Exploitation of resources modulates stopover behaviour of passerine migrants | Ausbeutung des Nahrungsangebotes beeinflusst das Rastverhalten ziehender Singvögel
2003
Starke-Ottich, Indra | Dierschke, Volker
The seasonal variation in the supply of ripe Elderberries (Sambucus nigra) on the offshore island of Helgoland (North Sea) was examined from the point of view of the effect of food supply and exploitation on the stopover behaviour of migrating passerines. Elderberries were completely exploited by birds by mid October, three weeks earlier than the last berries disappeared from exclosures. In five out of seven frugivorous passerine species the percentage of trapped birds settling (i. e. recaptured on days subsequent to initial capture) was significantly higher during the period of high availability of ripe Elderberries (early September to early October) than it was earlier (berries still unripe) and later (berries exhausted). No such pattern was observed in fuel deposition rates because rates of body mass change in recaptures differed between periods in only one out of five species. Therefore, in conjunction with the shortage of alternative food, berry supply had an impact on the stopover decision, but did not affect the refuelling itself. The energy demand of migrants as calculated from the number of birds present could not be covered by Elderberries from mid October onwards. Hence, exploitation by earlier migrants reduced the carrying capacity of the stopover site for later migrants, especially when high bird densities were involved. Continent-wide patterns of fuel deposition may have evolved in order to avoid such shortages of food for refuelling just before the crossing of ecological barriers. Passerine migrants would benefit from dispersed resources which allow the avoidance of crowding and exhaustion of supply.
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