Many birds have been trapped, ringed and weighed at migration sites, enabling their subsequent survival rates to be assessed from later surveys of the population. Several studies have revealed a link between the body condition of
Table 27.1 Examples of associations between body weights at migration time and subsequent re-sighting probability (reflecting survival)
Semi-palmated Sandpiper Calidris pusilla
Mallard Anas platyrhynchos
Lesser Scaup Aythya affinis
American Redhead Aythya americana
Canvasback Aythya valisineria
Great Reed Warbler Acrocephalus arundinaceus Red Knot Calidris canutus
Red Knot Calidris canutus
Bar-tailed Godwit Limosa lapponica
Pink-footed Goose Anser brachyrhynchus
Departure weight from a stopover site estimated from a previously established relationship between stopover duration and departure weight. Estimated heaviest birds were more likely to be seen at the same site in a later year Birds trapped, weighed, ringed and released soon after arrival at a wintering site. Lightest birds were more likely to be shot in the ensuing months Birds trapped, weighed, ringed and released soon after arrival at a wintering site. Lightest birds were more likely to be shot in the ensuing months Birds trapped, weighed, ringed and released soon after arrival at a wintering site. Lightest birds were more likely to be shot in the ensuing months Birds trapped, weighted, ringed, and released soon after arrival at wintering site. Lightest birds were more likely to be found dead in the ensuing months
Birds trapped, weighed, ringed and released at a migration/wintering site. Heaviest birds were more likely to be recaught at the site in a later year Birds trapped, weighed, ringed and released at a spring stopover site. Those that were above average weight were most likely to survive poor spring weather in breeding areas
Birds trapped, weighed, ringed and released at a spring stopover site. Heavy individuals were more likely to be re-sighted in subsequent years than lighter ones. In years when many birds failed to achieve normal departure weight, mean survival of ringed birds declined (by 37%)
Birds trapped, weighed, ringed and released at a stopover site. The heaviest individuals (with the most advanced plumage development) were most likely to be recaught at the site in a subsequent season Birds at a spring stopover site judged to be in good condition (from the abdominal profile) were more likely than poor condition birds to be later re-sighted in a wintering or migration area
Birds at a spring stopover site judged to be in good condition (from the abdominal profile) were more likely than poor condition birds to be later re-sighted in a wintering or migration area
Pfister et al. (1998)
Dufour et al. (1993) Pace & Afton (1999) Bain (1980) Haramis et al. (1986)
Nisbet & Medway (1972)
Prop & Deerenburg (1991)
individual birds at migration sites, whether autumn or spring, and their subsequent survival or, more strictly, their re-sighting rates (Table 27.1). In some such studies, searches for marked birds were not restricted to the capture site in subsequent migration seasons, but were made at several different points along the migration route, allowing the whole population to be surveyed thoroughly on repeated occasions in subsequent years. In these studies, therefore, we can be confident that lower re-sighting rates reflected poorer survival (as concluded by the authors).
Among various species of ducks, individuals which were light in weight when banded at autumn stopover sites, or soon after arrival in wintering areas, survived less well than individuals in good condition. After allowing for age and sex differences in body weight, such variation was evident in Mallards Anas platyrhyn-chos (Hepp et al. 1986, Dufour et al. 1993), Redheads Aythya americana (Bain 1980), Canvasbacks A. valisineria (Bain 1980, Haramis et al. 1986) and Lesser Scaups Aythya affinis (Pace & Afton 1999). In adult male Canvasbacks, lower survival of poor condition birds was due to natural factors (because the birds were not hunted at the time, Haramis et al. 1986), but in the other species it mainly resulted from greater proportions of poor condition birds being shot within the next few months. Relationships between body weight at migration time and subsequent survival were also noted among passerines, shorebirds and geese (Table 27.1).
Among geese and other waterfowl, food availability at wintering and spring migration sites has long been known to affect body condition and subsequent breeding success (Table 27.2). In a few species, the mechanisms have been studied. For example, among Brent Geese Bernicla branta in the Netherlands, females that had accumulated the greatest body reserves at a spring stopover site were more likely to return with young in the following autumn than were females that had accumulated smaller reserves, whereas males, which accumulated generally smaller reserves than females, showed no such relationship (Ebbinge & Spaans 1995). The favoured spring staging habitat was saltmarsh where nutrient-rich plants allowed the geese to fatten rapidly. However, the number of geese that could feed in saltmarsh was limited, so as the population grew over a period of years, increasing proportions of birds were relegated to less nutritious agricultural grassland. The geese used body reserves accumulated in spring for migration and reproduction, and individuals that had fed on saltmarsh showed better breeding success than those that had fed on grassland (Ebbinge 1992).
In some other species of geese, smaller proportions of females laid, and clutches were smaller, in years when feeding conditions in staging areas were poor than in years when they were good (for Barnacle Geese Branta leucopsis see Cabot & West 1973, for Lesser Snow Geese Chen c. caerulescens see Davies & Cooke 1983). These various studies show that the breeding success of some migratory geese depends partly on body reserves accumulated in wintering and spring staging areas, and that the effects of such reserves are evident both at the level of the individual and at the level of the population. Breeding success in some ducks is also influenced by body reserves, accumulated partly before and partly after arrival in breeding areas (e.g. Mosbech et al. 2006). The females of some species at high latitudes, such as Northern Pintail Anas acuta and Steller's Eider Polisticta stelleri, start laying within days after arrival in breeding areas, and so may be assumed to have accumulated the necessary body reserves elsewhere (Arzel et al. 2006).
