cannot be explained in the same terms.     

Space and habitat use by Red Kites Milvus milvus during winter in the Guadalquivir marshes: a comparison between resident and wintering populations

Differences in habitat selection, diet and behaviour of resident and wintering Red Kites Milvus milvus were studied in Donana National Park, southwest Spain. Adult resident Red Kites roosted at their nests, while immature residents and wintering birds gathered at communal roosts. Individuals remained on average for two consecutive nights (range 1–8) at the same communal roost. Wintering kites spent significantly more time foraging gregariously than residents. The use of the marsh was greater by wintering kites while residents more often used the forest. Wintering birds consumed more goose carrion than the residents. Wintering kites had larger core areas, moved farther from their roost sites to feed and changed foraging areas more frequently than residents. Among residents, adult females had the smallest home ranges (core area, distances travelled and time spent flying). We suggest that Red Kites wintering in Donana occupy the marsh because it provides high food availability, and is vacated by Black Kites M. migrans which exploit this profitable habitat during the breeding season.     

Consequences of the settlement of migrant European Robins Erithacus rubecula in wintering habitats occupied by conspecific residents

In wintering areas where migrant birds meet sedentary conspecifics, early settlement of local residents in the best habitat patches might reduce the availability of suitable sites for arriving migrants. We studied how sympatric migratory and sedentary European Robins Erithacus rubecula occupy two wintering habitats of different quality (forests and shrublands) in southern Spain, and how such a distribution affects individuals of each population sector. In September, before migrants arrived, Robins were only found in forests, and they had already saturated these habitats, so that rather than increasing Robin abundance in these habitats, the arrival of migrants caused a massive occupation of the previously vacant shrublands. During the winter, we captured Robins and identified them as migrants or residents using a discriminant function based on morphological traits. Residents always predominated in forests, and migrants in shrublands, but through the winter around 35% of residents (mainly juveniles) moved to shrublands, having been replaced by some migrants in forests. Although food was more abundant in shrublands, Robins had better body condition in forests, suggesting that other factors determined habitat preferences (e.g. shelter availability or food diversity, which were higher in forests). In addition, we observed a greater variance in body mass relative to body size in forests, suggesting that energy management was less constrained in this habitat (for example owing to a lower exposure to predators or a higher food predictability). Our results suggest that sedentary Robins benefit from an early occupation of the best habitats in the wintering grounds, forcing migrants to colonize apparently less suitable sites. This would explain the persistence of these small southern populations despite the yearly flooding of the area by huge numbers of migrant conspecifics.     

Altitudinal migration by birds: a review of the literature and a comprehensive list of species

Altitudinal migration is the seasonal altitudinal movement of birds from breeding areas to non‐breeding or wintering areas at different elevations. Although this type of migration is widely reported, questions remain concerning the number of species that perform altitudinal migration, possible variation among different taxa and geographic locations in the extent of altitudinal migration, and the foraging guilds of altitudinal migrants. We conducted an extensive bibliographic survey and compiled a list of altitudinal migrant birds worldwide. We characterized species in terms of their foraging guilds because the spatial distribution of food resources along altitudinal gradients is often evoked as a driver of bird altitudinal migration. We identified 1238 species of altitudinal migrants, ~10% of the ~10,000 extant species of birds. We found a strong geographic bias in publications focusing on avian altitudinal migration toward the United States and Costa Rica, and a paucity of studies in megadiverse regions such as the Afrotropical and Indomalayan realms, and areas in the Neotropics other than Costa Rica. We also found that most species of altitudinal migrants were invertivores rather than frugivores or nectarivores. This general pattern held true for all zoogeographic realms except the Neotropics, where nectarivores and frugivores predominated among altitudinal migrants. The prevalence of invertivore birds among altitudinal migrants is not unexpected because this is the most common foraging guild among birds worldwide. Overall, we found no prevalence of any specific foraging guild among altitudinal migrants across zoogeographic regions. The results of studies to date suggest that altitudinal migration by birds may be driven by a number of factors, including access to increased food resources for breeding or molting, weather conditions, and mating and nesting opportunities. However, to better understand the mechanisms underlying altitudinal migration, broadening the geographic scope of studies is paramount, with additional study of altitudinal migration especially needed in the megadiverse tropical regions of sub‐Saharan Africa, Southeast Asia, and South America.     

