Biology Reference
In-Depth Information
however, and the forest stands age, Kirtland's warblers are forced to pick up stakes and move
to the next patch or perish.
The firebird needs fire, and lots of it, but in the right places and at timely intervals—or, in
its absence, carefully managed harvesting in jack pine stands to mimic the effects of fire. Yet,
as I soon learned, there is more to the story of the Kirtland's rarity. Even if careless campers
or closet arsonists were to set the sandy areas of northern Michigan ablaze—leading to more
habitat—the species would still face an even greater threat.
On my second day in the grove, a new birdsong filled the warming air. It wasn't the explosive
melody of the Kirtland's but a more metallic and bubbly offering. The members of my bird-
ing party noticed that the songs grew louder until we found the source. Hidden behind a natur-
al screen of three-meter-tall jack pines sat a large cage containing six pairs of brown-headed
cowbirds. On top was the entrance, accessible to birds wishing to enter and join the cowbirds
inside, who were a most sociable lot. These, however, were bait birds, kept alive to attract
others of their kind and leave the Kirtland's warbler in peace. For the unsuspecting cowbirds
that entered this cage, there was no exit.
More than 100 species of North American songbirds, but especially breeding pairs of
warblers, thrushes, and vireos, suffer from nest parasitism by brown-headed cowbirds. Kirt-
land's warblers are an especially easy mark for them. The deed is quick, the effects long last-
ing. When a female Kirtland's begins to lay a clutch of brown-and-white-splotched eggs, she
steps briefly off the nest to feed. By the time she returns, a female cowbird, likely having
cased the nest from a lookout post, will have visited and deposited an egg or two of her own,
typically removing a host egg in the process. The cowbird's chick hatches a day or more be-
fore its adopted nest mates and grows more quickly. The adults fail to discriminate against
the monster chick in their midst. They keep feeding the imposter, whose larger size means it
gets more parental attention than the warblers' own young. Sometimes the more aggressive
cowbird chick will push the rival offspring out of the nest.
For the poor Kirtland's warblers, much depends on how many eggs the cowbird lays in
their nest; if only one, the pair have a chance to raise some of their own. If there are two cow-
birds, none of the Kirtland's chicks survive to fledging. Unfortunately, female cowbirds are
prolific egg layers. Over the May to July breeding season, one individual could conceivably
put one egg in each of forty nests. This behavior may sound like the handiwork of an exotic
species, able to exploit naive native birds, but while the brown-headed cowbird is a relatively
recent arrival to Michigan, it is no foreigner. It is a North American species native to the
South and West, where it was once known as the “buffalo bird.” When hunters killed off the
bison and ranchers replaced them with cattle, the cowbirds followed the domestic livestock
