As with many other bird species, the male, here on the left, is the more colorful when house finch couples are compared.

Courtesy of Alex Badyaev / university of arizona

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Babies!

Blood-sucking attackers!

Mothers to the rescue!

It's a fascinating story, and it's probably coming to a yard near you every spring, if you live nearly anywhere in Tucson, says a University of Arizona researcher.

The baby birds are not particularly adorable, with those huge, bulging eyes.

The blood-sucking fiends are just tiny mites, doing their disgusting parasitic thing.

But the mother house finches indeed are doing something remarkable, says Alexander Badyaev, a UA assistant professor of ecology and evolutionary biology.

What we wouldn't see, even if we could see the tiny mites feasting on the baby birds, is that the mother house finches have somehow reprogrammed their bodies to lay and hatch female offspring before males, giving the more vulnerable males less exposure time to the blood-sucking bugs.

The mechanism for setting the male and female hatching order is probably hormonal, says Badyaev, most likely triggered by "some external cue, such as ambient temperature or mite infestation."

He said the researchers detected a change in the hormone levels of the mother and yolks of eggs produced during mite infestation and showed that the hormones have an effect in determining gender.

Badyaev and fourth-year Ph.D. student Kevin Oh say the little house finches so common in Tucson — small brown birds, males having red breasts — offer an almost perfect experiment.

The birds nest twice a year, first between February and March and later between April and May. For reasons Badyaev says have to do with mite biology, the bloodsuckers are not present in the first nesting period, but are there in the second.

In the first period, they say, the male/female birth order is random. Typically, there are five eggs in a house finch clutch, and odds are roughly 50/50 whether a male or female will be born first, second, third, fourth or fifth.

But, female offspring nearly always move to the front of the hatching order when the mites are present for the second nesting.

"That's what's really unique about our study," says Oh. "Sex-biased hatching order in birds have been documented recently in lots of species. When we first looked, it appeared to be this unbiased order. But when we found the parasites were present, sex order became heavily biased."

Even more remarkably, the males — despite fewer days to grow outside the shell before they are all kicked out of the nest — will be at virtually the same stage of viability and maturity as the females, say Badyaev and Oh.

Since all the offspring will "graduate" from the nest at the same time, the males will have been exposed to less time with the weakening effect of the mites. The few males hatched first during the parasite infestation, Oh say, invariably die.

After that, says Oh, male and female siblings will have an even shot at living — at least until the males engage in the ornithological equivalent of teenage driving, war and heart disease.

Testosterone, in birds as in humans, may be just the thing for surviving short-term dangerous situations, but seems to increase vulnerability in other ways.

The researchers say females' higher resistance to disease is a fairly common animal trait, not just in finches.

There's no shortage of finches to study right on campus, says Badyaev. He estimates there are 12,000 house finches living at least part of their lives on campus, of which roughly 8,000 have been banded by Badyaev and his flock of students, graduate assistants and techs since 2002.

Their work with co-authors Terri L. Hamstra and Dana A. Acevedo Seaman, with the page-turning title of "Sex-biased Maternal Effects Reduce Ectoparasite-induced Mortality in a Passerine Bird," is published in the September edition of the Proceedings of the National Academy of Science.

On StarNet: Read more wildlife stories at azstarnet.com/wildlife

As with many other bird species, the male is the more colorful when house finch couples are compared.