Prettier tits (the bird!) get more help from their partners

After a long, cold winter, nothing says spring like the hopeful songs and dances of horny male birds looking for mates. Throughout Europe and western Asia, the blue tit is one of the most colorful birds to engage in this annual hormone-driven spectacle. The males bring their A game, flitting about, singing beautiful songs, and offering gifts, trying everything in their power to convince their potential mates they are the best man around. One thing is for certain when it comes to blue tit love: it?s ladies? choice. But, as a new study published today in Frontiers in Zoology found, the guys do have minds of their own: they?re better dads when they?ve landed an attractive mate.

While blue tit males will do their best to impress females, the females still rely heavily on looks when making their decisions. Male blue tits are ornamented with brilliant blue feathers that shine brightly in the UV range, while the girls? feathers are much duller. This difference isn?t meaningless; female tits strongly prefer males with the brightest UV crests. But not only does the guy?s looks matter in courtship: previous research has shown that if you dampen a male tit?s UV coloration after his chicks are born, his lovely mate will be derelict in her motherly duties, leading to weaker offspring.

Why should looks matter after the kids have been born? Well, from an evolutionary perspective, animals are attracted to individuals that make the best mates. Thus, in turn, attractiveness is a basic assessment of mate quality (though, certainly, other factors carry weight, too). Over a female tit?s life, she may mate with a number of different males that vary in their attractiveness. If the most attractive one she ever mates with is the healthiest, or the one with best genes, or in whatever way produces the best kids, it?s worth her while to make sure that any babies she makes with him are given the best odds of surviving ? which would mean putting more effort in to caring for her young when her partner is sexy, and less when he?s just so-so. This change in effort based on mate quality is known as the Differential Allocation Hypothesis (DAH).

Since the female tits are making the decisions, you might think their looks aren?t as important. But once the babies are born, both parents shoulder the burden of caring for their young ? and there?s reason to believe the guy?s parental care efforts may contribute more toward baby bird survival. While the female tits spend more time tidying their nest, evidence suggests that when it comes to bringing home the bacon, male blue tits bring in more food ? and specifically more high quality food ? than their mates. Furthermore, hungry baby tits beg dad for more instead of mom, suggesting that the young instinctively trust their father to feed them when times get rough. Which begs the question: do males slack on their fatherly duties if their mate isn?t pretty? That is exactly what Katharina Mahr and her colleagues at the Konrad Lorenz Institute of Ethology wanted to know.

To test the Differential Allocation Hypothesis, the research team took female blue tits and used UV-blocking chemicals in duck gland oil to dull their pretty color. On others, they placed the same oil, but no blocking chemicals, so their plumage still shone brightly. The UV-blocking chemicals didn?t alter the females behavior in any way, only made them look less ornate to their mates. So how did the males react?

While all males protected their mate and chicks with equal fervor, the males with the less attractive mates made significantly less foraging trips to feed their chicks. Less food means the young are not as strong, healthy and competitive as others, lessening their chance of surviving and reproducing themselves. ?The UV reflectance of the crown plumage of female blue tits significantly affected male investment in feeding nestlings,? explain the authors. This decreased parental investment wasn?t compensated for by the female, and thus the chicks are directly and negatively affected.

?This is the first study to show that male blue tit behavior depends on female ornamentation,? said Matteo Griggio, co-author of this study, in the press release. The male tits are likely using attractiveness as a measure of the health of their mate. ?Females in bad condition might not be able to provide sufficient parental care, which in turn affects nestling body mass and growth [of the young],? explain the authors. Since getting food for chicks costs the male both food and energy, the male can?t afford to waste his time feeding chicks that might not make it. Instead, he cuts his losses without completely sacrificing his young, and keeps himself healthy and strong for the next set of chicks that will hopefully be with more suitable mate.

Of course, it?s hard to resist the temptation to draw human parallels. After all, blue tits are considered monogamous, though they cheat on their partners and divorce bad matches like we do. However, no evidence for DAH in people has ever been presented, and designing such an experiment would be extremely difficult. Unlike many animals, though, humans are remarkable parents even in extreme biological circumstances. Adopted children and stepchildren receive a lot of parental care from their non-biological parents, for example. It?s unlikely that this kind of differential allocation plays a large role in human parenting. That said, this study of tits does make you wonder?

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Citation: Katharina Mahr, Matteo Griggio, Michela Granatiero and Herbert Hoi. Female attractiveness affects paternal investment: experimental evidence for male differential allocation in blue tits. Frontiers in Zoology (in press)

Photo of blut tits ?kissing? from Wikimedia commons.
Photo of an adult blue tit feeding its young by David Friel via Flikr.

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