Underuse of stopover site by migratory swans

Bart A Nolet,Abel Gyimesi

doi:10.1007/s10336-013-0934-7

Bart A Nolet, Abel Gyimesi

Open Access

https://doi.org/10.1007/s10336-013-0934-7

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Abstract

Many migratory birds use a chain of stopover sites to fuel their migration. Under time-minimizing migration, fuelling time and giving-up density at stopovers are predicted to depend on fuelling conditions. Fluctuations in food accessibility likely lead to changes in fuelling conditions, which should in turn be reflected in fuelling time and giving-up density. During their migration, Bewick’s Swans Cygnus columbianus bewickii refuel on belowground tubers of Fennel Pondweed Potamogeton pectinatus in shallow lakes. We studied giving-up density and stop-over use (expressed in bird-days) of Bewick's Swans at an autumn stopover site (Lauwersmeer, The Netherlands) during 1995–2008, as dependent on local environmental conditions. High water levels were hypothesized to restrict access to tuber stocks. High water levels at the stopover site were predicted to lead to higher giving-up densities and less bird-days spent at the stopover. Annual variation in giving-up densities and number of bird-days was strongly associated with year-to-year differences in initial tuber biomass density and number of days with high water levels. As predicted, giving-up density increased and bird-days decreased with the number of days with high water level. We conclude that, in line with time-minimizing migration, changes in fuelling conditions may lead to underuse of a stopover site. Underuse of stopovers by migratory birds has been reported before but only in the sense that more food was left at stopover sites than at wintering sites. In contrast, in our case, dealing with a given stopover site, more food is left behind in some years than in other years.

Highlights

Many migratory birds use stopover sites where they fuel for migration (Alerstam and Hedenstrom 1998)
High water levels at the stopover site were predicted to lead to higher giving-up densities and less bird-days spent at the stopover
Because fuel load is supposed to increase with fuelling time, diminishing returns of potential flight range are expected with increasing fuelling time (Alerstam and Hedenstrom 1998)

Summary

Introduction

Many migratory birds use stopover sites where they fuel for migration (Alerstam and Hedenstrom 1998). Predation risk affects site use, in particular for small to medium-sized birds, and is often traded-off against food availability (Clark and Butler 1999; Guillemain et al 2007; Pomeroy et al 2006; Duijns et al 2009; Cresswell 1994). These are the factors known to affect giving-up densities (GUDs), i.e., the food densities remaining after animals have left a food patch (Brown 1988; Nolet et al 2006a). This would result in the underuse of the stopover site in some years, in the sense that food is left unexploited, because it was (temporarily) inaccessible

Methods

Dic 3 h

Discussion

Findings

Di 1 h