Cold Weather, Warm Nest
Scientists have long studied the role of birds’ nests in avian reproduction, working to illuminate the relationship between nest structures and nesting success. Nests are not just a sturdy vessel in which to lay eggs; they also serve to conceal eggs and nestlings from predators, protect them from the elements, and for some species, nests even help females choose a mate with good genetic qualities. You might think that a stronger, denser nest would naturally result in higher nestling survival, but that’s not always the case. Consider the Mourning Dove: they build a flimsy platform nest which has little insulation value, and yet they are one of the most abundant nesting birds in North America. The Killdeer builds no nest at all, just deposits its eggs directly onto a graveled surface, and they are quite successful breeders. So we know that the relationship is not as straightforward as it might seem.
Researchers in Great Britain recently examined the relationship between local temperatures immediately prior to egg-laying and the thermal properties of the nests which were built, for two species of tits (a close relative of our chickadees). Analysis of temperature data from the seven days prior to egg-laying, when nest-building is most intense, revealed that the temperatures experienced by a female as she is building the nest did influence the amount of insulation that she added to the nest. If the nest was built during a cold period, females tended to add more insulating materials, and vice versa. The authors of this study point out that when tit nests are built during an early spring warming period, this could have negative impacts on success if temperatures drop again. And with climate change likely to make weather patterns less predictable over time, the consequences for nesting birds are unknown.
It remains to be seen whether less-insulated nests will actually result in fewer offspring for birds. So far, study results have been mixed, with some species experiencing a drop in success when insulation is lighter, and some species exhibiting no effects. We are often asked by nest monitors why some nests fail during severe weather and others survive just fine, and there is no simple answer. Perhaps it’s not the severe weather per se that is responsible, but the prevailing balmy weather during the nest-building stage. And perhaps some species are just not as reliant on their nests for insulation as others. It is important to be alert to the vagaries wrought by climate change, and every nest you report to NestWatch gets us closer to understanding whether–and how–nesting success is changing.
Reference: Deeming, D. C., M. C. Mainwaring, I. R. Hartley, and S. J. Reynolds. 2012. Local temperature and not latitude determines the design of Blue Tit and Great Tit nests. Avian Biology Research 5:203–208.