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BIRDWISE: Unique bird adaptations that improve their reproduction

Local bird expert Marshall Howe explains some of the interesting adaptations birds have developed to maximize their reproductive success.

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Barn owls exhibit asynchronous hatching, so their young are all different ages.
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Unless you’ve read and remembered most of my columns over the years or have taken a course in ornithology, you may not be aware of some of the interesting adaptations birds have developed to maximize their reproductive success.

Here are a few examples:

Did you know that, depending on the species, hatchlings are classified as either precocial or altricial? These are the two fundamental types of development in birds.

Precocial young, like ducks and grouse, are named such because they are sufficiently developed at hatching to be able to leave the nest within a day and feed on their own (though they do remain dependent on their parent or parents for keeping warm and safe from predators). They also are not capable of flight for several weeks.

Altricial species, on the other hand, like songbirds, are very poorly developed at hatching and require an extended period of parental care, during which their bodies and feathers mature. By the time they leave the nest (fledge) they are within just a few days of being able to survive on their own.

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Did you know that most songbird eggs within a clutch hatch at about the same time (synchronous hatching) even though the eggs are laid a day apart? This is because the parent doesn’t begin incubating the eggs until the last or next-to-last egg is laid. So the embryos begin developing at about the same time.

As a result, the chicks are equally competitive when the parents provide their food and are equally developed when it’s time to leave the nest.

In certain other species, however, like hawks and owls, incubation begins as soon as the first egg is laid. This results in chicks hatching a day or so apart (asynchronous hatching), so that the brood is composed of different aged young. This is believed to be an adaptation to uncertainty about the availability of their food prey.

Songbirds feed their young insects, which are normally abundant. But raptors are dependent on rodents for the most part, and rodent populations are prone to significant fluctuations from year to year.

By staggering the hatching sequence, raptors are able to raise all of their young in years of rodent abundance. But in poor rodent years, the older chicks, having an advantage over their younger siblings, will get the majority of the food provided and survive, while the younger chicks perish. (Yes, the real world is not for the faint-hearted).

Did you know that adult birds that incubate eggs develop a brood patch? The feathers on the abdomen where the eggs are contacted are shed and the bare patch becomes highly vascularized. This new network of blood vessels at the surface provides the heat necessary to enable the embryos in the eggs to develop as rapidly as possible.

Did you know that birds that nest in holes in trees (or boxes) remain longer in the nest than birds that have exposed nests? It is believed that this is a consequence of the extra safety from predators afforded to hole-nesters.

Exposed nests are highly vulnerable to predation, so the sooner the young can fledge, the more likely it is that they will survive. The safety afforded by hole nests makes it possible to delay fledging until the young are more fully developed and therefore better able to survive.

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I hope you’ve learned a thing or two from these examples and are beginning to understand how fascinating birds are!

Marshall Howe is a retired biologist with the U.S. Fish and Wildlife Service. He specialized in bird population studies. Howe has been a Park Rapids resident since 2010.

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