Gulf Crossings

Migratory Bird Stopover Habitat on the Gulf of Mexico

           (click to enlarge) 

The migration of nearctic-neotropical birds is one of nature's greatest spectacles.  In an earlier post, we discussed why birds migrate.  This post will highlight some general aspects of stopover habitat around the Gulf of Mexico.  GCBO is involved in initial research to create a landscape scale stopover habitat model to help in conservation efforts relating to stopover habitat.  The map at the left is an illustration of stopover habitats in Bird Conservation Region 37 (Partners in Flight).

Following is a conceptual framework for considering stopover habitat:

"Fire Escape”: Like fire escapes in human habitations, these stopover sites are infrequently used, but are utterly vital when they are. Habitat quality may be too low to allow birds to gain significant mass, but at least they will survive, can take shelter, and may be able to get fresh water. Fire escape sites are typically adjacent to significant barriers such as deserts or large bodies of water.

"Convenience Store”: Forested patches, such as small parks or woodlots, in a non-forested matrix and located along migratory routes. These sites offer a place where birds can briefly rest and gain some mass easily, perhaps between short flights to higher quality sites, or when migrants’ fuel stores are moderate. A given Convenience Store may be better able to serve the needs of some species than others. Convenience Stores are found in various areas like city parks, and small woodlots near agricultural fields.

"Full-service Hotel”: Forested sites in a forested landscape. Full-service Hotels are places where all needed resources (food, water, shelter) are relatively abundant and available. These places serve many individuals of many species. Bottomland hardwood forests are a good example.

After an exhausting overnight flight across the Gulf of between 10 and 20 hours (depending on wind), birds must find habitat to stop, rest, and refuel.  Recent research indicates that primary migration routes evolved in the last glacial period, 25,000 years ago, and are centered around 95 degrees west longitude (from just SW of Houston eastward to the central Louisiana coast).  The precise landing zone for a trans-Gulf migrant however, is largely dependent on weather, especially wind speed and direction, and the energetic condition of the bird itself.  A fat-depleted migrant, or one that encounters strong north winds or thunderstorms, will usually stop in a fire-escape (if the bird makes it ashore at all).  In most cases, during favorable weather, migrants will fly inland and settle in a patch of hardwood forest (convenience store or full-service hotel).  If the bird must use a fire-escape, it will usually spend the minimum time there necessary to regain strength (and possibly wait for favorable weather).  What happens after landfall - and what strategy the bird uses to find resources - will be the subject of the next post.

Bill   

What is a "Bird Observatory" anyway?

Most birders know that there are bird observatories out there.  Some are relatively well-known, such as Pt. Reyes in California, Cape May in New Jersey, and the Rocky Mountain in Colorado.  Bird observatories are not like astronomical observatories.  While one can usually see birds on the trails provided at most bird observatories, the focus of the institution is much more.  The primary focus of almost all bird observatories is the preservation of birds and bird habitat of the area.  There is also an emphasis on education and outreach, dissemination of information through newsletters and mailings, and often research projects.  But what about the Audubon Society, the Nature Conservancy, the American Bird Conservancy, EarthFirst!, and other organizations?  What do these others do?  Audubon Societies also restore and conserve habitiat for birds, and focus a lot on education and outreach.  Audubon groups are also advocacy groups - they involve themselves in environmental issues and represent a voice for people interested in preserving our natural environment.  EarthFirst! does the same, but in a much more dramatic and direct way.  The Nature Conservancy's main mission is to preserve land by acquiring and protecting it.  The American Bird Conservancy has a broad mission to coordinate and link groups and organizations together to address threats to birds and further research into combating those threats.

When you get the chance to be a member of a conservation organization, remember its mission and its purpose.  Try to join as many as you can - a lot of these groups depend of membership for a majority of their funding.  It's a great way to get involved in bird conservation.

Bill

Caracara photo by Joe Kennedy

Why do birds migrate?

Chestnut-sided Warbler photo by Charles Moores

I love basic questions like this where the definite answer is not known. We all know that birds migrate. We know why migration is a good idea, even though it's extremely hazardous. It's a good idea because it evolved. If it wasn't worth the risk, it wouldn't happen. There are other details, about food supply, weather, competition, etc. which are the reasons it evolved, but the fact is, it evolved. But why? Why is migratory behavior in some birds selectively advantageous? Some theories - pick your favorite:

1) At some point in time, environmental pressures induced some birds to move to find more food or to lessen competition for nesting sites. Pressures (or weather changes) in the new site may have forced them back. The cycle continued, and the birds that refused to move eventually died out as the migrant birds were more successful breeders and their genes predominated.

2) Advancing glaciers caused temperate zone birds to retreat to the tropics part of the year and return to their ancestral home when the glaciers retreated. Or conversely, retreating glaciers afforded tropical birds the opportunity to move to temperate zones to exploit food and breeding resources. Since glacier movement was not a yearly occurrence, this theory is generally not accepted, since it doesn't explain how the migration north/south cycle stabilized.

3) To me, the most plausible explanation is that groups of tropical species spread northward in warmer months to exploit new food and nesting resources to escape increased competition in their tropical homes. The temperate zones offered more daylight for feeding, and a wider range for selection of nesting territories. When the weather cooled, the birds returned to the tropics. Most bird families (taxonomically speaking) are thought to be of tropical origin, so this theory may hold true.

A nice mystery. We may never know the answer, but we enjoy watching the spectacle every spring and fall. There are many fascinating details about migration. I encourage you to find a resource and read more about it if you're interested.

Bill