Early breeding

Redstart. Photograph by Edmund Fellowes

The advance in Redstart laying dates is the greatest exhibited by any migrant; the species now breeds
a fortnight earlier on average than it did in the mid-1960s.

Data from the Nest Record Scheme provide strong evidence of shifts towards earlier laying in a range of species, linked to climatic change. We have now identified 39 species that, on average, are laying between three and 23 days earlier, on average, than in the mid 1960s.

The species now laying earlier in the year represent a wide range of taxonomic and ecological groups, including raptors (Kestrel – 9 days), waders (Oystercatcher – 3 days), migrant insectivores (Pied Flycatcher – 10 days, Swallow – 11 days), resident insectivores (Robin – 9 days, Blue Tit – 8 days), corvids (Magpie – 23 days) and resident seed-eaters (Greenfinch – 20 days).

For some species these shifts towards earlier laying may be insufficient to match seasonal advances in the peaks of food availability. Recent research has shown that significantly stronger phenological responses to climate change are displayed at lower trophic levels (such as the food birds eat) than at higher levels (such as the birds themselves), increasing the potential for disjunction and resulting productivity declines. However, the evidence for for a population-level effect of reduction in breeding success is mixed and more research is needed to determine the extent to which declines in abundance will result.

Only six species demonstrate a significant delay in average laying dates, of between two and 19 days: Woodpigeon, Turtle Dove, Barn Owl, Raven, Blackbird and Yellowhammer (taxonomic order). With the exception of Raven, all of these species initiate multiple breeding attempts per season and there is increasing evidence that species which are less reliant on seasonal peaks in resource availability may be able to extend their breeding seasons further into the summer, resulting in a later mean value for laying date. Raven typically initiates laying in February, long before most other species and prior to the early spring period in which climatic warming has been most pronounced.