Table 27.2 Examples of associations between body weight at migration time and subsequent breeding
American Redstart Setophaga ruticilla
Dusky Warbler Phylloscopus fuscatus
European Pied Flycatcher Ficedula hypoleuca
Red Knot Calidris canutus Pectoral Sandpiper Calidris melanotos
Mallard Anas platyrhynchos
Lesser Snow Goose Chen c. caerulescens
Greater Snow Goose Chen caerulescens atlantica Pink-footed Goose Anser brachyrhynchus
Barnacle Goose Branta leucopsis Brent Goose Branta bernicla Tundra Swan Cygnus columbianus
Black Kite Milvus migrans
Birds weighed soon after arrival in breeding areas. Heavier individuals laid earlier, and had larger clutches, egg volumes and chick weights
Birds weighed soon after arrival in breeding areas. Old males and males with high body mass had a greater chance of mating polygynously, while first-year males and males of low body mass often remained unmated
Birds weighed soon after arrival in breeding areas. Early-
arriving females were heaviest, with greater body reserves, laid earlier and had high hatching success
Birds at stopover sites in best condition produced most young
Birds weighed after arrival in breeding areas. Females that arrived early, with the largest body reserves, laid the largest eggs
Birds caught soon after arrival in breeding areas. Females in best body condition laid earlier, larger clutches
Birds shot after arrival in breeding areas. Heavier females, with greatest body reserves, had more developing egg follicles (taken to indicate larger clutches). Average spring body condition at a stopover site from year to year predicted autumn young: old ratios Birds at spring stopover site judged to be in good condition (from abdominal profile) laid earlier and larger clutches Birds at spring stopover site judged to be in good condition (from abdominal profile) were most likely to produce young Birds at spring stopover site judged to be in good condition (from abdominal profile) were most likely to produce young Birds at spring stopover site judged to be in good condition (from abdominal profile) were most likely to produce young Birds assessed at spring stopover site. Early individuals with high feeding rates were most likely to produce young
Birds weighed soon after arrival in breeding areas. Individuals in best condition occupied the best territories, laid earlier, and raised most young
Smith & Moore (2003)
Lundberg & Alatalo (1992)
Morrison (2006) Farmer & Wiens (1999)
Krapu (1981), Pattenden & Boag (1989)
Ankney & MacInnes (1978), Alisauskas (2002)
Bety et al. (2003), Reed et al. (2004) Madsen (1995, 2001), Drent et al. (2003) Prop & Black (1998)
Ebbinge & Spaans (1995)
Nolet & Drent (1998), Beekman (2005)
Sergio et al. (2007)
Among shorebirds nesting in the High Arctic, the extent to which body reserves are available for egg production may depend on climatic and other conditions in particular years, and may differ between protein and lipid. In one study, isotope analysis revealed that in several species egg protein was formed from terrestrial rather than marine foods, and hence was influenced by food eaten after arrival in breeding areas (Klaassen et al. 2001). On the other hand, in another study, eggs in the earlier clutches of Red Knots Calidris canutus and Ruddy Turnstones Arenaria interpres in the northeastern Canadian High Arctic were rich in 13C and 15N, which suggested that some residual marine nutrients were used in their production (Morrison & Hobson 2004).
Some studies of passerines have also suggested that conditions in wintering areas can influence both the timing of spring migration, and also the timing and success of reproduction (Chapter 26; for American Redstart Setophaga ruticilla see Marra et al. 1998, for Black-throated Blue Warbler Dendroica caerulescens see Bearhop et al. 2004, Norris et al. 2004). In such small species, the weight of a clutch is so large, relative to body weight, that body reserves could provide at most a tiny proportion of the material necessary for egg formation. Presumably they serve some other function, such as maintenance, and the nutrients required for eggs must come mainly from food eaten at the time (Sandberg & Moore 1996). However, both sexes of American Redstarts arrived to breed in northern Michigan with surplus fat, but females arrived with more than males in two out of three years (Smith & Moore 2003). Individuals of both sexes, but especially females, that had more body fat on arrival also showed higher reproductive success, with larger clutch sizes, egg volumes and nestling weights than birds arriving with little or no fat. In cases like this, where the reserve is small, it is hard to tell whether the association with subsequent breeding success reflects a cause-effect relationship, or merely a correlation, with birds that achieve good body condition on migration also achieving good body condition at other times, and hence showing high survival and reproductive success.
All the studies known to me that have related body condition at migration times to subsequent survival or breeding success have shown clear, positive relationships, although their numbers are still small. Possibly other species showed no such relationships, and have therefore less often been reported in the ornithological literature. However, if we are to assess their frequency, it is important that studies showing no relationships are published, so that they can be assessed along with those that do.
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