Results from the North American Breeding Bird Survey, 1966-1994
Reprinted from: Purple Martin Update 8(1): 26-27
Bruce G. Peterjohn and John R. Sauer
US Geological Survey, Biological Resources Division
Patuxent Environmental Science Center
Laurel, Maryland 20708
The only current source of continental population trend data for Purple Martins is the North American Breeding Bird Survey (BBS). BBS data is used here to summarize the 1966 to 1994 population trends and relative abundance of Purple Martins in the United States and Canada.
The North American Breeding Bird Survey: The BBS consists of nearly 4,000 randomly located routes along secondary roads throughout the United States and Canada. All birds heard or seen within 0.25 mile of each of fifty 3-minute stops are recorded along each route. Routes are surveyed once annually during the peak of the breeding season.
Relative Abundance: Fig. 1 shows the relative abundance of Purple Martins in the United States and Canada based on BBS data from 1966-1994. The abundance categories reflect the average number of Purple Martins seen or heard per route per year in various regions. As expected, martins prefer relatively low elevations where aquatic habitats and their associated insect populations are plentiful. Martins are less numerous in mountains and where forested habitats prevail.
Population Trends 1966-1979: Despite local declines associated with weather-related mortality, the 1966 to 1979 period was generally favorable for Purple Martin populations. Significant increases occurred in 10 states (AR, FL, IN, KY, MI, MS, MO, NY, OK, TX), the Eastern and Central BBS regions, United States, and survey-wide. Other regional trends were also positive, except in the Western BBS Region. Significant declines occurred in California. Increasing populations predominated throughout eastern and central North America. Declining populations tended to be locally distributed, primarily within the northern portion of the Purple Martin's range. Weather-related mortality may have contributed to some of these declines, especially in the vicinity of the Appalachian Mountains.
Population Trends 1980-1994: After 1980, population trends for Purple Martins have generally been in a negative direction. Significant declines occur in 10 states (AL, FL, IL, IA, MI, MN, MS, NY, OH, WI), the Eastern BBS Region, United States, and survey-wide. The other regional trends are also generally in a negative direction, except in the Western BBS Region. Increases are limited to populations in North Carolina and Saskatchewan. From 1980 to 1994, fairly consistent declines were seen throughout the northern United States. Declines also predominate along the Gulf Coast from Texas to Alabama and Florida. Increases prevail along the Atlantic Coast from Virginia to Georgia, and from portions of Tennessee and Kentucky westward into Kansas and Oklahoma.
Population Trends 1966-1994: With early increases followed by subsequent declines, the long-term trends are generally nonsignificant at the regional level except for an increase in Canada. Significant increases are evident in 7 states (KY, MD, MO, NC, OK, TN, VA), while declines occur in 9 states (AL, IL, IA, KS, MI, MN, NY, OH, WI).
The trend map shows definite regional patterns to the Purple Martin population trends (see Fig. 2). Decreasing populations prevail across the northern portion of its range, from New York and Pennsylvania westward across the Great Lakes region to the Great Plains. Increasing populations predominate across the southern half of its range, although an area of decline extends from eastern Texas east to Alabama and north through eastern Tennessee to eastern Kentucky.
Temporal Patterns in Population Trends: Declines during the late 1960's and early 1970's apparently correspond with the weather-related mortality associated with Hurricanes Abbey (in 1968) and Agnes (in 1972). The continental population recovered during 1973-1977, followed by fairly stable numbers until a sharp decline during 1983. Their numbers quickly recovered, but a gradual decline has been apparent since the late 1980's.
Discussion: Several patterns clearly emerge from these BBS data. Purple Martin populations in the northern portion of their range are undergoing a long-term decline. Weather-related mortality may have contributed to this decline during some years prior to 1974. However, this decline has continued throughout subsequent years and is fairly consistent over a large geographic region, an indication that it is not solely a weather-related phenomenon. Populations in the southern portion of their range tend to be increasing during the entire survey period, although recent declines are evident in some states. There are no consistent temporal patterns to these positive trends, suggesting that local factors may be influencing the trends in each state.
While the BBS data describe geographic and temporal patterns in population trends, they provide no information on the factors responsible for these trends. Certain short-term population changes have been well documented, such as mortality associated with cold wet weather during the breeding season. However, the factors responsible for the long-term declines in the northern portion of its range and increases in the south remain poorly understood. Additional information is needed to determine if factors on the breeding range and/or wintering range are contributing to these trends. The systematic collection of data on the size and reproductive success within colonies throughout the breeding range is essential to determine if the breeding pairs are raising enough young to sustain current population levels. Given the accessibility of most colony sites, these data could be collected fairly easily. Obtaining data on martin populations from their winter range will be considerably more difficult, and require collaboration with South American ornithologists. Until the factors responsible for the observed long-term population trends can be identified, the development of meaningful strategies to reverse population declines cannot be undertaken.
This is a condensed version of an article published in Update 6(2):2-8 in the summer of 1995.
Copyright 1997 by Purple Martin Conservation Association. All Rights Reserved.
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