From my study window I can see the nest box that I attached to the studio last autumn. It is a robust affair, hand-made and presented to me by a visitor, but it has everything that a cavity-nesting bird could require. Over the winter the box has been used by roosting birds, most likely blue tits, and it is a pair of blue tits that has occupied the box for this spring’s breeding attempt. From my upstairs vantage point I have been able to watch the female – the male does not help with nest construction in this species – taking in moss and grass, a process that has taken many days.
Initially the building was a stop-start affair; the bird would be busy on one warm morning but not the next. Then things became more focussed and the nest building efforts more intense. There is a suggestion, not just from this box but from others elsewhere across the county, that blue tits are running a little late this year. We’d normally expect the first eggs from mid-April but I’d be surprised if this particular pair gets to that stage much before the month’s end.
The timing of a blue tit nesting attempt is important, not least because the birds seek to time things so that the period of peak food demands of their growing chicks matches the peak abundance of the caterpillars on which they feed. The peak in caterpillar abundance follows soon after bud burst, when young leaves are at their most tender and not packed with deterrent chemicals. Of course, bud burst is also running late this year, so the blue tits should not be too far out in their timing. There is a wider concern, however, that a changing climate might eventually lead to a more serious mismatch between the availability of caterpillar prey and the timing of blue tit nesting attempts. This may occur because insects, like moths, are able to respond to a changing climate more rapidly than larger, more complex organisms like birds. We know that, on average, blue tits now nest more than a week earlier than they did a few decades ago, but just how much earlier now is the peak in caterpillar abundance?
My blue tits also face one additional challenge. They are not nesting in a wood – the preferred habitat – but in an urban setting, where fewer caterpillars are available. Urban birds are often less productive and, on average, rear fewer chicks than their country cousins. It may be that many urban pairs are young birds, not able to secure a breeding territory in a better quality habitat, so the option to breed in a garden still provides their best chance of success.