For the Birds Radio Program: Baby Bird Biology, Part I
Laura talks about the biology of baby birds.
(Recording of a Mallard)
In June, eggs are bustin’ out all over. Ordinary eggs crack open to reveal baby eagles, egrets, and Eastern Bluebirds. The miracle of new life is even more awe-inspiring when you understand the biology of the newly-hatched bird.
Baby birds are classified into 2 broad categories. Precocial birds are those that hatch with their eyes open and their bodies covered with soft down. These birds are ready to leave their nests within 2 days, and sometimes within hours of hatching. Baby ducks and chicks are examples of precocial birds.
Altricial birds are those that hatch with their eyes closed and their bodies pretty much naked. Where precocial birds are fluffy and cute, altricial birds are the kind of babies only a mother could love. They’re utterly helpless at first, capable only of opening their mouths and begging for food, and must remain in the nest for weeks or sometimes even months.
The kind of babies a species has depends on the habitat and on the species’ food requirements. Precocial birds tend to be those that nest in marshes or near inland waters where the large quantity of predators would pose a threat. Also, even in upland habitats, the heavier a ground nester is, the more likely it is to have precocial young—probably in part because heavy birds need enough landing space that they cannot alight on or right next to their nests. If parent grouse or pheasants were to continually be running back to the nest to feed their young, they would certainly wear down a revealing path that predators would follow. For these species, evolution has favored larger eggs, allowing the babies to complete much of their development before hatching, while the mother sits quietly on the nest incubating, and only occasionally getting up to feed herself. She pays a physiological price for laying such rich and heavy eggs. Smaller ground nesters like sparrows and warblers that can effectively hide their babies from predators for a few weeks bear altricial young.
Altricial young include all our songbirds, herons, hawks, eagles, and owls. The larger species among these, like Great Blue Herons and eagles, nest in trees or on inaccessible cliffs, and so their young are not as vulnerable to predation as helpless baby ducks or pheasants would be. Egg production is less taxing for the mother, since altricial birds don’t develop as much in the egg, and don’t need as much nourishment, but as a tradeoff, once the eggs hatch, the parents of altricial young must work incredibly hard to feed their nestlings for weeks or even months.
Altricial young don’t leave the nest until they are feathered out, have reached their adult body weight, and are almost ready to fly. Many warblers reach this point in 8-10 days. It takes about 17 days for Blue Jays to leave the nest, 35 days for Great Horned Owls, 2 months for Great Blue Herons, and 70-98 days, or about 3 months, for Bald Eagles. Once they leave the nest, both precocial and altricial birds remain with their parents until they can manage on their own. In songbirds this may be for only a few days or weeks, but many larger birds remain together until fall migration, or even throughout the winter.
The miracle of baby birds is one you can watch in your own backyard. If you have an interesting experience with a baby bird this summer, drop us a line.
(Recording of a Mallard)
This is Laura Erickson, and this program has been “For the Birds.”