Fall Migration in Arizona and the importance of Important Bird Areas
The migration of large numbers of birds through Arizona, spring and fall, is one of nature’s most breathtaking spectacles. As a concept, it seems simple enough. As the year wanes, the weather turns frosty and food becomes much more difficult to find. To survive, birds undertake the long and dangerous journey south to warmer latitudes and more abundant resources. A never ending cycle, they return again in spring. For years, migration was generally thought to be this simple. Recent research results have shown that migration is far more subtle. This is especially true of fall migration in the American West. While details of migration in eastern North America are relatively well-known, recent studies have revealed that patterns of bird migration in the Southwest are unique, perhaps coming about through the relative lack of geographic barriers, such as those found in the east and the naturally fragmented nature of habitats in the west.
Recent investigations have shown several distinctive differences between spring and fall migration in the west. Spring migration can be characterized as a race to the breeding grounds in an attempt to claim the best nesting sites and to ensure young are in the nest during a time of peak food availability. In part this is because young birds that fledge earlier in the season have a higher survival rate than those fledging later. So spring migrants head north quickly, and in larger groups than in their fall return.. After breeding is over, birds begin their return journey south in a much less hurried way and in a far more dispersed pattern. A satellite tracking study of Swainson’s Hawks showed that they migrated south on a much broader front than when they headed north in the spring, made more stopovers and stopped for longer periods of time during the fall migration.
The routes, or “flyways” as they are popularly known, are complex in the west and differ between spring and fall. Conservationists have long emphasized that riparian areas are vital to migration in the arid West and serve as the general pathway the birds follow. While this seems to be generally true for spring migration, it is less so in the fall. In the spring, riparian areas are productive when conditions are relatively cool and moist at low elevations. In late summer and fall, the productivity of these areas is heavily dependent on structure of habitat and availability of water.
Riparian areas that usually flow year round, such as Lower San Pedro River Global IBA, can remain somewhat productive into the summer, but such habitats are increasingly scarce in the American West. Higher elevation habitats, which may not be very productive early in spring, remain productive longer into summer and fall than low elevation areas due to cooler temperatures and higher retention of moisture at the higher locations. These conditions result in the plants at these higher elevations flowering later in the season, providing food in the form of nectar and insects for migrants in the late summer when food resources are far less abundant at lower elevations. Many species, passerines and hummingbirds especially, have been observed traveling north through low elevation riparian areas in the spring and in the fall returning south through the Sky Islands such as Chiricahua Mountains Global IBA.
Our Sky Island habitats are also vital to those western fall migrants that have an interrupted molt strategy. More eastern species tend to complete their molt on their breeding grounds before heading south. Some western species however leave their breeding grounds and head south and then interrupt their migration to stop in favorable habitat to molt their feathers. Our Sky Islands, such as the Huachuca Mountains IBA and Santa Rita Mountains IBA, are two such molting areas. This pattern of interrupted molt and migration is more prevalent in birds of the west–with approximately 50% of species and subspecies using this strategy–than the east where only about 10% exhibit this behavior. It is thought that this difference is due to arid conditions found throughout the west in early summer which diminishes available food when birds are undergoing this physiologically demanding process. The abundance of food found in the Sky Islands during the late summer and fall fill the gap in available resources and the birds travel to these areas to take advantage of these resources. These birds are following a chain of stopover sites and we need to preserve the integrity of this chain to ensure their survival. Some of the species that show interrupted molt and migration are Lazuli Bunting, Painted Bunting, Western Kingbird, Lucy’s Warbler, Western Tanager, Lesser Goldfinch and Bullock’s Oriole.
Migration and molt are very taxing on birds, and for some species migration is the time of greatest mortality. However, the benefits of superabundant resources and decreased competition at distant breeding locations outweigh the high costs of migration from winter quarters. The success of this gamble however depends on suitable migratory stopover sites along the way.
Special Thanks to University of Arizona Professor Charles Van Riper III for his help with this article.
A Matter of Perspective. Arizona IBAs at Very Different Scales.
Arizona’s Important Bird Areas are part of a global network, with more than 200 countries participating. The massive size and multi-layered nature of this program make it very interesting to look at IBAs at different scales.
To become a state-level IBA, a site must have significant populations of either species of conservation concern–species with a very restricted range–or species that are vulnerable because they congregate in high densities. An independent committee of researchers and managers from around the state decide if an area meets these state-level criteria.
A Global IBA is part of a much bigger picture. To qualify as a Global IBA, a state IBA has to support a specified number of individuals from any of an explicit list of species. An IBA can also qualify if a certain proportion of the species’ global population can be found in that location.
The Arizona IBA team recently requested that several of our State IBAs be considered for Global IBA status. Three new Global IBAs were designated in Arizona, raising the total number to eight.
The first newly elevated IBA is the Salt/Gila Riparian Ecosystem IBA, which qualified for the numbers of Yuma Clapper Rail and Neotropical Cormorant present. This IBA is very close to Phoenix and is a particularly productive and beautiful riparian area within easy visiting distance of a large metropolitan area.
The other two new Global IBAs are Whitewater Draw State Wildlife Area IBA and Wilcox Playa/Cochise Lake IBA both of which qualified for the huge numbers of Sandhill Cranes that famously winter in there. Both are located within the Sulphur Springs Valley southeast of Tucson and act as “loafing” sites—places to assemble while not feeding–for Sandhill Cranes that forage during the day in nearby agriculture fields.
The remaining Global IBAs in Arizona are scattered throughout Arizona. Marble Canyon IBA, which qualified because of the California Condors in residence there, is north of the Grand Canyon. Anderson Mesa IBA is south of Flagstaff and was made a Global IBA for its population of Pinyon Jays. Bill Williams NWR IBA is a marvelous riparian area located near Lake Havasu and qualified for Global Status for both Bell’s Vireo and Black Rail. Closer to Tucson, the Lower San Pedro River IBA is a well known riparian area that qualified for Global Status for the high numbers of Bell’s Vireos that breed here. The last, but not least, Global IBA in Arizona is the Chiricahua Mountains IBA which qualified for the numbers of breeding Spotted Owls that have been documented in the range.
On a much closer scale, there is news regarding a part of a single IBA. At its last meeting in October, the Arizona IBA Science Committee approved the expansion of the existing Sabino/Bear Creeks IBA to include the Tanque Verde Wash and was renamed the Tanque Verde Wash/Sabino Canyon IBA.
This latter IBA offers a valuable proximity to urban Tucson and ease of access, and illustrates the concept that urban habitat can still be valuable for native birds if it is preserved and enhanced. We look forward to working with local birders and conservationists in developing threat assessments and conservation actions to enhance this area for birds. It is an excellent place to bird and very easy to visit year round. There are IBAs all over the globe and right in our own backyard and we hope to engage you in their protection.
These articles originally appeared in the January-March 2012 Vermilion Flycatcher, the quarterly magazine of Tucson Audubon.