BIRDS AND BEYOND

Why rare birds are rare

By Claire Curry | Published Wednesday, December 28, 2011

Have you ever wondered where our summer birds go in the winter? To find some of them, like a Grasshopper Sparrow in winter, is merely unusual. To find others, like a Swainson’s Hawk, would be crazy rare. What’s the difference, and why?

These are birds that are rare because of the season. They nest and winter in different places, so their appearance out-of-season is an anomaly. It would be very unusual to see an American Goldfinch here in July, while in Oklahoma they sometimes nest so it wouldn’t be so noteworthy. Likewise, orioles are common migrants, but an Orchard Oriole in November here would be extremely unusual.

Other birds are rare because of geography. Pileated Woodpeckers are unusual in our region because they are at the edge of their range, but there are a few sites (like Rucker’s Pond and Black Creek Lake in the national grasslands) where they are regular because there is appropriate habitat there. Other birds aren’t even on the edge of their range here and therefore, are more rare.

The difference, then, between the sparrow and the hawk is a bit of both geography and season. Both species winter somewhere else, but the Swainson’s Hawks winter in the South American pampas. The sparrow merely winters farther south in Texas. In fact, some locations in central Texas have them year-round. So to find a Grasshopper Sparrow here in the winter seems like it would be much less of a stretch than finding a Swainson’s Hawk. Not surprisingly, Swainson’s Hawk is rare in winter even in South Texas, while we often see one or two Grasshopper Sparrows every winter.

This difference in degree of rarity is also due to the fact that some birds wander more than others. Red Crossbills show up every few years despite living in the mountain west and north eating pinecones because they wander with the highly variable pinecone crop. Because they are already prone to wander, we get more of them than, say, Cactus Wrens, which are residents in the desert southwest.

Finally, there are rare birds that are rare in the world at large but may be common locally. This would be a species such as the Texas endemic Golden-cheeked Warbler. Within appropriate habitat in the Hill Country, it can seem quite abundant, but that is the only place in the world that they nest. Birds like this are often threatened or endangered because their habitats are so localized and may also coincide with conflicting human usages.

Why do birds occur in these patterns? Where birds regularly occur is partially due to their preferences in food and habitat. For birds that have specific requirements and adaptations to those habitats or for gathering those foods, they must live where a particular resource is found. Woodpeckers live where there are trees in which to excavate nests and search for wood-dwelling insects. Ospreys live by water where they can catch fish.

For nesting, the Golden-cheeked Warbler requires strips of bark from mature Ashe Juniper trees and so only lives where Ashe Junipers of appropriate age are growing (hey, some critters are pickier than others, and they can’t just change on a dime!).

Even if the habitat is appropriate for a particular species, it may not occur there. This tends to be evident at large scales, like the differences in species found on different continents and islands. For example, House Sparrows are obviously doing quite well in urban and suburban North America, but they had to be brought here by humans to get their start.

Hawaii is full of introduced species that are thriving but that would have never found their way to the remote islands without human intervention. Hummingbirds only occur in North and South America despite loads of probably fantastic habitat across the tropics in the rest of the world! These differences between where a bird could occur and where it actually does occur are due to accidents of history – where the birds’ relatives originated and how good those birds are at dispersing across uninhabitable regions (like an ocean or even just inhospitable habitat).

So, while as the saying goes that birds have wings and anything is possible, some possibilities are more likely than others. It doesn’t depend just on the fact that all birds have wings. Some are more likely to use them, some are more proficient at using them when they do and some just live so far away that they won’t get here unless humans tote them around.

The study of where birds live (and all critters and plants, really) is fascinating because it combines their biology and their history. Plus, it does narrow the choices a bit when you are trying to identify that unknown bird in your yard or on your pond. Common birds are common and rare birds are rare but all for different reasons. Keep your eyes peeled for all our delightful birds of all abundances!


The next monthly field trip on the Lyndon B. Johnson National Grasslands will be Wednesday, Jan. 4. We will depart at 9 a.m. from the Forest Service District Office in Decatur. For more information, contact Mary Curry (see below) or the Forest Service District Office at (940) 627-5475.

Claire and Mary Curry are nature enthusiasts based in Greenwood. If you would like to contact them, please email them at larksparrow@eeclaire.com.

Leave a Reply. Note: As of March 24, 2011, all posted comments will include the users full name.

WCMessenger.com News and Blog Comment Guidelines

You must be logged in to post a comment.

Login

Username:

Password:


Recover password | Create an Account