About the birds and the fish - bees are another story

Posted: Sunday, April 29, 2007

The robins are back. Soon they'll be nesting up in Sheep Creek valley, on the moraines near the Mendenhall Glacier, and in our backyards.

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In a few weeks, every pair will be feeding nestfuls of loudly begging chicks. Robins are good parents, and both adults actively care for the chicks.

Chances are, however, that the male may not be the father of them all. He may even have fathered offspring with other females, and those chicks are being fed by another male.

Robins, like most songbirds, are apparently monogamous, and males usually help rear the chicks in the nest. However, studies have now shown than the apparent monogamy in many species conceals a much more lively program.

Yes, there's plenty of hanky-panky in birdville.

It turns out that males cruise around in search of opportunities to mate with neighboring females when their mates are not looking. The females are cruising too, visiting neighboring males on the sly.

That means many nests contain chicks fathered by more than one male. Thus, an apparently monogamous male often raises chicks that are not his own, while some of his offspring are reared by neighboring pairs.

Success of males in finding a mate and in clandestine couplings can be related to certain male attributes.

The common yellowthroat, a marsh nester sometimes found in the brushy meadow on the Back Loop and in the big marsh along the Amalga Trail near Eagle River, provides a good example.

Males have a black mask across the face, and males with big black masks are more attractive to females than males with smaller masks. As a result, big-masked males not only have better pairing success but also better success in copulating.

Birds, like people, aren't the only species that fool around. The so-called alternative mating strategy is well known in certain fish.

Pacific salmon males vary in size more than females, and males of three species (chinook, coho, and sockeye) also vary in their mating strategies.

Typical, 'standard' males go to sea for several years, achieving large body size and developing hooknoses, toothy jaws, and deep bodies in the spawning season. They compete for females in the spawning stream, and large males are usually dominant over smaller ones.

However, some salmon known as 'jacks' spend only one year at sea and return to the spawning grounds at a smaller size than typical males.

Some populations of chinooks also have individuals that don't go to sea at all and become sexually mature at very small sizes without the typical adult breeding coloration.

These alternative male types don't spend much energy fighting. Instead they produce more sperm relative to their body size than standard hooknose males.

Alternative male types achieve their reproductive success by sneaking. Standard males expend lots of energy fighting other males and courting the females, and they release their sperm as the females release their eggs.

That's when the sneakers streak in and emit a cloud of sperm, which will fertilize some of the eggs. In some cases, these sneaker males are very successful.

Although female salmon prefer large, standard males, their opportunities for mate choice are small when sneakers are present, because the sneakers move in so quickly.

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.



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