Blue tit: climate change and breeding behaviour

Blue tits are breeding earlier, because Dutch springs are getting warmer.

Trends

The warmer the spring, the earlier the blue tit lays its eggs (see left-hand graph). In the last thirty years the springs have been getting warmer and so the trees have come into leaf earlier and insect populations have peaked earlier. The blue tit reacts to this by breeding earlier. Nowadays, it is breeding about 10 days earlier, on average, than in 1986 (see right-hand graph). This synchronises the chicks' growth with the maximum availability of food.

The blue tit is able to take advantage of the warmer springs by breeding earlier, because it stays in the Netherlands during the winter. Migrants that overwinter in Africa cannot react as easily. The number of breeding blue tits in the Netherlands is continuing to increase somewhat.

Technical note

The laying date is expressed as the day number (day 1 = 1 January, and so on). The laying date is the average of the earliest 25% of all clutches found. The spring temperature is the average of the daily means for February through June, as recorded at the met station at De Bilt.

The data are from the Ecological Monitoring Network's nest-mapping project.

References

• Majoor, F., R. Foppen, F. Willems en D. Zoetebier (2001). De waarde van het nestkaartenproject voor signalering en beleid. Intern Rapport. SOVON. Beek-Ubbergen.