Nongeographical Migrant Traps

Dave La Puma, over at woodcreeper, had some smart analysis today about local scale movements of migrant passerines, and how that may account for reports of new arrivals even following nights of unfavorable weather. The premise is pretty straightforward; lacking favorable conditions to move on, birds are likely to shift from areas of substandard habitat to prime foraging grounds. Remember that a migrant trap typically works by virtue of its geographic location. The classic example of course is Cape May island, essentially the bottom of the bird funnel that is the state of New Jersey. With thousands of birds packed in to such locations, its only a matter of time before competition, or merely a need to seek more fertile feeding grounds, disperses birds across a wider area. Often the tried and true migrant traps are left desolate. If you don't believe it, goto Sandy Hook after four days of south winds sometime this autumn.

Understanding what makes up prime foraging habitat can make your birding more consistent. No longer do you have to wait for those perfect cold fronts, you can take advantage of "slow days" by visiting sites that concentrate migrants left in the area during the last wave. To follow up on Dave's post, I thought I'd give a quick punch list of the sort of attributes that tend to be common among such locales. Of course these aren't hard and fast rules, but sort of a general set of guidelines.

1. Diverse habitats. Lots of habitat types will provide prime foraging for a greater number of different species. Paulinskill Wildlife Management Area; a locale I like, contains upland forest, open woodlands, meadows, shrubby tangles, a riparian corridor, a freshwater wetland, etc. Equally important are the edges. Its a well accepted pillar of ecology that an edge between two ecosystems tends to be more diverse than the interior of either (the edge effect). A sunlit edge between forest and field is precisely what I think of when asking myself, "where would I find lots of warblers?"

2. Structure. Much like diversity across the landscape, lots of variation in a forest profile can be the difference between poor migrant habitat and a productive birding area. Birders are very familiar with the strong preferences some species have for a specific band in the forest spectrum, such as the Mourning Warbler (almost always associated with dense thickets <5 m above the ground) and the Tennessee Warbler (an infamous source of neck pain). The more strata you take out of the forest, the fewer species you can expect to find. Even for birds with a more generalist approach to the forest profile a diverse structure is beneficial, providing more feeding and roosting opportunities.

3. Oaks. Really any native trees, though Oaks are certainly among the most productive. Such trees are absolutely teeming with insects. Your average oak can support more than five times as many insect larvae than a tree not native to the northeastern United States. Why? Well, I've discussed this at length before. Oaks, by virtue of their evolutionary roots in this region, support a far greater number of different species of insects.

4. Fruit. As the breeding season marches on, fruiting shrubs and trees become incredibly important sources of food for migrant birds. Cold weather snaps may kill off insect food sources before later migrating species, such as Black-throated Blue Warblers, pass into our area. These birds have adapted an ability to switch their diet over to primarily fruit (as opposed to obligated insectivorous species such as Cerulean Warblers, which as a result have to migrate much earlier). Areas with lots of fruiting shrubs and trees can be magnets for migrants. Celtis (Hackberry), Prunus (Cherry), and Cornus (Dogwood) are just a few native genera that are especially favored.
Though it is worth noting that the nativity of the fruit does not matter much. You can certainly see migrants feeding on Ligustrum (Privet), Lonicera (Honeysuckle), Rosa multiflora (Multifloral Rose), and other invasive plants. In fact, passerine migrants likely play a big role in moving invasive species from one region to another. Aralia elata (Japanese Angelica Tree) a newly emerging invasive species in the Midatlantic region, seems to be (based on pure anecdotal evidence only) distributed primarily along major migration routes in New Jersey. If you go to the parking lot of the Chimney Rock hawk-watch in September, you can witness dozens of birds gorging themselves on Aralia berries.

5. Sun. This, for me, was an overlooked ingredient of prime feeding habitat until last fall. Look for the little spots that: (1) get sunny the earliest in the morning and (2) stay sunny latest in the evening. Even a little increase in air temperature will get insects moving and birds from hundreds of yards away will converge on the area.

So thats it, five things to look for when seeking good foraging habitat. Of course this is only a starter list, so if anyone has additional suggestions please leave a comment. Make sure to get out and find some feeding sites, then check them on that next slow day. You may end up pleasantly surprised!