The Breeding Distribution and Habitats of the Pied Flycatcher (Muscicapa Hypoleuca) in Britain

Examination of the plumage of Palaearctic warblers as migrants or wintering birds at Kampala, Uganda, provided useful material for a consideration of moult in relation to the timing of spring and autumn migration. Reasons are suggested for observed interspecific differences in the moult pattern, and for the variation reported from different wintering areas in the African continent.     

Migration patterns and habitat use by passerine and near-passerine migrant birds in eastern Africa

Palaearctic migrant passerines and near-passerines which visit eastern Africa can be divided into six groups based on the latitude of their final wintering area. Species wintering further north in Africa tend (a) to breed in more southern parts of the Palaearctic, (b) to prefer drier habitats and (c) to feed more from the ground than those wintering further south. Many species use quite narrow passage routes through eastern Africa and, for some species, passage is centred further east in spring than in autumn. Southward migration can take more than 4 months from the Palaearctic breeding grounds and many birds stopover in the northern tropics from September to November. By contrast, the return migration takes only about 6 weeks. The timing and strategy of migration within Africa can be broadly related to seasonal patterns of rainfall and vegetation.     

Foraging ecology of a tropical aerial feeding bird guild

The feeding ecology of the constituent members of the aerial feeding bird guild was studied in Malaya. Although numbers changed with the movements of migrants, more birds were seen feeding over forested habitats than open areas. Methods of ecological isolation were seen in morphology, flight behaviour and food selection. Seasonal changes in foraging behaviour associated with breeding and migration were found in Pacific Swallows and Barn Swallows. Migrant species were observed to mix freely with residents and no evidence was found for competitive exclusion between the two groups.     

MIGRANT SHOREBIRDS IN THE SEYCHELLES

The aim of this paper was to assess the status of migrant shorebirds in the Seychelles. Most observations were concentrated on two areas: a 6 ha mudflat at Victoria, Mahé, and open areas of Bird Island. A systematic list of all shorebirds that have been recorded in the Seychelles is given, including data on abundance and their incidence on other islands of the Malagasy region. Most of the Seychelles migrants probably originate from the north and east, i.e., the central and eastern Palaearctic, and arrive after a sea-crossing of over 1600 km. Highest numbers of species and of individuals occurred in the Seychelles during the northern winter, but about one third of the species recorded were also present, in reduced numbers, in the summer. Few birds in breeding plumage were seen; probably most birds, especially those which summered, were immature. Numbers of both summering and wintering birds varied from year to year, these variations probably being related to breeding success at higher latitudes.     

Palaearctic migrants in the central Sudan

A brief account is given of the topography and climate of the Sudan and a more detailed survey is made of the vegetation and habitats afforded to wintering Palaearctic migrants in the Sahel and Soudan savannah zones in the central Sudan. Climatic conditions during the migration seasons are mentioned and the utilization of habitats by wintering migrants is broadly surveyed.
The status of 196 Palaearctic migrant species is treated briefly. The basic observational data are tabulated to summarize observed patterns of migration in terms of date, relative abundance and region (for which purpose the central Sudan is divided into four from west to east). Hitherto unpublished field notes of the authors and their collaborators are used, together with dated observations (including specimens collected) of earlier authors, to provide the fullest data on each species.
Moreau's (1967) paradox is discussed as it relates to the Sahel and Soudan savannah zones in the central Sudan, together with the degree of competition with resident African species in those zones. Some disparities between the spring and autumn abundance and d. istribution of Palaearctic migrants are discussed in relation to 40 species ana some tentative views are advanced on the possible reasons for such disparities. Evidence is adduced which points to Lanius minor and Coraczus garrulus (and possibly Circus pygargus , partially at least) having loop migrations, south through the Sudan and north through Arabia, and Falco wespertinus south through the Sudan and north through West Africa. An explanation is offered for the greater volume of water birds observed migrating down-Nile in spring compared with their volume up-Nile in autumn, and for the reversed situation observed in the region west of the Nile for these same species.
Shelduck Tadorna tadorna is included in the Sudan list for the first time.     

