On the ecology and evolution of partial migration : a field study on migrant and resident European blackbirds

Abstract

Partial migration is defined as a within-population variation in migratory behaviour, meaning that some individuals migrate while others remain year-round residents in a given habitat. Studying a partially migratory population is the ideal system to test hypotheses concerning the evolution of migration and to elucidate costs and benefits of the two strategies (migration versus residency). I studied a wild partially migratory population of European blackbirds (Turdus merula) using a combination of radio tracking and classic capture-mark-recapture methods.Most of the migratory songbird species are diurnal, but they migrate at night. How these birds shift their daily rhythms from diurnal to nocturnal during the migratory seasons remains unknown. To answer this question, in the second Chapter I studied how migrant blackbirds organize their daily rhythms prior to departure in the fall. Using an automated radio telemetry system, I monitored, quantified and compared the activity of migrants and residents every 30 minutes, seven days before the departure of the migrants. I found that activity patterns between migrants and residents did not differ during daytime or night-time. Furthermore, migrants did not change their daily rhythm in a progressive manner, as previously shown in the lab, but instead shifted abruptly on the night of departure. I found no signs of nocturnal activity exhibited prior to migration.Activity of an animal is the most basic descriptor of its behaviour. Animals vary their activity levels throughout the year in response to environmental fluctuations and the specific requirements of the different stages of the annual cycle. In the third Chapter, also using an automated radio telemetry system, I compared the seasonal activity patterns of migrants and residents when individuals of both groups were present at the breeding grounds (from spring until fall). The main objective was to understand whether the constraints and requirements of migration as a strategy would have an effect on the activity levels of migrants after their arrival to the breeding grounds in spring, during the breeding season and prior to migration in the fall. I expected that after arrival in the spring, migrants would exhibit different activity levels compared to residents; either to recover from migration (decreased activity levels) or to compete for breeding mates and territories (increased activity levels). Similarly, during the pre-migratory period I also expected differing activity levels between migrants and residents because migrants prepare themselves for migration. Contrary to my expectations, I found that migrants and residents do not differ in their activity levels when both groups were present at the breeding grounds. Additionally I described the annual activity pattern of resident birds. Even when the photoperiod is taken into account, activity follows a seasonal pattern: there is high activity during the breeding season, decreasing activity during the moulting period, and increasing activity towards winter.One hypothesis for the evolution and maintenance of partial migration is that migration offers individual fitness benefits. Migrants would travel during the fall to overwinter in habitats that offer better survival chances than the breeding grounds. To test whether migration offers survival benefits, I used a combination of capture-recapture and radio telemetry methods to generate a presence/absence matrix with data collected over seven years. Using multi-event survival models, I estimated seasonal and annual survival probabilities for individuals of both groups. I found that during the non-winter season (March – November) migrants and residents have a similar probability of survival. However, during the winter (November - March), migrants have higher survival probability than residents. These results support the notion that migration offers fitness benefits.