Species Summering In Wintering Areas

In some long-distance migrants, non-breeding individuals can be seen in 'winter quarters' in every month of the year. These are mostly young individuals that do not breed until they are two or more years of age. They stay in their wintering areas beyond their first winter, returning to the breeding area in a later year (deferred return). In this way, the age groups are separated geographically in the breeding season, and the young birds, which would in any case not reproduce in their first year, avoid the costs and risks of an unnecessary return journey. Alternatively, the immatures may migrate only part way from their wintering areas, passing the summer at sites en route to the breeding area, sometimes getting nearer year by year (graded return). Once sexual maturity is attained, these birds make a full return migration, and usually settle to breed close to where they were hatched (Chapter 17). From then on, they normally show the usual twice-yearly migrations.

Over-summering in 'winter' quarters is regular in at least 15 families of birds, being best known among raptors, shorebirds and seabirds. It is not always certain that the same individuals stay throughout, and the continued presence of birds in winter quarters may be due to overlap in the departure and arrival dates of different individuals, with some returning before others have left. However, some species have now yielded enough ring recoveries, or sightings of colour-marked birds, to indicate that some individuals remain in 'wintering' areas year-round, and that the younger age groups are not present in breeding areas in spring-summer.

Among raptors, the Osprey Pandion haliaetus has been studied in detail. Almost all ring recoveries from the winter range during the northern breeding season refer to first-year birds, younger than the usual age of first breeding, while no first-year recoveries come from the breeding range. This is true in both Old and New World populations, and implies that such birds stay in wintering areas all their first year, rather than returning north for brief periods varying in timing between individuals (Henny & van Velzen 1972, Saurola 1994), a finding also supported by recent radio-tracking studies (Chapter 8). The same is true for Egyptian Vultures Neophron percnopterus, judging from the virtual absence of the distinctive first-year individuals among the birds migrating north through Israel in spring (Shirihai et al. 2001), and in Oriental Honey Buzzards Pernis ptilorhynchus judging from a single radio-tracked individual (Higuchi et al. 2005).

Among practically all migrant raptor populations that winter in Africa, a proportion of individuals remain there through the northern summer (the local wet season) (Thiollay 1989). They are found even in small species such as the Common Kestrel Falco tinnunculus, but are relatively more numerous in larger species, such as the Short-toed Eagle Circaetus gallicus, in which individuals do not breed until they are two or more years old. There is also a northward shift within Africa of these 'summering' birds, with many more individuals in the Sahel and Sudan zones in the summer wet season than in the winter dry season (tantamount to a part-way return, coincident with a northward migration of intra-African migrants). Similarly, most Black-crowned Night Herons Nycticorax nycticorax migrate from Europe to winter in the Sudan-Sahel zones of Africa, where a high proportion of juveniles remain during their first 2-3 years (Brosselin 1974). The same holds for White Storks Ciconia ciconia and others.

First-year seabirds that perform long migrations often also remain in 'winter quarters' for the summer or at least in sea areas separate from those used by the breeding adults. As they get older, the non-breeding immatures migrate progressively nearer to breeding areas (as found, for example in Fulmars Fulmarus glacialis and Northern Gannets Morus bassanus). As the years pass, increasing proportions begin to return to nesting colonies during summer, 'prospecting' for nest-sites, but they usually arrive later in spring and depart earlier in summer than the adults. For example, at a colony of Common Terns Sterna hirundo nesting in Germany, fledglings were marked with transponders, allowing the automatic registration of their subsequent return. Most individuals returned to the colony

November December January February March

Figure 15.7 Attendance patterns of Wandering Albatrosses Diomedia exulans at a nesting colony in relation to experience, South Georgia Island. Each horizontal bar spans the median arrival to median departure dates of males and females. In successive seasons after first arrival, the date of arrival became earlier (F6371 = 43.0, P < 0.001) and the number of days spent ashore increased (F3371 = 63.3, P< 0.001). Redrawn from Pickering (1989).

November December January February March

Figure 15.7 Attendance patterns of Wandering Albatrosses Diomedia exulans at a nesting colony in relation to experience, South Georgia Island. Each horizontal bar spans the median arrival to median departure dates of males and females. In successive seasons after first arrival, the date of arrival became earlier (F6371 = 43.0, P < 0.001) and the number of days spent ashore increased (F3371 = 63.3, P< 0.001). Redrawn from Pickering (1989).

for the first time as two-year-olds, and remained as 'prospectors' for a year or more before starting to breed. Prospectors arrived later than breeders (too late to breed), and first-time breeders arrived, on average, 17 days later than experienced breeders (laying, on average, 19 days later). The arrival dates of older Common Terns got progressively earlier with increasing age and experience of the colony but, on average, males arrived earlier than females (Dittman & Becker 2003).