Numerical and behavioural responses of migrant passerines to experimental manipulation of resident tits (Parus spp.): heterospecific attraction in northern breeding bird communites?

Summary We studied experimentally interspecific competition among foliage-gleaning passerine birds by manipulating the density of resident tits. In 1988 tit density was experimentally increased on three small islands in a central Finnish lake, and decreased on three other islands by tit removal. In order to avoid the effects of between-island differences in habitat quality, the role of the islands was reversed when the experiment was repeated in the following year. Censuses and observations on foraging and feeding behaviour were conducted to assess the numerical and behavioural responses of migrant conguilders (mainly chaffinches and willow warblers) with respect to the manipulated abundance of the tits. We also measured whether variation in food consumption of tits affected the frequency with which the migrants found food by calculating average intervals between successful prey captures, time lags to prey-capture and giving-up times. Our results indicate that interspecific competition is of minor importance in structuring breeding bird assemblages and species feeding ecologies on the study islands. No consistent difference in foraging or feeding niches of chaffinches and willow warblers was found between low and high tit density conditions. Niche overlap analysis showed no avoidance by chaffinches and willow warblers of the microhabitats which tits used. Tit abundance had no significant effect on feeding success or behaviour. Experimentally increased abundance of resident birds was associated with increased abundance of breeding migrants, however. This pattern was found not only in the foliage gleaning guild but also with all passerine birds, indicating that food was not an important contributor to this pattern. We elaborate a hypothesis suggesting heterospecific attraction in northern breeding bird assemblages. Habitat generalist migrants may use the presence of residents as an indicator of safe and/or productive breeding sites in northern unpredictable circumstances.     

Clutch size of passerines at mid-latitudes: the possible effect of competition with migrants

The clutch sizes of the passerines of Israel and the Cape Province, South Africa, which lie at similar latitudinal range, were compared. Mean clutch sizes in Israel and the Cape Province are 4.09 and 2.87, respectively. Mean clutch size of Israeli migrants is larger than that of residents (4.45 and 3.93, respectively), but no such difference exists in the Cape Province.
It is suggested that the larger clutch size in Israel is a result of two factors: (1) the higher proportion of wintering birds in Israel in comparison with the Cape Province and the presence of many transients there which may compete with resident birds and cause high winter mortality among them and (2) a higher proportion of migrants in the Israel avifauna, which suffer heavy losses during their trans-Saharan migration in comparison with Cape Province migrants, which travel shorter routes. The resulting reduced competition for food during the breeding season in Israel enables passerines there to lay larger clutches as predicted by Ashmole (1963) and Ricklefs (1980).     

Winter habitat use by Loggerhead Shrikes (Lanius Ludovicianus) in Mexico: separating migrants from residents using stable isotopes

Several species of migrant birds overlap in range on their wintering grounds with non-migrant conspecifics or other species that occupy a similar niche. Very little is known whether such overlap results in competition and subsequent habitat segregation since it is usually impossible to separate resident from migrant individuals. The Loggerhead Shrike (Lanius ludovicianus) is a declining grassland species in North America that winters in the southern United States and Mexico. Using stable-hydrogen isotope (δD) analysis of feathers, we identified resident and migrant shrikes wintering in northeastern Mexico based on a latitudinal gradient in precipitation and feather δD values. Indicator species analyses showed that migrants occupied areas where bare ground was less available than those occupied by residents, a pattern which held when a more restricted set of birds from the extremes of the δD distribution were considered. This provides evidence for conspecific habitat segregation. Habitat differences were also found between sites occupied by shrikes and apparently suitable but unoccupied sites. Shrikes occupied more open sites that contained shorter tall shrubs and huisache (Acacia farneasiana) and fewer tall shrubs, mesquite (Prosopsis glandulosa) and huisache than unoccupied sites. The availability of suitable winter habitat and the potential competition between migrants and residents may be factors that influence the population dynamics of migrant shrikes in North America.     

