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Partial Migration in blue tits

A part of the population of blue tits (Cyanistes caeruleus) at the Revinge area in the south of Sweden migrates and another part remains resident, thus qualifies blue tits as partial migrants. Although, the migrants follow a south-western direction and start migration movements at the same time as obligate migrants, a number of features differ between migrating blue tits and obligate migrants.

Blue tits only move short distances, median distance 82 km, which is much shorter than so called short-distance obligate migrants. Furthermore blue tits proceed on their migration very slowly, only averaging 13 km/day which is also much slower than obligate migrants. It is not even sure that the migrating blue tits return to their summer ranges. Three out of 18 blue tits with known wintering and breeding grounds, stayed to breed at their wintering grounds, thus undertook directed dispersal movements rather than true migration.Also, migrating blue tits have a lower metabolic rate and have bolder personalities than resident ones, in both cases contrary to obligate migrants and residents.

Still, little is known about the decisions to migrate or remain resident.

Who is staying?

Blue tit singing
Photo: Johan Nilsson

Earlier results seem to indicate that it is the sub-dominants that migrates, thus implying that migration is part of a “best-of-a-bad-job” strategy, i.e. a conditional strategy. However, a definite answer has to await measures of fitness consequences of the two strategies. When is the decision taken? If migration is a conditional strategy, the decision to migrate has to be taken well before the effects of being sub-dominant starts to affect condition.

More behavioural studies is needed

Clutch of blue tits in the nest
Photo: Johan Nilsson

We do neither know much about the behaviour of individuals using the two strategies before the migration period. Do residents and migrants form different social groups already during summer and early autumn? Is migration merely a directed movement when searching for food? Another interesting set of questions is to determine to what extent migrants in partially migrating populations have the same adaptations for flight, navigation and decisions for when to migrate as obligate migrants.

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Jan-Åke Nilsson
Johan Nilsson

Centre for Animal Movement Research
Evolutionary Ecology, Department of Biology
Ecology building S-223 62 Lund Sweden