At a Wandering Albatross Diomedia exulans colony, the variability was even greater, as recruitment to the breeding population took 2-8 years after first return to the natal colony (Pickering 1989). In successive seasons, from first return to pairing, the date of arrival became earlier, and the number of days spent ashore and interacting with other birds increased (Figure 15.7). Birds that paired arrived earlier and spent more time ashore than birds with similar experience which did not pair that year. In the season following pairing, birds returned at the same time as breeders, but most did not produce eggs. They left in mid-season before other non-breeders and bred the following year. In this and other seabird species, the successive arrivals of established breeders, new breeders and then prospectors can be spread over periods of weeks or months each year, as can their subsequent departure (for Manx Shearwater Puffinus puffinus see Perrins et al. 1973, for Razorbill Alca torda see Lloyd & Perrins 1977, for Common Guillemot (Murre) Uria aalge see Halley et al. 1995, for Common Tern see Ludwigs & Becker 2002).

In all marine terns from high latitudes, most juveniles stay in their 'wintering' areas for 1-2 years before returning part way or all the way back to breeding areas. In the Arctic Tern Sterna paradisaea, the majority of immatures remain in the southern hemisphere in the northern summer, extending from equatorial cold water areas south to the pack-ice. Some appear to circumnavigate Antarctica, possibly taking up to two years before moving north towards the nesting colonies where they breed at 3-4 years old (P. Monaghan, in Wernham et al. 2002). Most young skuas (jaegers) also remain in the southern hemisphere, although some young Great Skuas Stercorarius skua and Arctic Skuas S. parasiticus return not just to the breeding areas, but spread to even higher latitudes in the northern summer (B. Furness, in Wernham et al. 2002).

Similar patterns of (a) pre-breeding birds over-summering in 'wintering' areas, (b) returning part way towards breeding areas, or (c) returning to breeding areas for a shorter time than nesting adults, occur in some shorebirds, especially among populations that migrate the longest distances and winter in the southern hemisphere (Table 15.2). Some colour-dyed Short-billed Dowitchers Limnodromus gri-seus were absent from their wintering site in Apalachee Bay in Florida for less than a month in the period late June to early July (Loftin 1962). If these birds reached their breeding areas, they could have spent only a few days there before returning.

Most waders seen during summer in their 'wintering' areas are in their first year of life, but some are older. In the Bristle-thighed Curlew Numenius tahitiensis that breeds in Alaska and winters on central Pacific islands, individuals do not leave their wintering areas until they are three or even four years old (Marks & Redmond 1996). In some other large shorebird species, immatures leave the wintering areas in increasing proportion, and progressively earlier each year as they grow older, as shown in individually marked Oystercatchers Haematopus ostralegus, Grey Plovers Pluvialis squatarola and Pied Avocets Recurvirostra avosetta (Goss-Custard et al. 1982, Evans & Davidson 1990, Hotker 2002). Not all migrating immature shorebirds travel as far as breeding areas, but little is known of their fattening patterns. In one wintering population of Oystercatchers, individuals began to return to breeding areas from about the fourth summer on, but males did not begin to breed until ages 5-8 years and females until 3-6 years (Goss-Custard et al. 1982).

Comparing different shorebird species, a higher tendency to over-summer in winter quarters occurs in those populations with the longest migratory flights that winter furthest from their breeding range (Summers et al. 1995, Hockey et al. 1998). The same is true for some gulls, cormorants and others (Dolbeer 1991). Perhaps the longer journeys are more arduous and hazardous, tipping the balance in favour of staying rather than returning to the breeding range. Such latitudinal trends are apparent within species. In the Western Sandpiper Calidris maura, most juveniles wintering in California and western Mexico gain fat and migrate north in their first spring. In contrast, nearly all juvenile Western Sandpipers wintering in Panama do not gain fat or migrate north. Instead they spend their first summer in predominantly winter plumage in non-breeding areas (Nebel et al. 2002, O'Hara et al. 2005). The difference between these sites was assumed to reflect a latitudinal trend within Western Sandpipers, with the birds showing an increasing propensity to over-summer the further from the breeding areas they

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