Biology Reference
In-Depth Information
too, feasting on the insects churned up by the hooves of the large grazers drifting through the
grass. When logging and agricultural development began in earnest in Michigan in the eight-
eenth and nineteenth centuries, cowbirds followed, later spurred perhaps by the sharp decline
of the bison in the 1880s.
How has the Kirtland's warbler held up under this home invasion by cowbirds, whose
chicks hatch first and outcompete the natural nestlings for parental attention? Why has this
invasion greatly affected rare species such as the Kirtland's yet abundant species hardly at
all? To the second question, the short answer seems to be that some bird species have evolved
ways to resist the nest parasites. American robin parents, for example, detect the strange eggs
and roll them out. Others may abandon their nest and lay more eggs elsewhere or build an-
other nest on top of the cowbird egg. Kirtland's warblers lack such behaviors, perhaps be-
cause, compared with these other birds, they have not yet had the time to evolve better egg re-
cognition. Or the small size of the Kirtland's population may have reduced genetic variation
available for selection of the defensive behaviors that have evolved in species with much lar-
ger populations and gene pools. Or both. In any case, the impact on Kirtland's reproduction
grew to alarming levels. At its nadir in the early 1970s, less than one-third of warbler nests in
the Grayling population produced any young, largely attributable to the influence of cowbird
parasitism.
In 1972, the US Fish and Wildlife Service, along with other federal and state agencies,
began controlling cowbirds with large live traps, such as the one we'd seen, placed in Kirt-
land's warbler nesting areas during spring and early summer. Wildlife technicians check the
cages daily and euthanize any trapped cowbirds—an average of 4,000 per year.
“No one wants to kill cowbirds, but the trapping has worked a small miracle,” Sarah Rock-
well related. Nest parasitism rates had dropped sharply, from 69 percent in the late 1960s,
before trapping began, to less than 5 percent. Even more inspiring, average clutch size had
increased from 2.3 eggs per nest to more than 4, and the average number of young Kirtland's
warblers fledged per nest increased from fewer than 1 to almost 3 birds during the same peri-
od. Cowbird rustling is an expensive task, and euthanization seems gruesome to some. But
for the attending biologists, watching the last few Kirtland's warblers unwittingly feed super-
sized cowbird chicks seemed at first surreal and then shameful. Cowbirds have their right to
live, too, but at what point do we intervene when their hardwired behavior threatens to drive
other species to extinction?
Over coffee with Sarah, I posed a hypothetical question about the Kirtland's future: “If
half of northern Michigan were turned into a jack pine preserve and immature stands were
increased by a factor of ten, through either fire or management, would we still need cowbird
control?”
“Well,” Sarah replied, “what you need to answer your question are data from the last time
we had reproductive success information collected before cowbird control.” Larry Walkin-
