Ok, maybe I’m overstating things just a bit. I’m not talking about one of those American Gladiator kinds of things. I don’t mean the pro-wrestling kind either. I’m talking about foraging competition <cue the round 1 bell>.
While walking around near the Catawba River last week, I was just sort of keeping track of things that I was seeing and hearing. I wasn’t carving the list into my hand or anything, just a mental checklist: Cardinal, Titmouse, Chickadee, Red-Shouldered Hawk, American Crow, Downy Woodpecker, Junco, Jay, Towhee, Nuthatch, Pine Warbler and Yellow-rumped Warbler. Two little warblers, hanging out in the same place? Well, I didn’t so much think that right then. Like I said, at the time I was just check-listing. But on my way to the car, I did get a good view (and a not-too-bad picture of) a Pine Warbler. Contrary to what I would have expected, though, this Pine Warbler wasn’t hanging out on the trees, it was foraging in the needles around the base of a tree. And it really was foraging, not gathering nest material. I watched the bird for about 10 minutes.
<Ding! Round 2> When I came home, I looked up some stuff on these little birds and that’s when the two warbler issue hit me. There’s been all sorts of work, historically, on competition in warblers that shows how their foraging behaviors are affected by competition (more to come on that). My first reaction to my observations was surprise at the ground-feeding. As I began to dig around (in the literature, not on the ground), I found information about an interesting interaction between the Pine Warbler and the Yellow-throated Warbler. They do seem to compete for foraging space in some places. The question, of course, is how does competition affect their use of space? There are Yellow-throated Warblers in the area, locally. And the Yellow-rumped Warblers that are saw in the same genus. Hmmm….Was the Pine Warbler on the ground because the Yellow-rumped was using the trees? There seem to be a few answers to this question (which is why it’s fun!).
<Ding! Round 3> The first answer came from a paper that noticed this overlap of species in a forest in Maryland. The authors suggested that these two species of birds actually use different parts of the habitat, because the Yellow-throated Warblers can stick their beaks into pine cones for insects, but the Pine Warblers can’t. This difference allows the resources to be partitioned among the birds.
Now this paper only presents a partial answer to the question. By looking at the two species in one place, you can see how they use space their. The follow-up question is: would they use space in the same way if they weren’t both present in the same area?
<Ding! Round 4> Morse was the one to continue asking questions about this particular situation and found that in areas where Pine Warblers lived with no Yellow-throats around, the birds still foraged in the same kinds of areas. ”hunh?’ you say. ”What does that mean?”. Well, it means that foraging patterns of these birds (how they use space) is not affected by the presence of the Yellow-throated Warblers.
These two birds may be able to co-exist in an area not because they change their behaviors when they are both present, but because they already use slightly different parts of the habitat in these forests. Cool! But wait! There’s more to this story. If habitat use patterns don’t change with respect to the local competitors, then that must mean that the birds have a strict preference for foraging location and should always use it. But hey! Other studies have found that Pine Warblers in different places use different portions of the habitat.
<Ding! Round 6> What if the differences in habitat usage is not a function of learning by individual birds, but a more hard-wired sort of behavior? An interesting little experiment on Pine Warblers from two different islands in the Bahamas with different foraging patterns shed some light on this particular question. Since birds on the two islands use habitats in different ways, the researchers wanted to see if the foraging patterns were just a function of resource availability. By bringing birds into captivity and giving them standardized resource distribution, they were able to show that the birds from the two islands had different foraging patterns, regardless of resource distribution.
How cool is that? Now this point takes us all the way back to round 1. Maybe the reason these species are able to live in the same habitats are because the populations have intrinsic habitat preference differences. These differences could be the ghost of competition past- (individuals with different foraging patterns were able to survive and reproduce better than individuals without those preferences). Certainly a possibility. And that means that the current distribution of birds would sort of an amicable, neighborly relationship (Hey Frank, good to see you foraging up in the trees. I’ll just keep hanging down here), rather than active, direct competition for access to space. Neato!