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BIRDWISE: Nest location is important

The northern cardinal builds a cup nest in a dense shrub or low tree. (Stock photo)1 / 2
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No matter how much we may be tempted to interpret bird behavior in human terms, birds are little more than feathered machines powered by hard-wired brain circuits. Except for a few species with more advanced brain development, like crows and ravens, learning plays only a very small role. Most everything they do is purely instinctive, including their "decisions" on nest type and location.

The consequences of nest placement over millions of years of bird evolution are quite profound and have influenced other aspects of birds' reproductive biology in interesting ways.

Closely related families of birds tend to have similar ways of building and locating their nest sites. For example, nests of most warblers, thrushes and vireos are open cups placed on the ground or in bushes or trees. The nest contents of these species are visible to potential predators from above.

Other common songbirds (e.g., chickadees, nuthatches, house wrens, tree swallows, purple martins) place their nests in holes in trees or in nest boxes. Their nests are not readily visible to many predators.

Studies have shown that predation rates on nests in the first category are much higher than predation rates on nests in holes.

This variation in vulnerability is reflected in slightly different incubation periods (time from complete egg-laying to hatching) and fledging periods (time from hatch to leaving the nest) between open-nesters and hole-nesters.

For the vulnerable open-nesters, it's important to get nesting over with as quickly as possible. Hence, their incubation and fledging periods are relatively short.

Hole-nesters, being relatively safe during nesting, have evolved slightly longer incubation periods and considerably longer fledging periods, resulting in the young being more fully developed when they leave the nest.

I am occasionally asked about the incubation and fledging periods of birds. There is no simple answer, and even within the categories of open nesting and hole-nesting there is variation. The chart shows some examples.

There is also great variation among species in the length of time that the young continue to be dependent on their parents after fledging. But that's another story.

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