Looking at bird migration on the North Shore

Chel Anderson is a North Shore naturalist. She lives in Cook County and joins us periodically to talk about phenology or what’s going on in the woods right now. Welcome back, Chel.

Anderson: Hello!

The spring bird migration is in full swing, and everyday there seems to be a new bunch moving in, but there is also a bunch moving out, and then another bunch comes in. So, why is this such a popular breeding ground?

Anderson: Yeah, why do we have this privilege of all of these beautiful, colored and song-filled creatures among us? What an amazing thing to be a part of. So—why? Good question. Minnesota, as it happens, is part of kind of an east-to-west-oriented swath of the North American continent that has the highest bird diversity of any region north of Mexico. So, it’s not just our imaginations. We really are right in the heart of the richest bird diversity around. That is pretty much because of the wide, amazing array of wetland and forest and open habitats. This incredible mosaic that we live in supports all these different habitats for birds. And, birds are incredibly diverse as a group in terms of the kinds of habitats that they can take advantage of. Even within what, to us, might look like the same kind of habitat, so dense conifer forest, can be occupied by many different types of birds, because they individually as a species focus on one aspect of that, even one level of the forest. So, in the case of warblers, there was a very famous study done by a researcher named MacArthur, showed that there are up to seven species of warblers that are using different levels of the canopy of a single conifer. So, in that way, they don’t interfere with each other. So, one is using just the outer tips of the branches for feeding. Another is just feeding right in close to the trunk of the tree. Some are down low in the branches closer to the understory. Others are way up high. So, this was a landmark study that showed how birds capitalize on habitat.

We have this diversity, and this is starting to make sense to me now, but also these various habitats, you know, are up against each other. And, if I’m not mistaken, edge is a big deal for birds. Can you talk a little bit about edge?

Anderson: Sure. Edge is a big deal. And how edge affects birds is really species-dependent. So, within this vast and marvelous family that we label birds, there are birds that are strictly interior forest birds. So, they are birds that really, they feed, they nest, they—even when they’re migrating, you know, when they stop and rest for the day—they want to be in, deep in the forest. And, then there are other birds that utilize resources that are more available at the margin. It might be a soft margin, so some place where instead of big, tall dense tree canopy; it’s more thick with shrubs and not much for shady tree canopy. Other birds might really like that place where there’s an abrupt change, where the forest comes right down to the water and there’s just a nice little edge of shrubs right at the shore edge. Other birds really use those edges of riparian areas strictly for feeding, but they nest in more interior conditions. And then, other birds really want to be on open ground. We don’t have a lot of birds like that, because that’s not a common feature typically on our landscape, but we do have, some people would likely be familiar with the woodcock. Well, woodcocks like more open conditions, and if they need to move around over time from one place that has an open kind of situation and then to another that is created by some kind of natural disturbance, than that works for them. The other way that edge has an impact on birds, and particularly on interior forest birds, is that where there’s edge, that tends to be a place where they can be more impacted by, for instance, non-native bird species, like cowbirds, which are actually nest parasites on other birds. They are also impacted by differences in climate at the edge of something, versus in the interior forest, and on their prey in the edge versus the interiors.

So, what I’m taking from this by inference is that if we enjoy birds and want to keep our bird populations coming in that diversity of our environment is an important thing, that we need to keep forests and open areas and marshes and all the kinds of things that we have around here. And, if we keep those in good shape, then we should have pretty good bird population during migration time.

Anderson: That’s exactly right. It’s essential. Without these things, we wouldn’t have these birds. These birds are basically coming here—the birds that don’t stay here all year—are basically coming through here or passing through here, because, in a very simple way, because there is great food resources here. And the migration, the “why” of migration, still has a lot of mystery attached to it, but it’s very certain that migration as a habit, as a behavior, predates the big glacial epochs, like of the Quaternary and the Pleistocene, and that migration as a habit developed in response to a need during a period of time when the climate was very different than it is now. One theory that birds were mostly living south, but as climate improved in the north, they moved themselves north to take advantage of improving conditions, more habitat. Another theory says that it’s just the opposite. Birds were more widespread into the north, but that with changes in climate that made the winters unsurvivable, that birds were pushed to the south. Then, as things improved, they came back to the north, because in the north we have this incredible explosion of vegetation and insects accompanied by very long days, and most birds feed during the day. So, both as individuals for survival this is a huge benefit, and as importantly, a much better shot at being successful with nesting. We never know what really was the fundamental to migration becoming a habit of so many birds, but it’s clear that that’s why they are here.

You’re absolutely right. We do have an explosion of vegetation, and particularly insects.

Anderson: Yeah!

As we get further into the summer, Chel, I know we’re going to talk about bugs. Chel Anderson, botanist and plant ecologist, thanks for helping us understand spring migration.

Anderson: You’re welcome!

Photo courtesy of tjsalo via Flickr.