The Robins are Back!

Things are finally starting to warm up out there, and suddenly these guys are all over the place:

American Robin (Turdus migratorius), St. Louis Zoo,
St. Louis, MO  3/2/2012

This is, of course, an American Robin (Turdus migratorius), one of our most famous birds. They're well known for chasing worms across our nicely mowed lawns -- in fact, there's evidence that mowing actually makes their prey more visible, and mowed lawns are actually preferred for feeding in. (1) And here's a shot of one doing just that:

American Robin (Turdus migratorius), Illinois Beach State Park,
Lake Co, IL  5/19/2012

So, when the winter rolls in, and the snow's on the ground and the worms are down deep and the bugs are hibernating, then everyone knows that the robins head south for the winter, to return as the first sign of spring, right?

Well, not exactly! Apparently our breeding robins do head south for the winter, but if you wander the right forest preserves in the winter time, you'll likely see something like this:

American Robin (Turdus migratorius), Lyons Woods Forest Preserve,
Lake Co, IL  12/11/2012

This guy's hanging out in what looks like a Hawthorn tree, in Lyons Woods Forest Preserve, in December. And you can still find them in various locations around the area throughout the winter. Turns out they're winter visitors from even farther north, but either way, they can't be chasing worms!

In fact, they're mostly eating fruit -- this guy's sitting in his favorite diner at the moment. Quite a few birds that we think of as mostly insectivores turn to fruit in the winter, including many of the thrushes, Yellow-rumped Warblers, Eastern Kingbirds, and even Tree Swallows. Others, including Cedar Waxwings and tanagers, will happily consume fruit all year round.

In most cases, though, they won't digest the seeds -- they'll pass right through instead, possibly to germinate wherever they're deposited. This dispersal is quite important for the plants, (2) and appears to be the primary reason for the evolution of sugary or oily fruits in the first place. (All of that sugar or oil is energy the plant could be using for growth.) It also has important implications for conservation. First, fragmented habitats can reduce seed dispersal by birds and affect recruitment and gene flow in plant populations. (3) Second, many successful invasive plants are dispersed by birds. (4) In Lake County, that includes some particularly problematic species: 10 different species (including hybrids) of Honeysuckle (Lonicera sp.), and European Buckthorn (Rhamnus cathartica). (5)

Obviously, wiping out frugivorous birds to aid in eliminating Buckthorn would be throwing out the baby instead of the bathwater. Even if we didn't care about the birds, we'd be causing even more problems for a number of native plants. But it does throw an extra wrinkle into our conservation efforts when our native birds are part of the problem.

(1) Eiserer, L. A. (1980). Effects of grass length and mowing on foraging behavior of the American Robin (Turdus migratorius). The Auk, 576-580.

(2) Nathan, R., & Muller-Landau, H. C. (2000). Spatial patterns of seed dispersal, their determinants and consequences for recruitment. Trends in ecology & evolution, 15(7), 278-285.

(3) Levey, D. J., Bolker, B. M., Tewksbury, J. J., Sargent, S., & Haddad, N. M. (2005). Effects of landscape corridors on seed dispersal by birds. Science,309(5731), 146-148.

(4) Gosper, C. R., Stansbury, C. D., & Vivian‐Smith, G. (2005). Seed dispersal of fleshy‐fruited invasive plants by birds: contributing factors and management options. Diversity and Distributions, 11(6), 549-558.

(5) http://www.invasive.org/species/list.cfm?id=152