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Flycatchers use early birds (great tits) to crowd-source clutch size

A bird that tries to raise too many chicks this year may not survive to reproduce again. So there's a tradeoff between current and future reproduction. Earlier, I discussed how stress hormones over-worked mothers pass to their chicks decrease survival of this year's chicks, whereas the reduced work-load can increase the mother's life-long reproduction. But how many chicks is too many? It depends on food supply and other conditions that vary from year to year.

Rather than researching conditions themselves, flycatchers could wait to see how many eggs other flycatchers lay. This is the approach many humans use to decide how much to invest in stocks, but it doesn't work any better for the flycatchers than it does for us. By the time they figure out what others are doing, it's too late to take advantage of the information.

An alternative crowd-sourcing approach is to see how many eggs were laid by another species of bird that finishes egg-laying a bit earlier. Indeed, Jukka Forsman and colleagues report, in a recent paper in Biology Letters, that "Observed heterospecific clutch size can affect offspring investment decisions."

They placed simulated great-tit nests, containing either 4 or 13 imitation eggs, near likely nesting sites for flycatchers. Older flycatchers near 4-egg fake nests laid an average of 10.4 grams of eggs, while those near 13-egg nests laid 11.5 grams of eggs, about 10% more. The increase was partly due to laying more eggs and partly due to laying heavier eggs. Younger know-it-alls were less influenced by the imitation nests.

The direction of the change in investment in current-year reproduction is consistent with the hypothesis that flycatchers rely on clutch size of great tits as one measure of their own optimal clutch size for this year's conditions. But a 10% change seems too small a response to 13 eggs versus 4, assuming the great tits know what they're doing. Then again, conditions change over the season, so relying 100% on what an earlier species did would be risky.

Also, this year's conditions aren't all that matters. I have explained earlier that, to maximize one's proportional genetic representation in future generations, it can be better to produce one offspring this year than two next year (if the population is increasing), but it can also be better to produce one offspring next year than two this year (if the population is decreasing). So, ideally, bird decisions about reproductive investment should be based on predicted population trends, not just maximizing the lifetime number of chicks fledged.

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