![]() 2017) while, others stress variation in environmental conditions, especially temperature and the availability of food (Haila et al. ![]() Some hypotheses focus on intraspecific differences in sex, body size, or dominance status (Ketterson and Nolan 1976 Gauthreaux 1982 Lehikoinen et al. Several not mutually exclusively hypotheses have been proposed to explain why some individuals migrate while others do not. As crop failure or mast in the most common tree species is synchronous across large spatial scales, food availability for various frugivorous species is also likely to be synchronous. 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. 2017) as well as southern latitudes (Griffioen and Clarke 2002 Mackay and Gross 2019).ĭue 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). ![]() 2011 Suhonen and Jokimäki 2015 Suhonen et al. The year-to-year variation in food availability affects the migration behaviour of some migratory species, such as frugivorous and granivorous (hereafter frugivorous) birds at northern latitudes (Haila et al. In the boreal zone, the seeds of several tree species are an important but fluctuating food resource for migrating and wintering birds (Haila et al. 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.Įach 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 autumn migration in the frugivorous birds depends on the availability of food and is weakly connected with the variation in temperature. Increasing temperature and crop sizes advanced spring arrival dates. We also observed a temporal trend towards later departure. The increased crop sizes of trees delayed autumn migration dates whereas, autumn temperature did not show a significant connection. 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. ![]() On the other hand, especially spring migration dates have been negatively connected with increasing temperatures. 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. 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. Migration has evolved to tackle temporal changes in availability of resources. ![]()
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