Ecology is a beautiful thing. Organismal interactions are multifaceted to a degree very few, if any, of us can fully understand. Luckily, some are simple and interesting enough that even an intern can write about them during rain delays from fieldwork. Most folks that know anything about Kirtland's warblers know first about their strong dependence on Jack Pine trees. The next ecologically-relevant factoid most recollect is their battle against the dreaded cowbirds. Since we study birds here, and not trees, I'm opting to write about the latter.
Brown-headed cowbirds are nest parasites—they lay their eggs in the nests of other birds. While this behavior isn't unique among North American birds, cowbirds are our only obligate nest parasites. They never make their own nests and are entirely dependent upon other birds to provide parental care to their young.
Some ducks, coots, and others are facultative nest parasites, making nests of their own to go along with the eggs they slip into the nests of neighbors. It's an odd behavior, but perfectly reasonable viewed through the lens of natural selection. If you evolved to make use of grubs kicked up by buffalo and had to fly miles back to your nest every few minutes to keep your kids happy, you'd hire a nanny too.
You may notice a problem here. There are no bison in Michigan and bison don't use Jack Pine forests. Well, we humans have a special knack for redecorating our surroundings at the behest of our neighbors. The expansion of agriculture on this continent by European settlers brought the cowbirds knocking at the Kirtland's warblers' doors.
A healthy majority of nests were parasitized by cowbirds in the 1960s. A cowbird egg takes 12 days to hatch while a Kirtland's egg takes 14. If the baby Kirtland's warblers still manage to hatch, they're at an extreme disadvantage having to compete with the much bigger cowbird nestlings for food. Coupled with the lack of forest fires creating room for fresh Jack Pine habitat, this drove the Kirtland's warblers to near extinction. So, what was the solution?
In order to restore the Kirtland's back to safe levels, the cowbirds had to be removed, and that is accomplished with cowbird traps. How does one trap a cowbird? Well, If you put 5 or 6 male and female cowbirds in a cage, give them food and water, and let them sing, other cowbirds join the party. If you adjust the openings in the top of the cage so that they're big enough for a cowbird to squeeze through, the cowbirds fall in and have a lot more trouble finding their way out. At the very least, the new cowbirds stick around long enough for someone to check a trap each day and remove them. Basically, we use their own kind to lure them in and keep them away from our warblers.
A few thousand cowbirds are removed each year, and while that may seem like a lot, I can assure you cowbirds are doing just fine. Working with vireos last year in Massachusetts, I ran into my fair share of cowbirds, but I didn't exactly bump into many (any) Kirtland's warblers. The good news is, through roughly 50 nests we've found this year, we haven't bumped into a single cowbird egg yet.
It's easy to view this as a triumph of wildlife management and conservation, and, by all means, it certainly is, but it is important to remember how we got here—we opened the door for the cowbirds to wreak havoc. Kirtland's warblers may be off the proverbial cliff hanging over extinction, but we had to put a lot of work into saving these birds. Cowbirds are just doing what any species would presented with a fitness-oriented opportunity. To view them as the bad guy, is a tad misguided. We'd do to make sure that the wonderful players in the game of ecology, molded by generations that evolved over thousands of years, don't disappear in a flash.
On the next episode…How to radio track a fledgling and look like you're contacting aliens at the same time. Stay tuned.