Despite observational evidence of carry-over effects (COEs, events occurring in one season that produce residual effects on individuals the following seasons), to our knowledge no experimental studies have been carried out to explore how COEs might affect reproductive output. We simulated an environmental perturbation affecting spring-staging migrants to investigate COEs in greater snow geese (Anser caerulescens atlanticus). During three consecutive years, 2037 females captured during spring staging (approx. 3000 km south of their Arctic breeding grounds) were maintained in captivity (with or without access to food) for 0-4 days. Duration of captivity (but not food treatment) negatively affected reproductive success, probably through stress response. Reproductive success was reduced by 45 -71% in 2 years, but not in a third year with unusually favourable breeding conditions. This unprecedented manipulation indicates that COEs can have a strong effect on individual reproductive success in long-distance migrants, but that this effect can be partly compensated for by good environmental conditions on the breeding ground.
Externally mounted transmitters or loggers may adversely affect migration performance for reasons other than the effects of added mass. The added frontal area of a payload box increases drag, and if the box triggers separation of the boundary layer over the posterior body, the drag coefficient could also be increased, possibly by a large amount. Any such effects would lead directly to a decreased migration range and reduced energy reserves on completion of migration. We measured the body drag coefficients of Rose-coloured Starlings in the Seewiesen wind tunnel by the wingbeat-frequency method. The speed at which the wingbeat frequency passed through a minimum was taken to be an estimate of the minimum-power speed (V mp ), from which the body drag coefficient was calculated in turn. Dummy transmitter boxes were mounted on the bird's back by attaching them with Velcro to a sideloop harness pad. The pad alone projected 6 mm above the bird's back, and increased the drag coefficient by nearly 50%, as compared to the ''clean'' configuration with no harness. Adding boxes (square-ended or streamlined) produced no further significant increase in the drag coefficient, but the addition of a sloping antenna increased it to nearly twice the clean value. These increases are attributed to separation of the boundary layer over the posterior upper body, triggered by the payload. We then ran computer simulations of a particular Barnacle Goose, for which detailed information was available from an earlier satellitetracking project, to see how its migration range and reserves on arrival would be affected if its transmitter installation also caused flow separation and affected the body drag coefficient in a similar way. By representing the range calculation in terms of energy height, we separated the effect of the transmitter's mass, which reduces the fat fraction (and hence also energy height) at departure, from that of flow separation, which steepens the energy gradient. The effect of the mass is small, and increases only slightly with increasing distance, whereas a steeper energy gradient not only reduces the range but also reduces the reserves remaining on arrival, to an extent that increases with migration distance. Energy height is related to the fat fraction rather than the fat mass, and is therefore preferable to energy as such, for expressing reserves in birds of different sizes.
Evaluating consequences of habitat selection is an important step in understanding life history strategies and behavioural decisions of animals. Kilpi and Lindström (1997) found that incubating common eiders Somateria mollissima on exposed, treeless islands lost weight faster than females nesting on wooded islands and proposed that this difference was due to adverse incubation conditions at exposed nests. Therefore, we tested whether common eiders gained an advantage when nesting in sheltered habitats by placing artificial shelters over randomly-selected females after the onset of incubation within an eider colony in arctic Canada. We predicted that sheltered females would be heavier on completion of incubation than control hens lacking shelters. Females nesting in artificial shelters experienced a more moderate thermal environment at both cold and warm temperature extremes. Eiders nesting in shelters were heavier than control females during mid incubation, consistent with habitat-specific rates of weight loss reported by Kilpi and Lindström (1997). Natural overhead cover was available at potential nests but few eiders used those sites. We suspect that microclimatic advantages offered by sheltered sites may be offset by costs of increased female vulnerability to predators. Further work is needed to test this hypothesis, and to determine mechanisms responsible for lower weight loss in eiders attending well concealed nests.
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