Last year's cold spring weather added an average of three days to the breeding time of great and blue tits. Nests also averaged one egg fewer compared to the period 1980-2012. For its latest report, the Dutch NESTKAST ('nestbox') network collected data from some 9000 broods.

Great tits and blue tits lay their eggs in April. In many cases they do so in nestboxes, which makes it easy to keep track of the broods.

Both professional researchers and volunteers followed the fortunes of some 9000 tit broods last year. An analysis of part of the resulting data suggests that the cold April weather had a major impact on the birds' breeding.

There was even some snow last April, and nature developed more slowly than it normally would. In anticipation of this, the tits temporarily halted their egg laying and brooding, adding an average of three days to the process.

Fewer eggs

The annual report by the Dutch working group NESTKAST collates breeding data from 16.950 nestboxes, demonstrating once again that tits are mong the Netherlands' most thoroughly studied bird species.

According to the report, the average number of great tit eggs in the 2016 season was 7,2. Blue tits laid 9,1 eggs on average.

That puts the birds one egg short of the long-term average over the period 1980-2012 for the third year running. Researchers have yet to find an explanation for the decline.

New season

Altogether, 2016 was a bad year for tits in terms of breeding. But we've just had a mild winter, in which the availability of beech nuts - the tits' staple winter diet - has been far above average.

Because of this, researchers are expecting that the 2017 season will bring more breeding success for the birds.