Underuse of stopover site by migratory swans

Authors: Bart A Nolet Abel Gyimesi

Publish Date: 2013/02/07

Volume: 154, Issue: 3, Pages: 695-703

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Abstract

Many migratory birds use a chain of stopover sites to fuel their migration Under timeminimizing migration fuelling time and givingup 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 givingup density During their migration Bewick’s Swans Cygnus columbianus bewickii refuel on belowground tubers of Fennel Pondweed Potamogeton pectinatus in shallow lakes We studied givingup density and stopover use expressed in birddays of Bewicks 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 givingup densities and less birddays spent at the stopover Annual variation in givingup densities and number of birddays was strongly associated with yeartoyear differences in initial tuber biomass density and number of days with high water levels As predicted givingup density increased and birddays decreased with the number of days with high water level We conclude that in line with timeminimizing 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 yearsViele Zugvögel nutzen aufeinanderfolgende Rastplätze um Energie für ihren Zug aufzunehmen Um die Zugdauer zu minimieren sollte deshalb die „Auftankzeit“ und die Nahrungsdichte beim Verlassen eines Rastplatzes „Aufgabedichte“ von den örtlichen AuftankBedingungen abhängen Fluktuationen in der Nahrungserreichbarkeit können zu Änderungen in diesen Auftankbedingungen führen und sollten sich in Auftankzeit und Aufgabedichte niederschlagen Singschwäne Cygnus columbianus bewickii ernähren sich während ihres Zuges von Wurzelknollen des Kammlaichkrauts Potamogeton pectinatus im Flachwasserbereich Wir untersuchten die Nahrungsaufgabedichte un die Rastplaztnutzung gemessen in VogelTagen von Singschwänen und deren Abhängigkeit von den lokalen Umweltbedingungen an einem Herbstrastplatz in den Jahren 1995–2008 Hohe Wasserstände sollten den Zugang zu den Wurzelknollen begrenzen und damit zu höheren Nahrungsaufgabedichten und einer geringeren Anzahl an VogelTagen auf diesem Rastplatz führen Die jährlichen Variationen in den Nahrungsaufgabedichten und den Anzahlen an Vogeltagen waren stark mit Unterschieden in der Wurzelknollendichte bei Ankunft am Rastplatz sowie der Anzahl der Tage mit hohen Wasserständen verbunden Mit der Anzahl an Hochwassertagen erhöhte sich wie erwartet die Nahrungsaufgabedichte und verringerte sich die Anzahl der Vogeltage In Übereinstimmung mit den Vorhersagen zur Minimierung der Zugdauer schlussfolgern wir dass Änderungen in den Auftankbedingungen zu einer Unternutzung von Rastplätzen führen können Solche Unternutzungen von Rastplätzen waren zwar bereits berichtet worden doch nur insofern dass an den Rastplätzen mehr Nahrung zurückblieb als an den Winterplätzen Wir dagegen zeigen dass die nicht genutzte Nahrung innerhalb eines Rastplatzes von Jahr zu Jahr schwanken kannMany migratory birds use stopover sites where they fuel for migration Alerstam and Hedenström 1998 During migration more time is commonly spent on stopovers than travelling particularly in birds using flapping flight Hedenström and Alerstam 1997 Hence stopover duration largely determines migration speedTheory predicts that timeminimizing migrants should stay at a stopover as long as the rate of increase in body fuel enables a faster migration than the overall migration speed on their journey Alerstam 1991 Alerstam and Lindström 1990 In flying animals fuel load comes with a flight cost Pennycuick 1975 Kvist et al 2001 and therefore potential flight range and hence migration speed is expected to increase at a decelerating rate with fuel load Alerstam 1991 Alerstam and Lindström 1990 Because fuel load is supposed to increase with fuelling time diminishing returns of potential flight range are expected with increasing fuelling time Alerstam and Hedenström 1998 At the stopover carrying and maintaining a given fuel load already comes at a cost and this will further slow down the increase in flight range with fuelling time Klaassen and Lindström 1996 Moreover food depletion might cause diminishing returns of potential flight range with fuelling time because depletion often leads to lower fuelling rates Nolet and Drent 1998 but see Lourenço et al 2010 Together fuel load and food depletion determine the optimal fuelling time at a stopover Alerstam and Lindström 1990 Alerstam 1991 If conditions are unfavourable at a particular stopover site migrants have been shown to move on Newton 2006 and references therein and to depart with low fuel loads as expected when migrants aim to migrate as fast as possible Lindström and Alerstam 1992The main factors determining the fuelling conditions and hence fuelling time or stopover duration of a stopover site are its food abundance Newton 2006 the costs of harvesting that food Van Eerden et al 1997b and food accessibility Zwarts et al 1992 Predation risk also affects site use in particular for small to mediumsized birds and is often tradedoff against food availability Clark and Butler 1999 Guillemain et al 2007 Pomeroy et al 2006 Duijns et al 2009 Cresswell 1994 These are also the factors known to affect givingup densities GUDs ie the food densities remaining after animals have left a food patch Brown 1988 Nolet et al 2006a Hence GUDs at a stopover site have been proposed to reflect the longterm fuelling rates along the migration route Van Gils and Tijsen 2007Bewick’s Swans Cygnus columbianus bewickii Yarrell fuel their migration by feeding on aquatic macrophytes especially tubers of Fennel Pondweed Potamogeton pectinatus Beekman et al 1991 Nolet et al 2001a b During aquatic foraging of these large birds c 60 kg predation risk can be neglected which partly explains why they prefer to forage on water over field feeding Nolet et al 2002 For instance the largest avian predator in their aquatic habitat the Whitetailed Eagle Haliaeetus albicilla only exceptionally takes birds of more than 25 kg Cramp and Simmons 1979 Therefore aquatically foraging Bewick’s Swans are well suited to study the effects of fuelling conditions on stopover use without the confounding effects of predation risk

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