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.

迁移已发展为解决资源可用性的暂时变化。事实证明，气候变化会影响物种的迁徙日期，这引发了一个问题，即迁徙时间的变化是否取决于气候或资源？气温和食物供应量是春季和秋季季节迁移行为的驱动因素，其相对重要性尚未得到很好的研究。在这里，我们调查了食肉和食肉鸟类（以下称食肉鸟类）中的这些模式，这些鸟类被假定在有足够食物的情况下推迟了秋季迁徙，这也可能会推动即将到来的春季迁徙。另一方面，特别是春季迁徙日期与温度升高负相关。我们使用芬兰29年的数据，测试了11只普通食肉类鸟类的秋季和春季迁徙日期是否取决于树木的作物大小或环境温度。树木产量的增加延迟了秋季的迁徙日期；而秋季温度没有显示出明显的联系。我们还观察到了以后离开的时间趋势。温度升高和农作物大小增加，春季到达日期提前。我们的结果支持以下假设：在食肉类鸟类中秋季迁徙的时机取决于食物的可获得性，并且与温度的变化之间存在弱关联。重要的是，农作物的大小会影响作物的生长，并影响春季的到来，这可能是因为在丰收的一年之后，鸟类越靠近繁殖地越冬。