There’s no need to tell you how cold it’s been recently. But please bear with me as I beg your sympathy. Last night we recorded minus 14.5 degrees at the Pinfold. We have ice so thick on our windows that we cannot see out, have reached a limit on the number of clothes we can wear indoors and still move, and the family are complaining of chilblains. But compare this with our wildlife without resource to heaters and extra layers, with food covered in snow and water frozen. Within an hour of putting out warm steaming water this morning for the birds, enjoyed quickly by three eager robins, it was frozen. I guess many birds have departed west where it has been a little warmer, but those that remain have to be ever more resourceful.
The Bramley apple tree in our garden has had a bumper crop this year and we have struggled to keep up with the windfalls, despite consuming quantities of stewed apple, apple snow, apple crumble and apple pie. This has left a lot of half bruised, half rotten apples scattered under the tree - until the cold snap, these appeared of little interest to the birds. However in the last few days redwing and blue tits, but mainly blackbirds, have been gorging on them with increasing squabbling over the best ones. This gave me the opportunity to look more closely at the Blackbird (Turdus merula), and I was struck by one male that had a particular orange/red beak with a matching eye ring. Other blackbirds had either bright yellow or brown colouration. I have to be honest and say that I had never noticed this colour variation before and so wanted to investigate why.
The colour of the blackbird’s beak and eye ring is controlled by plasma carotenoid levels. There are many studies, with widely different species, that have considered how females use such colouration to evaluate the quality of a potential mate. It would seem that more colourful males are considered more attractive - But why? Researchers, Bright, et al. (2004) proposed that larger male blackbirds may have larger territories or be better at defending territories during male-male interactions, ensuring access to carotenoid food sources. So, a more colourful beak would equal a more successful male. More recent research however by Biard, et al. (2010), found that there was a relationship between bill colour and relative intensity of different parasitic infections, such as lice. They suggested that these could then act as colour signals for females to assess the relative condition and health of males to help make mating choices.
Now it just so happens that apples have a particularly high concentration of carotenoids. I wonder therefore if it is likely that the males most successful at eating our Bramley’s will be more likely to catch the eye of females in spring.
.....and so to re-coin a phrase, ‘the early blackbird that snatches the apple, catches the mate’.
Biard, C., Saulnier, N., Gaillard, M. and Moreau, J. (2010) Carotenoid-based bill colour is an integrative signal of multiple parasite infection in blackbird. Naturwissenschaften, 97:987–995
Bright, A., Wass, J.R., King, C.M., and Cuming, P.D (2004) Bill colour and correlates of male quality in blackbirds: an analysis using canonical ordination. Behaviour Processes, 27; 65(2):123-32.