Abstract
Migration has evolved to tackle temporal changes in availability of resources. Climate change has been shown to affect the migration dates of species, which raises the question of whether the variation in the timing of migration is climate or resource dependent? The relative importance of temperature and availability of food as drivers of migration behaviour during both spring and autumn seasons has been poorly studied. Here, we investigated these patterns in frugivorous and granivorous birds (hereafter frugivorous) that are assumed to postpone their autumn migration when there is plenty of food available, which may also advance upcoming spring migration. On the other hand, especially spring migration dates have been negatively connected with increasing temperatures. We tested whether the autumn and spring migration dates of eleven common frugivorous birds depended on the crop size of trees or ambient temperatures using 29 years of data in Finland. The increased crop sizes of trees delayed autumn migration dates; whereas, autumn temperature did not show a significant connection. We also observed a temporal trend towards later departure. Increasing temperature and crop sizes advanced spring arrival dates. Our results support the hypothesis that the timing of autumn migration in the frugivorous birds depends on the availability of food and is weakly connected with the variation in temperature. Importantly, crop size can have carry-over effects and affect the timing of spring arrival possibly because birds have overwintered closer to the breeding grounds after an abundant crop year.
Highlights
Each year, a large number of animals conduct migration journeys due to temporal changes in resources (Dingle 1996)
Our results support the hypothesis that the timing of both autumn and spring migration in the birds depends on the availability of food resources during autumn (Newton 2008)
Our results were in line with earlier findings suggesting that advancing spring arrival dates are linked with increasing temperatures
Summary
A large number of animals conduct migration journeys due to temporal changes in resources (Dingle 1996). The seeds of several tree species are an important but fluctuating food resource for migrating and wintering birds (Haila et al 1986; Newton 2008; Meller et al 2016). Due to year-to-year variation in food resources, frugivorous birds may adjust their autumn migration period according to crop size of trees (Jenni and Kery 2003; Newton 2008). Recent work has shown that crop sizes of four common tree species are synchronous in space and time, and this synchrony is partly linked with annual changes in weather conditions (Meller et al 2016; Gallego Zamorano et al 2018). As crop failure or mast in the most common tree species is synchronous across large spatial scales, food availability for various frugivorous species is likely to be synchronous
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