The impact of tree crops and temperature on the timing of frugivorous bird migration

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.

Extrinsic factors, endocrine mechanisms, and behavioral indicators of migratory restlessness in wintering whooper swans (Cygnus cygnus)

“…Changing weather conditions were also reported to influence the migration of palae-arctic migrants. Spring migration is mainly related to increasing temperatures, while autumn departure corresponds to decreasing air temperature in the breeding range 18 – 22 . Such behavioral responses correspond to changes in the corticosterone level to meet physiological needs in the early stage of migration, such as an increase in foraging frequency to enhance the body fat content (refueling) for the upcoming spring migration 3 , 23 .…”

Facultative migration in two thrush species (Fieldfare and Redwing): Rowanberry abundance is more important than winter weather

“…Snow cover may be a smaller disadvantage for species foraging in trees than for species feeding on the ground (Newton 2008), whereas cold spells will increase energy demands and may trigger southward migration and limit wintering in northern areas independent of foraging site (Ulfstrand 1963, Zuckerberg et al 2011. Kanerva et al (2020) found that temperature had little influence on the timing of autumn migration in Finland for a variety of irruptive species and facultative migrants. The study of Kanerva et al (2020) is also one of the few that have assessed the relative importance of crop size and weather, and found that crop size was most important for determining timing of autumn migration, including for all the rowanberry specialist species.…”

“…Kanerva et al (2020) found that temperature had little influence on the timing of autumn migration in Finland for a variety of irruptive species and facultative migrants. The study of Kanerva et al (2020) is also one of the few that have assessed the relative importance of crop size and weather, and found that crop size was most important for determining timing of autumn migration, including for all the rowanberry specialist species. Similarly, partial migration in many Finnish landbird species was influenced more by tree seed and berry crops than temperature (Meller et al 2016).…”

“…Bullfinches occur irruptively in autumn and winter in southern areas when there is crop failure further north (Fox et al 2009), Pine Grosbeaks move south to exploit abundant crops every 2-3 years (Dale 2023a), whereas waxwings show a more complex pattern with southward migration mostly in years with good crops, but sometimes also in years with poor crops (Dale 2023b). In facultative migrants such as the Fieldfare, parts of the populations stay in northern areas when there is high rowanberry availability, but most individuals migrate south in other years (Tyrväinen 1975, Haila et al 1986, Kanerva et al 2020.…”

Timing is critical: consequences of asynchronous migration for the performance and destination of a long-distance migrant

“…However, earlier spring migrants may experience worse weather conditions during migration, increasing their energy expenditure [ 10 ]. The consequences of variability in timing of the autumn migration are less well understood [ 11 , 12 ] and may be species specific [ 13 , 14 ], thus more challenging to determine. Nevertheless, autumn migration is marked by high mortality of juveniles on their first migration [ 15 ], as well as by the choice of wintering location [ 16 ] that may have carry over effects to the remaining annual cycle [ 17 ].…”