Moult of three Palaearctic migrants in their West African winter quarters

Some theories about moult strategies of Palaearctic passerine migrants assume that birds adapt timing of moult to environmental conditions such as rainfall on their African wintering grounds. Species wintering in the northern tropics should limit moult to the period shortly after their arrival at the end of the rainy season. Passerine migrants wintering in West Africa should also moult more rapidly compared to related species or conspecific populations that moult elsewhere. We investigated the moult of melodious warblers Hippolais polyglotta, willow warblers Phylloscopus trochilus and pied flycatchers Ficedula hypoleuca wintering in Comoé National Park, Ivory Coast, between October 1994 and April 1998. In contrast to previous studies we did not restrict our analyses to moult of flight feathers but also included moult of body feathers. The results differed partially from the general assumptions of previous authors. Melodious warblers moulted twice: a complete moult shortly after their arrival, and a moult of body feathers and in some cases some tertials and secondaries in spring. Willow warblers moulting flight feathers were found between December and March with the majority moulting in January and February. Primary moult was not faster compared to populations moulting in central Africa and South Africa. Body feather moult varied strongly among individuals with birds in heavy moult between December and April. Pied flycatchers moulted body feathers and tertials between January and April. Birds with growing feathers were found throughout the whole period including the entire dry season. Moult strategies are thus not readily related to a few environmental factors in general and our results show that factors other than mere resource availability during certain times on the wintering grounds are likely to govern the timing of moult.Communicated by F.Bairlein     

High juvenile mortality during migration in a declining population of a long‐distance migratory raptor

Many populations of long‐distance migrants are declining and there is increasing evidence that declines may be caused by factors operating outside the breeding season. Among the four vulture species breeding in the western Palaearctic, the species showing the steepest population decline, the Egyptian Vulture Neophron percnopterus, is a long‐distance migrant wintering in Africa. However, the flyways and wintering areas of the species are only known for some populations, and without knowledge of where mortality occurs, effective conservation management is not possible. We tracked 19 juvenile Egyptian Vultures from the declining breeding population on the Balkan Peninsula between 2010 and 2014 to estimate survival and identify important migratory routes and wintering areas for this species. Mortality during the first autumn migration was high (monthly survival probability 0.75) but mortality during migration was exclusively associated with suboptimal navigation. All birds from western breeding areas and three birds from central and eastern breeding areas attempted to fly south over the Mediterranean Sea, but only one in 10 birds survived this route, probably due to stronger tailwind. All eight birds using the migratory route via Turkey and the Middle East successfully completed their first autumn migration. Of 14 individual and environmental variables examined to explain why juvenile birds did or did not successfully complete their first migration, the natal origin of the bird was the most influential. We speculate that in a declining population with fewer experienced adults, an increasing proportion of juvenile birds are forced to migrate without conspecific guidance, leading to high mortality as a consequence of following sub‐optimal migratory routes. Juvenile Egyptian Vultures wintered across a vast range of the Sahel and eastern Africa, and had large movement ranges with core use areas at intermediate elevations in savannah, cropland or desert. Two birds were shot in Africa, where several significant threats exist for vultures at continental scales. Given the broad distribution of the birds and threats, effective conservation in Africa will be challenging and will require long‐term investment. We recommend that in the short term, more efficient conservation could target narrow migration corridors in southern Turkey and the Middle East, and known congregation sites in African wintering areas.     

宁夏罗山国家级自然保护区鸟类区系特征及群落结构

自2010年10月至2011年12月对宁夏罗山国家级保护区鸟类区系及群落结构进行了调查研究,共记录到鸟类15目46科98属164种,占宁夏已知鸟类总种数的48.81%。其中留鸟51种(31.10%),夏候鸟68种(41.64%),旅鸟38种(23.17%),冬候鸟7种(4.27%)。繁殖鸟119种,其中以古北界鸟类占优势,有88种,占繁殖鸟总数的73.95%;东洋界种15种,占12.61%;广布种鸟类16种,占13.45%。研究发现不同季节、不同生境中的鸟类群落特征差异较大。林地鸟类物种数和多样性指数最高,水域鸟类具有最高的G-F指数。相似性分析显示,山地荒漠草地和废弃村庄的鸟类群落,林地和浅山灌丛鸟类群落,分别具有一定的相似性。春季鸟类群落物种数、G-F指数、多样性指数和均匀度指数最高。     

