Gulf Crossings

New Research on How Birds Orient Themselves During Migration

Researchers have long known that birds figure out where to go during migration by using a combination of the earth's magnetic fields, the stars, the sun, the moon, and polarized light pattern cues at the horizon at sunrise and sunset.  The exact nature of how each compass system integrates with the others, and how these systems are calibrated and recalibrated has been a mystery.  The magnetic compass changes with latitude, and others systems may be altered by weather or time of day.  To be effective despite these alterations, it would be necessary for all compasses to be calibrated to a common reference.

A paper published in Science magazine in August, 2006, "Polarized Light Cues Underlie Compass Calibration in Migratory Songbirds", by Muheim, Phillips, and Akesson provides experimental evidence that the polorized light cues at sunrise and sunset are the common reference that other systems calibrate to.  The actual reference value is an average of both sunrise and sunset cues.  These researchers were able to re-orient the magnetic compass of Savannah Sparrows by presenting a false polarized light cue.  Earlier orientation experiments often did not provide the birds a view of the horizon.  In these cases, the magnetic compass became the reference, and other systems were calibrated with respect to the magnetic field.  It is unknown if all species of birds use this same sequence of cues to orient in migration.  Future work will no doubt discover if this system is universal in birds, or if other combinations have evolved in different bird groups.

Bill

Photo of Colima Warbler by Greg Lavaty

Wind Farms in Spain

Here is a post from GCBO's Research Coordinator John Arvin:

I spent the previous two weeks in southern Spain and Gibraltar. This area is one of the most important bird migration corridors in the world because birds migrating between tropical Africa and western Europe are essentially squeezed into a narrow stream to cross the Mediterranean at its narrowest point. An outstanding feature of the migration in this area is that of raptors, storks, and other large bodied species that migrate using rising air currents to move by soaring to conserve energy. It is also an area of extensive wind energy production.

I was struck by how closely the situation there mirrors our own situation on the Gulf coast. We are also located in a globally important migration corridor with a strong component of migrating raptors. The latter is especially concentrated along the western Gulf coast where the flight path is narrow by virtue of being squeezed between the ocean and the north-south mountain chains in northern Mexico. It occurred to me that students of Gulf coast migration and conservationists could learn much about the interaction of large numbers of migrating birds and the wind energy schemes that are planned or actually under development along the Gulf coast, particularly the western Gulf coast of southern Texas.

I tried to meet with a local ornithologist who is a life-long student of the migration in the region of the Straits to talk about their experience but he was away. I will be establishing contact with him and probably other ornithologists in Spain to learn what I can from their experience since most of the published research on the subject of bird – wind turbine interaction has not been carried out in a major migration corridor.

Follow the Radar...

Here are five radial velocity radar images from the Gulf Coast today at around 12:30 pm.  With a south wind, this is about the time most birds arrive after crossing the Gulf.  Don't forget, however, that a lot of birds are circum-Gulf migrants - they fly northward around the east coast of Mexico instead of crossing water.  A sixth image show percipitation at Oklahoma City for comparison. 

Let's follow the radar around the Gulf:

Below is radar at Brownsville, TX on the Mexico border.  Note the northward movement of "stuff" parallel to the coast.  The scale on the right indicates the speed of any objects moving.  This should be compared to the wind speed.  The radar displays from ground level at the station, to around 10,000 ft. at its limit - 130 miles.  Most migrants under these conditions would be flying at about 4,000 ft.  The winds at Brownsville and Corpus at this altitude were about 30 kts, which explains the dark colors.  Birds flying would be measured at 30 kts, plus the speed of the bird.

Further up the coast at Corpus Christi, still paralleling the coast:

At Houston, showing the typical pattern of direct flight from the Gulf.  The wind speed was 7 kts - notice how much faster the "objects" here are moving.

Notice as we go eastward, the number of birds diminishes greatly. At New Orleans:

And, finally, Jacksonville, FL. Nothing much:

One last velocity image for comparison from Oklahoma City showing thunderstorms moving north northeast.

There you have it. If you'd like to check the images yourself anytime, they can be found Here .

Bill

I've flown across the Gulf - now what??

In the previous post, we discussed the different types of stopover habitat available to a trans-Gulf migrant on its journey to and from its wintering grounds in the tropics.  This discussion will examine what happens when the migrant makes landfall.

Most trans-Gulf migrants reach the coast before dawn, and probably continue inland until forest is encountered after sunrise.  Depending on the weather encountered during flight and the energetic condition of the bird, stopover options may be limited and birds may have to land immediately after reaching the coast.  Regardless, birds will usually undertake a “morning flight” soon after arrival or after sunrise. These flights are of short duration, low altitude (often treetop to treetop), and in flocks (often mixed species).  It is assumed that the birds are “evaluating” their immediate surroundings to determine the suitability of the habitat for feeding and resting.  There is great pressure on the bird to find suitable habitat quickly – time spent in migration searching for optimum habitat is time that the bird is not on the breeding grounds and is not competing for a breeding territory.   It is speculated (in most cases) that birds first stop based on gross habitat features like vegetation density.  The decision to search for resources or move on may depend on more specific habitat features or on the behavior of other migrants.  It’s important to realize again that most of these decisions are probably based on the migrant’s energetic condition – birds with remaining fat are most interested in finding a place to rest, while fat-depleted birds must find food quickly.  The exact methods by which migrants make decisions about habitat use during stopover are poorly understood.

Bill

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   

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

The Strange Journey of the Cerulean Warbler

Very little is known about the migration pathways of most Neotropical migrants but even less is known about the Cerulean Warbler. There have been very few records of this species away from the breeding and wintering grounds. It wasn’t until the late great tropical ornithologist, Ted Parker, observed as many as 100 Ceruleans foraging with mixed species flocks over the course of 10 days in 1994 in the Maya Mountains of southeastern Belize that we had our first clue. Parker theorized that the reason there were so few Cerulean Warbler records in migration between South America and the coast of the U.S. was because the birds were making two over-water flights; one across the western Caribbean Ocean and the other across the Gulf of Mexico (Click on map to enlarge). To complete this journey the entire global population of Ceruleans needed to stop in northern Central America to build up additional fat reserves.

 

Subsequent surveys in the last four years by GCBO Research Associate Melinda Welton have confirmed Parker's hypothesis, and have documented several other localities in Honduras and Guatemala.  It appears that the birds are only in Central America for a few weeks in April, in forests between 100 and 1000m.  This type of habitat is similar to their wintering grounds in South America.

After the 2007 field season, Melinda will be working with other ornithologists to develop a model which may be able to predict the spacial distribution of stopover habitat for Cerulean Warbler.  Such a model has been developed by Dave Buehler, Melinda, and others for breeding habitat in Tennessee.  Rigorous field-testing of this model could lead to the identification of critical stopover locations and help prioritize future on-the-ground conservation actions.