Tits on the move: exploring the impact of environmental change on blue tit and great tit migration distance.

1. In response to warmer spring conditions in Central Europe many migratory bird species have shifted their timing of breeding. Environmental change has also led to warmer winters, shortening the distance between the breeding grounds of migratory birds and their overwintering areas. 2. Here, we show that in response to warmer winters, blue tits (Cyanistes caeruleus), but not great tits (Parus major), breeding in Germany decreased their migration distance between 1964 and 1996. Understanding this difference provides insight into possible constraints and selection pressures involved in how species respond to environmental change. Here, we focus on their breeding ecology. 3. In a nest box population in southern Germany, both species laid their first clutch earlier with increasing spring temperature, but over the study period (1974-1999) blue tits showed a significant and stronger advancement in laying date than great tits. For both species, selection for earlier breeding did not vary with environmental change, indicating that early laying pairs did not do better than later laying pairs as spring temperature increased. 4. Blue tits in the nest box population were single-brooded and existing hypotheses state that single-brooded species likely advance their laying date to match timing of reproduction with the advancing food peak in spring. We hypothesize that this might be one reason why blue tits adjusted their migration strategy as closer proximity to the breeding grounds in winter allows better prediction of the onset of spring. Ten per cent of great tits successfully produced second broods and their first clutch laying date is a compromise between first and second clutch laying date, which might be why great tits had not advanced their laying date nor altered their migration strategy.