Geographic analysis of population change

for groups of species

Introduction

Major conservation programs tend to focus on groups of species of birds that share a common attribute. For example, the Partners in Flight program (Finch and Stangel 1993) focuses on birds that breed in North America but migrate to the neotropics in winter, and the North American Waterfowl Management Program (Sparrowe 1988) is primarily devoted to waterfowl and wetland species. These programs are motivated by public perceptions and monitoring results that suggest common patterns of population decline among these species (e.g., Robbins et al. 1989, Caithamer et al. 1993). Other groups of bird species are also generally considered to be declining (e.g., grassland birds, Johnson and Schwartz 1993) and provide impetus for conservation programs such as the Conservation Reserve Program that will conserve habitat for these species.

Groupings we use in analyses

Our BBS analyses have used a series of groupings of birds, or "guilds," as a basis for multispecies summaries. These groups are defined in Peterjohn and Sauer (1993), and include groupings based on Breeding Habitat (grassland, wetland, scrub/successional, woodland, urban), Nest Type (cavity and open-cup), Migration Form (short-distance and Neotropical migrant, permanent resident), and Nest Location (ground-low and mid-story-canopy). We attach a complete list of species that comprise these groups. We also summarize BBS analyses for all species.

Philosophical issues associated with the analysis of groups

Actually documenting consistent patterns of population change among groups of species can be very complicated. For example, any choice of species to use as a "guild" for analysis can be criticized because each species has a unique set of environmental factors that influence survival and reproduction, and the characteristic chosen to define the guild may not be of equal importance for all species, or may even be confounded with another characteristic that is the actual causal factor influencing change (Jaksic 1981, Mannan et al. 1984). For example, hummingbirds and waterfowl can both be grouped as neotropical migrant birds, and both neotropical migrants and nonmigratory species breed in grassland habitats.

The species that comprise the groups also differ regionally, thus the results from different regions may reflect the trends of very different species. Sometimes, this can lead to heterogeneity in the group results. For example, the western Neotropical migrant species generally winter in different parts of the tropics than the eastern Neotropical migrants, and their trends also tend to be more positive than the eastern species (Sauer and Droege 1992).

Approaches to estimating change for groups of species

Apart from the conceptual problems associated with the analysis of population change in groups of bird species, there are many technical issues to consider in summarizing trends from large-scale survey data. Here, we provide regional summaries of percentage of species with increasing populations.

We first estimate weighted composite trend estimates for each species in the group for a specific region and time period, as described in Geissler and Sauer (1990). Then, we examine the set of regional trend estimates to determine if a disproportionate number of species are declining.

We use a procedure that is based on empirical-Bayes methods and incorporates the relative variance of the component trend estimates. A numerical procedure is used to estimate the percentage of the precision-adjusted trend estimates that are greater than zero. See Link and Sauer (1995) for a detailed description of the procedure.

Literature Cited


Caithamer, D.F., J.A. Dubovsky, F.A. Johnson, J.R. Kelley, Jr.,
     and G.W. Smith.  1993.  Waterfowl status and fall flight
     forecast, 1993.  U.S. Fish and Wildl. Serv., Annual Rept.
     37pp.
Finch, D. M., and P. W. Stangel.  1993.  Introduction. 
    Pages 1-4 in D. M. Finch and P. W. Stangel, eds.  
    Status and management of Neotropical migratory birds.  USDA
    Forest Service General Technical Report RM-229.
Geissler, P. H., and B. R. Noon.  1981.  Estimates of avian
    population trends from the North American Breeding Bird
    Survey.  Pages 42-51 in C. J. Ralph  and J. M. Scott,
    eds. Estimating the numbers of terrestrial birds. 
    Stud. Avian. Biol. 6. 
Geissler, P. H., and J. R. Sauer.  1990.  Topics in route-
     regression analysis.  Pages 54-57 in  J.R. Sauer and S.
     Droege, eds. Survey designs and statistical methods for 
     the estimation of avian population trends.
     U.S. Fish Wildl. Serv., Biol. Rep. 90(1). 
Jaksic, F. M.  1981.  Abuse and misuse of the term "guild" in
     ecological studies.  Oikos 37:397-400.
Johnson, D.H. and M.D. Schwartz. 1993.  The Conservation Reserve 
     Program and grassland birds.  Cons. Biol. 7:934-937.
Link, W. A., and J. R. Sauer.  1994.  Estimating equations
     estimates of trend.  Bird Populations 2:23-32.
Link, W. A., and J. R. Sauer.  1995.  Estimation of empirical
      mixing distributions in summary analyses.  Biometrics 
      51:810-821.
Link, W. A., and J. R. Sauer.  1996.  Extremes in ecology: avoiding
       the misleading effects of sampling variation in summary
       analyses.  Ecology 77:1633-1640.
Mannan, R. W., M. L. Morrison, and E. C. Meslow.  1984.  The use
     of guilds in forest bird management.  Wildl. Soc. Bull.
     12:426-430.
Peterjohn, B. G., and J. R. Sauer.  1993.  North American
     Breeding Bird Survey annual summary 1990-1991.
     Bird Populations 1:1-24.
Robbins, C.S., J.R. Sauer, R.S. Greenberg, and S. Droege.  
     1989. Population declines in North American birds 
     that migrate to the neotropics.  Proc. Natl. Acad. 
     Sci. USA 86:7658-7662.
Sauer, J. R., and S. Droege.  1992.  Geographic patterns in
     population trends of Neotropical migrants in North America.
     Pages 26-42 in J. M. Hagan, III and D. W. Johnston, eds.
     Ecology and conservation of Neotropical migrant landbirds.
     Smithsonian Inst. Press, Washington, D. C.

Sauer, J. R., B. G. Peterjohn, and Link, W. A.  1994.  Observer
     differences in the North American Breeding Bird Survey.  Auk
     111:50-62.