The Problem of Partial Migration - the Case of the Blue Tit

In the evolution of bird migration, partial migration is assumed to be an important intermediate step between migration and residency. Partial migration is characterised by the existence of both migratory and resident individuals in a given population. Compared to regular migration, relatively little is known about partial migrants and their migratory performances.

In my thesis, I have studied the partially migratory blue tit Cyanistes caeruleus in Scandinavia. The numbers of migrating blue tits have increased during the last decades, according to systematic, long-term observations at Falsterbo, a migratory passage site in southern Sweden. Global warming and subsequent climate change predict that the proportion of resident individuals should increase in partially migratory populations. However, in relation to its breeding densities, the blue tit has maintained its migratory activity indicating that the effect of environmental factors on partial migration may be complex.

Migrating blue tits differ from more regular migrants in several aspects. Compared to regular migrants, blue tits migrate relatively short distances with extraordinarily low speeds. They also have a strong response to weather en route and, in contrast to regular migrants, they are to a large extent affected by certain specific weather cues, e.g. absence of cloud cover, associated with favourable migratory conditions. Thus, partial migrants seem to choose the safest occasions for migratory flights with all orientation cues available.

Partial migrants have the ability to change behavioural strategy. Young birds often switch from migration to residency with increasing age. Therefore, partial migrants need adaptations for both migration and residency. In experimental comparisons between migratory and resident blue tits, I have found differences with respect to behavioural dominance, personality and metabolic rate. Migrants were dominant under clear skies. Thus, weather conditions seem to have a surprisingly large effect on partial migrants. Furthermore, fast explorers of new environments are more often migratory. Residents, on the other hand, had higher basal metabolic rate (BMR) than migrants, probably as a consequence of harsher conditions during winter.

Hence, we need to study the dual demands of migration and residency in partial migrants and the resulting trade-offs between adaptations to the two strategies to understand the evolution of migration.