From egg to adulthood in weeks

Fast-paced growth keeps birds from being eaten in wild

Humor me for just a moment and consider the following scenario. A child is born on New Year's Day. Congratulations. Valentine's Day finds the boy topping 100 pounds.

So much for the stroller.

On April Fool's Day of the same year the child, now a strapping lad of 160 pounds, leaves home to start a life of his own. Three months from birth to adult size! Think about the food bill. And how do you keep a child like that in clothes?

Ridiculous you say, but this is how it works in the bird world.

The growth rate of our hypothetical hero reflects the growth of the golden eagle. From hatching to full size takes about 12 weeks. When a young bird takes its first flight from the nest, it weighs as much as mom and dad.

The growth rate is even more impressive for smaller birds, such as horned larks and Wilson's warblers, which spend only nine to 11 days in the nest. That's egg to adult size in less than two weeks.

The rapid growth of baby birds is something we often fail to appreciate. Almost every year someone asks me about a young "great horned owl" in the backyard. It's usually a full-grown western screech owl. The conclusion that a western screech owl might be a young great horned owl is understandable given humans' more leisurely approach to growth.

For some insight as to why rapid development is important to nesting birds, especially small songbirds, visit New Zealand, where native birds have had some challenges.

First the Maoris arrived and ate 11 species of moas to extinction. Moas were large, flightless and apparently tasty birds.

Next came the Europeans, who brought with them a range of predators, including cats, dogs, two kinds of rats, stoats (weasels), ferrets and others, plus a host of plants and some backyard birds to remind them of home.

New Zealand, being a remote set of islands, had no native land mammals apart from a couple of bats. The native land birds, in the absence of predators, gave up the frenetic pace of development observed elsewhere. Young typically spend a leisurely 17 to 21 days in the nest, nearly twice as long as similar birds from Europe. This means the period of vulnerability to nest predators is nearly twice as long, and this has huge consequences.

Predation from introduced rats and stoats decimated the populations of many New Zealand land birds. In contrast, the introduced backyard birds from Europe, with their rapid development, are doing fine. They suffer nest predation, too, but the period of high vulnerability is a lot shorter, which keeps losses to an acceptable level.

It's a different story for birds that nest in the relative safety of a tree cavity or nest box, both here and elsewhere in the world. Crows and jays can't penetrate the defenses, and most mammal predators are stymied. In this relatively secure setting small birds typically take closer to three weeks to rear their young. They even may add an egg or two to the clutch for good measure.

Stewart Janes is a biology professor at Southern Oregon University. He can be reached at janes@sou.edu.


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