Compost-Pile Nests

The bird nest underground

Australian Brush-turkey by Marie Read (2)  marieread.com

Of all the bird nests in the world, the huge mounds constructed by “megapodes” such as the Australian Brush-turkey are perhaps the most unusual. Male brush-turkeys use their oversized feet (“megapodes”) to scratch together a pile of dead leaves, branches, and other fallen vegetation. Even small brush-turkey mounds require a lot of labor to construct; large mounds, those 12 feet in diameter and 3 feet high, require a Herculean effort.

Brush-turkey mounds appear to be only messy piles of old leaves, but they function like sophisticated egg incubators. Just as in a garden compost pile, the decomposing vegetation inside a brush-turkey mound gives off heat, often raising the temperature inside to 98° F or higher.

Every few days the female brush-turkey visits a mound, digs a hole, lays an egg, covers it over, and leaves. Incubation proceeds without any further attention from her. Weeks later the newly hatched brush-turkey digs itself out of its birth-mound and runs off into the nearby forest, already on its own.

Research published in 2007 by Australia-based scientist Ann Göth (Auk 124: 253?263) helps explain why male Australian Brush-turkeys go to all this mound-building effort. Göth’s team carefully dissected many brush-turkey mounds, counted and measured the eggs they contained, and took a temperature reading from each egg’s subterranean location. Some mounds are substantially warmer than others, and some have more constant temperatures. The best incubation environment for brush-turkey eggs is a stable temperature of 90-95° F, and mounds in this range contained the most eggs. These particularly good mounds also contained, on average, larger eggs, indicating that the highest-quality brush-turkey females were gaining access to the best-quality incubation mounds.

Surprisingly, the overall size of the mound does not much matter: a small mound that contains lots of fresh, rapidly rotting vegetation may be much warmer inside than a massive but older mound with a base of already-decomposed material. Studies in the 1990s by Sharon Birks, then a Cornell graduate student, showed that female brush-turkeys frequently visit mounds even when they are not laying eggs. Presumably these visiting females are comparing different mounds to see which offer the best incubation environment.

Male brush-turkeys have a strong incentive to attract females to their own mound: before a male allows any female to lay an egg in his mound, he exacts a “toll” in the form of a mating event. Female brush-turkeys usually lay eggs in many mounds over the course of a breeding season; the more visits a male receives at his own mound, the more young brush-turkeys he will sire overall. Female brush-turkeys appear to care much more about the quality of a male’s mound than about his physical traits. Building and maintaining an optimally rotting mound is a male’s best means of fertilizing lots of eggs.

—Irby Lovette, director, Fuller Evolutionary Biology Program

For permission to reprint all or part of this article, please contact Laura Erickson, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-1114. email: lle24@cornell.edu