parental investment: blue tit

Blue tits are common birds in many parts of the world. Here’s one:

Apart from having a fun name, blue tits are interesting for a number of reasons:

(1) Unlike peafowl, they co-parent. That’s true of about 90% of bird species, it turns out. The female incubates the eggs – big clutches, up to 16 eggs – while the male feeds her, and then both parents feed the young when they hatch. However, this split of responsibilities results in a metabolic game where the female benefits from synchronous hatching (all chicks born together = greater feeding effort by Daddy) and the male benefits from asynchronous hatching (more incubation effort by Momma, less stressful provisioning by Daddy). The females, as the incubators, typically win this game.

(2) They appear monomorphic (i.e., the males and females look alike) to our eyes, as we might expect in a species where both males and females invest in parenting (see peafowl post for a fuller explanation). But they can see ultraviolet light, and it turns out that the blue crown reflects UV light, and females favor males with highly reflective crowns. (Imagine if men had glow-in-the-dark penises, and the brighter it glowed, the better he was at cunnilingus, and you begin to get the idea.)

(3) Here’s the really interesting bit: the UV reflectance trait might not be primarily selected for by sexual selection, but rather secondarily selected for; Momma feeds her highly reflectant nestlings more than the lesser reflectant ones; in an environment of intense sibling competition such reflectance means that the more you glow in your mother’s eyes, the better you’ll get fed by her. Interestingly, Daddies are not significantly affected by this.  So the female preference for reflectant males might be a byproduct of her preference for reflectant offspring! Neat, eh?

And I’ll be honest with you: I learned all of the above pretty much just so I could post this video, which the animators set in a blue tit birdhouse, but which is otherwise unrelated. I thought it was too funny not to find some excuse to post it: