Wednesday, November 3, 2010

Ramblings on Migration in Tropical America

What images come to mind when you think of migration? Geese flying south in V-formation? Exhausted warblers resting on Gulf Coast beaches? Broad-winged hawks kettling their way south along the Appalachians? I think about the familiar cycle of migrating birds that breed in temperate forests and winter further south.

I must say that spending time in the tropics – most recently on HMANA’s Costa Rica trip – has given me a different perspective on migration. I no longer refer to the local New Hampshire breeders in my area as “our birds”. It’s hard not to when a Baltimore oriole arrives in your backyard in spring, spends the summer there raising its young and keeping you company with its lovely song and bright plumage. We feel connected to this and a sense of this bird and its young “belonging” here. But think about how much time that oriole spends with us versus time spent on migration and in the tropics throughout the winter. A mere three months, maybe four? Baltimore orioles were all over many of Costa Rica’s tropical forested habitats this October, and many of them are settled in until spring! In truth, the oriole (and many other familiar breeding birds of the Northeast US) spends the majority of its time in the tropics and comes north to the temperate forests to take advantage of a very brief food supply and a place with fewer competitors.

The more I learn about migration, the more questions I have. One thing that is certain is that migration is a constantly changing and widely variable phenomenon. Throughout North America, we have short and long distance migrants, complete and partial, altitudinal migrants, and irruptives, to name a few types of migration strategies. But what about those species which carry out their migrations within tropical latitudes? HMANA’s recent trip to Costa Rica to witness migration touched on this spectacle.

On a drizzly afternoon, our group watched four resplendent quetzels feeding in an avocado tree at 2000m on the road to Volcan Irazu. They were taking full advantage of this fruiting tree, but in typical falls, these birds will migrate further south on the Caribbean slope and mainly be found between 500-800m.
From our hotel room in San Jose, my husband and I watched a distant fruiting Jamaican Plum tree through our scope for one hour. Within that time, 20 different species of birds fed at this tree, most of which were migratory species.

Historically, most migration research has focused on movements of birds between temperate and tropical habitats, but the ‘intrartropical’ migrants – those birds moving within the tropics – haven’t received as much attention. According to Gary Stiles, coauthor of Birds of Costa Rica, approximately half of the bird species found in Costa Rica show some evidence of seasonal movement. These movements likely reflect changing food availability and/or weather changes (wet vs dry seasons). There are even a few tropical species that travel long distances to follow burned areas – ‘fire followers’. Food is still the underlying reason for their movement.

It’s easy for us to notice migrating flocks of swallow-tailed and plumbeous kites (large birds moving through the open sky along ridges and coasts), but most other movements happen very quietly, which is part of why this movement was overlooked for so long.

Today, more tropical research has taken place within Costa Rica than any other area in the neotropics, and it is thought that the local migration patterns between mountains and lowlands are probably typical of the rest of Central America. Birds are making localized short distance movements up and down in elevation. If you’re a bird that doesn’t shift its diet when food becomes scarce, then you have no choice but to move. Interestingly, the species most likely to migrate are the fruit and nectar eaters since this food supply changes seasonally more than insects do. Hummingbirds, parrots, toucans, quetzals, and bellbirds are just some bird families that employ this strategy seasonally.

If you are as fascinated as I am about migration in the tropics, or just curious about general behavior and breeding of tropical birds, be sure to check out Birds of Tropical America by Steven Hilty. It’s a terrific book and has answered so many of my questions about migration and more.
(Photo: plumbeous kite at the Kekoldi Hawkwatch)

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