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There are 361 species of Neotropical migrant birds ranging from herons and raptors to swallows and warblers. Over 130 species breed right here in New Jersey, and nearly 80 of these are songbirds, many of which you can see in your own backyard during spring migration. If you are lucky enough to live near large forests, you can hear many different species in a chorus of song - these are usually the males singing to defend their breeding territories from other males and to attract females.
Over the past 20 to 30 years, biologists have been documenting the alarming declines of many Neotropical migrant bird populations. Habitat loss and fragmentation, caused by development and other human activities, are the main causes of population declines.
Forest birds of the Northeast are the group for which population declines are the best documented. Unfortunately, we know that many other groups of species have suffered population declines for the very same reasons -- habitat loss and fragmentation severely impacts many animals such as mammals, reptiles, amphibians, and invertebrates. (See: Endangered and Threatened Species List; Avian Species of Special Concern in New Jersey.
The Landscape Project for Rare Species Protection began in 1994 as a method to help stem the decline of wildlife populations in New Jersey. The mission of the Landscape Project is to:
ENSP biologists used this bird survey data in two ways. First, survey data for forest-dwelling Neotropical migrants was combined with location data for endangered and threatened forest species to identify and map critical forest habitats in the Delaware Bay and Highlands Landscapes. These forested areas are critical because they provide habitat for a diversity of species with many different life-history requirements. Species diversity is an indicator that a habitat is a healthy, functioning ecosystem -- these are the areas where we first focus conservation efforts. Over the last two decades the Northeast has experienced an increase in forested land, however, forested land now generally tends to be more fragmented. This has serious consequences for bird species that require large, contiguous forest tracts for successful breeding like worm-eating and Kentucky warblers, as well as threatened and endangered raptors like the barred owl and red-shouldered hawk.
In addition to mapping critical areas for multiple species, the Neotropical migrant survey data will be used as a predictive tool for identifying suitable habitat for forest, scrub-shrub, and grassland birds. This process involves using Geographic Information Systems (GIS) Software to quantify the characteristics of the habitat patches surrounding each survey location. We then use statistical methods to determine if there is a pattern of habitat use by individual bird species detected at the survey locations. Using this pattern (or model) as a guide, we can identify and map areas that have the same favorable habitat characteristics, and thus have a high probability of species occurrence. The areas predicted to have "high occurrence probability" would be targeted for survey, long-term monitoring, and conservation efforts. ENSP biologists have performed this analysis for forest species. The next two groups targeted for analysis are scrub-shrub and grassland birds.
Mapping of critical areas for endangered and declining wildlife, as well as mapping of suitable habitats for forest birds, will be available through the ENSP's Landscape Project web page in early 2000.
Over the past 30 years, extensive research in North American breeding areas, particularly in northeastern forests, has clarified the major factors that make Neotropical migrants vulnerable to population declines. Many species require particular habitat types for breeding (i.e., "habitat specialists"), require large tracts of habitat for successful breeding, or share both of these characteristics. As habitats are lost and become more fragmented, there is less suitable habitat available, and threats to adult birds and their young increase. Mammalian and avian predators and brown-headed cowbirds tend to be more abundant in fragmented landscapes. In addition, tower strikes during migration and predation by free-ranging house cats have been estimated to kill tens of millions of birds each year.
Migration itself is hazardous - bad weather, predators, radio towers, and tall buildings with reflective glass claim very large numbers of birds each year. Migration is physically demanding - birds require quality habitats along migration routes to rest, replenish fat reserves and escape predators. The loss of habitat along migration routes reduces chances for survival. Migration is also very time consuming - this means that migrants have little time in which to breed, generally late April to mid-June. Many species usually have only one chance at successfully fledging young. If the first nest is destroyed or preyed upon, a second nesting attempt usually has a much lower probability of success.
Research on Neotropical wintering grounds has revealed that, as with breeding habitat, many species require specific habitat types during winter months. Males and females of many species, individually, defend their own winter foraging territories and return to these same territories year after year; this is called "site fidelity". The continuing loss of habitat in the Neotropics has a grave impact on overwinter survival of birds and decreases the probability of successful migration and breeding the following spring.
Web Resources:
* Cats and Wildlife: A Conservation Dilemma J. S. Coleman, S. A. Temple and S. R. Craven. 1997. University of Wisconsin, e-publications: http://wildlife.wisc.edu/extension/catfly3.htm.
* Managing Stopover Habitat for Migratory Land Birds J. P. Tash, E. Stiles and L. Widgeskog. 1999. New Jersey Division of Fish, Game and Wildlife, Endangered and Nongame Species Program.
* Landscaping for Birds - a series of pamphlets describing how to landscape for migratory birds on barrier islands, and large and small parcels of land.
The Birder's Handbook: A Field Guide to the Natural History of North American Birds P.R. Ehrlich, D. S. Dobkin, and D. Wheye. 1988. Simon and Schuster, New York, NY.
Neotropical Migratory Birds: Natural History, Distribution and Population Change R. M. DeGraaf and J. H. Rappole. 1995. Cornell University Press, Ithaca, NY.
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* Copies can be obtained from the Endangered and Nongame Species Program, P.O. Box 400, Trenton, NJ 08625 (609) 292-9400.
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