A pile of Turdus

There seem to be a lot of male blackbirds. Maybe four or five and two females. I started to wonder if this was chance or if there was something else at play.

Turns out that various things impact the preponderance of one sex or the other in birds.

A study of Turdus merula, the European blackbird concludes

In our population of Blackbirds, the difference in the parents’ age could affect offspring sex, and paternal age could influence offspring fitness. Younger females mated with older males tended to produce more sons because of the greater fitness of male descendants with “good genes”—the sons of older fathers enjoyed a higher breeding success. In contrast, older females mated with younger males produced more daughters: this could have been due to the lesser attractiveness of a younger male and the poorer condition of an old mother.

Hmmm... and maybe that is a pile of shite. I find it to be a questionable hypothesis that females are, as it were, the cheap choice. Maybe older females are better at mothering and so more of the females survive, in which case females could be more costly? And none of this considers the fact that many paired females have sex with other neighbouring males so that their children have different fathers anyway. Another paper tells a completely different story:

In years 2006–2007 secondary sex ratio of blackbird nestlings in Stefan Żeromski park in Szczecin was studied. 190 nestlings were sexed by genetic analysis. Female: male ratio was found to be 1 : 0,94. No relationships were found between hatching date and sex of nestling and female age and sex of offspring.

This second paper did consider just one year, maybe the first one took a longer period. If seems that (female) birds can control the sex of their offspring... this article suggests in poor conditions birds produce more females, again suggesting that females are less costly to produce - but what if they produce more females to maximise the chances of their being another generation as you actually don't need that many males. I mean, if an area can only support 10 birds, you only need three of them to be male for most of the females to get laid. To lay eggs. Whatever. Though, of course, they need males to help with the feeding... so that doesn't work. But maybe in poor conditions, the role of females choosing mates is more important: ie only the strongest of the strong females lay eggs.

Or, maybe there are many males around me because there was plenty of food (thank you bird food suppliers) and EXTRA males survived, so all the females can be sure of excellent mates, and excellent alternative lays.

Anyway. They are very handsome.