Remember the Gouldian Finch? I wrote about it a few months ago. Sarah Pryke, a behavioral ecologist at Macquarie University in Sydney, Australia has conducted some amazing research showing that female Gouldian Finches can control the sex ratio of their broods.

Now, there’s no denying that Gouldian finches are weird; males and females come in three head-color morphs, and this “sex ratio manipulation” comes into play only when a female is paired with a male bearing a different head color than her own – a situation in which the average fitness of male and female “mixed-morph” offspring differs dramatically. So, given all the peculiarities of the Gouldian Finch, is the manipulation of offspring sex ratio widespread among more “normal” birds?

A good strategy to address this question is to look for other situations in which the expected fitness of male and female offspring differ. Lincoln’s Sparrows, somber-toned residents of high-altitude and high-latitude breeding grounds in North America, provide just such an example. Male Lincoln’s Sparrows compete for mates using elaborate songs, and producing a high-quality song requires a certain bill shape. A deep, narrow bill allows males to produce a sexier song than does a wide, shallow bill.

As it turns out, the shape of a Lincoln’s Sparrow’s bill is related to its hatching date (the exact reason for this odd relationship is unclear). Chicks that hatch later in the season tend to have shallower, wider bills than those hatching early in the season. This holds true for both males and females, but females don’t need to sing to impress potential mates, so only late-hatching males suffer the cost of this seasonal shift in bill shape – when these males reach adulthood, their songs will be less attractive than those of earlier-hatching males.

Emily Graham and colleagues at UNC-Chapel Hill tested the hypothesis that Lincoln’s Sparrow parents manipulate the sex ratios of their broods based on their laying date. Later in the season, they predicted, broods ought to contain more female chicks. In a wild population of Lincoln’s Sparrows in Colorado, Graham and colleagues observed 35 nests and used genetic markers specific to the avian sex-determining chromosomes to identify the sex of each chick.

Sure enough, the later-laid clutches contained more female offspring. In fact, over a period of just 19 days from the earliest-laid to latest-laid clutches, the proportion of males in each brood declined from about 0.8 to about 0.4. This means that there were about twice as many male chicks per brood early in the breeding season as there were later in the season!

This study adds to the growing body of evidence that parents invest selectively in offspring of different sexes – they invest preferentially in male offspring when males are likely to have higher fitness, and in females when female offspring are likely to have higher fitness. The marked seasonality of sex-ratio bias in Lincoln’s Sparrows is an interesting wrinkle on this story. Seasonal variation in sex ratio is a clever solution to an evolutionary problem: what do you do when the complex relationship between the environment and the phenotype (i.e. the external, observable characteristics of an organism) causes a transient, but predictable difference in the fitness of male and female offspring?

What if the fitness of human males and females could be predicted by their date of birth? Loads of people (about a third of Americans, it turns out) believe that astrology is at least “sort of scientific,” so let’s all suspend our disbelief for a moment and imagine that people really have personality traits that can be predicted by the date of their birth. Some of these traits might affect reproductive success, and perhaps not equally for both sexes. For example, I am a Pisces. According to the Internet, this automatically qualifies me as “mysterious.” Does this characteristic make me irresistible to the opposite sex? Perhaps so.

But while my exaggerated level of mysteriousness makes me dreamy, a comparably mysterious woman might not gain the same reproductive advantage – maybe men don’t like mystery. In this case, natural selection would favor humans who tended to produce sons from February 19-March 20. You could test this hypothesis, as Graham and colleagues did, by examining hospital records and asking whether birth date predicted sex as predicted.

Of course, that test would be futile, because astrology is bogus. Sorry, 33% of Americans!

Literature cited:

Graham, E., Caro, S., & Sockman, K. (2011). Change in offspring sex ratio over a very short season in Lincoln’s Sparrows: the potential role of bill development Journal of Field Ornithology, 82 (1), 44-51 DOI: 10.1111/j.1557-9263.2010.00306.x