Simulation of migratory flight and stopover affects night levels of melatonin in a nocturnal migrant

Several species of diurnal birds are nocturnal migrants. The activation of nocturnal activity requires major physiological changes, which are essentially unknown. Previous work has shown that during migratory periods nocturnal migrants have reduced night-time levels of melatonin. Since this hormone is involved in the modulation of day-night rhythms, it is a good candidate regulator of nocturnal migratory activity. We studied whether melatonin levels change when nocturnally active blackcaps (Sylvia atricapilla) are experimentally transferred from a migratory to a non-migratory state. We simulated a long migratory flight by depriving birds of food for 2 days, and a refuelling stopover by subsequently re-administering food. Such a regimen is known to induce a reduction in migratory restlessness ('Zugunruhe') in the night following food reintroduction. The experiments were performed in both autumn and spring using blackcaps taken from their breeding grounds (Sweden) and their wintering areas (Kenya). In autumn, the food regimen induced a suppression of Zugunruhe and an increase in melatonin in the night following food reintroduction. In spring, the effects of the treatment were qualitatively similar but their extent depended on the amount of body-fat reserves. This work shows that the reduction of night-time melatonin during migratory periods is functionally related to nocturnal migration, and that fat reserves influence the response of the migratory programme to food deprivation.     

High within‐winter and annual survival rates in a declining Afro‐Palaearctic migratory bird suggest that wintering conditions do not limit populations

For migratory birds, it is necessary to estimate annual and overwinter survival rates, identify factors that influence survival, and assess whether survival varies with age and sex if we are to understand population dynamics and thus inform conservation. This study is one of the first to document overwinter and annual survival from the wintering grounds of a declining Afro‐Palaearctic migrant bird, the Whinchat Saxicola rubetra. We monitored a population of marked individuals for which dispersal was low and detectability was high, allowing accurate estimates of survival. Annual survival was at least 52% and did not differ significantly across demographic groups or with habitat characteristics or residency time in the previous winter. Overwinter survival was very high and monthly survival at least 98% at some sites. Although winter residency varied spatially and with age, lower residency did not correlate with reduced annual survival, suggesting occupancy of multiple wintering sites rather than higher winter mortality of individuals with shorter residency. Our results suggest that mortality occurs primarily outside the wintering period, probably during migration, and that wintering conditions have minimal influence on survival. The similarity between survival rates for all age and sex classes when measured on the wintering grounds implies that any difference in survival with age or sex occurs only during the first migration or during the post‐fledging stage, and that selection of wintering habitat, or territory quality, makes little difference to survival in Whinchats. Our findings suggest that the wintering grounds do not limit populations as much as the migratory and breeding stages, with implications for the conservation of declining Afro‐Palaearctic migrants more widely.     

Resource buffering and the evolution of bird migration

Variation in migratory behaviour among birds is thought to reflect the relative variability in abundance of resources used by different species. Here I present evidence that increased migrancy among North American passerines is associated with greater reliance on food resources that vary in abundance because they occur in microhabitats that are exposed both to consumers and environmental fluctuation, and that residency is associated with use of food resources that occur in more ‘buffered’ microhabitats, and so are relatively invariant in abundance. Evidence from a variety of sources suggests that migrants and residents also differ in their use of foraging actions and methods of exploring the environment, with migrants using a rapid, extensive method of exploration, and a plastic, opportunistic foraging mode, while residents explore more methodically and use more stereotyped foraging actions. This suggests that the degree of migrancy exhibited by a species may be part of a suite of coadaptations for exploitation of foraging microhabitats at a particular position on the buffering scale, and I propose a selective typology involving feedback loops that could generate such coevolution. I also explain established patterns of difference between migrants and residents in morphology, and in resource use and distribution, as outcomes of the evolutionary scenario described.