Sunday, August 5, 2012

Great tits make warmer nests up north

A well-known and highly-contested “rule” among biogeographers is Bergmann’s rule, which basically says that animals in the north have larger bodies than animals in the south because it allows them to stay warmer. Sometimes, this is true.  Sometimes, in order to stay warm, animals behave differently too. 

UK researchers have discovered that great tits and blue tits living in the northern parts of the Isle build warmer nests than those in the south by adding extra feathers, grass or animal hair to the inner cup that holds the eggs. Why study tits? They seem to actually like nesting in wooden boxes that humans make especially for the purpose of studying bird breeding. This makes it easy to borrow their nests when the birds are done with them and test how well nests would insulate their potential residents. The scientists found that the extra weight that birds add in the north was enough to keep up with the gradual decline in spring temperatures from southern sites to more northerly ones.

The sites where the researchers tested nests spanned only 5 degrees of latitude, which corresponds to a change of just 1.5°C in average spring temperature when the birds are laying eggs. It seems incredible that this slight variation has even elicited any reaction by the tits. Clearly, 1.5°C to a bird is not the same as 1.5°C to a human.  If 1.5°C didn’t affect the survival of eggs and chicks there would be no pressure for mother tits to change their nests.  But, because they do, we know that it matters and we also know that additional increases in temperature that result from global climate change will matter too.

Just as baby birds can get too cold, they can also get too hot. To think of it a different way; perhaps birds in the north are not building warmer nests, but it is the birds in the south that are building cooler ones.  A two-year-old knows that there are only so many clothes you can take off to cool off; nests can only get so light before not much is left to hold the eggs. How hot before nests get naked?  2°? 3°? Over the next hundred years, we may find out. Or, we may not- Great tits living as far south as north Africa seem to have found a way to keep their eggs cool enough. Extinction by overheating seems unlikely, but more changes to how the tits build nests, a certainty.

You can find this article at:

ResearchBlogging.orgM. C. Mainwaring, I. R. Hartley, S. Bearhop, K. Brulez, C. R. du Feu, G. Murphy, K. E. Plummer, S. I. Webber, S. J. Reynolds, & D. C. Deeming (2012). Latitudinal variation in blue tit and great tit nest characteristics indicates environmental adjustment Journal of Biogeography DOI: 10.1111/j.1365-2699.2012.02724.x