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 present 2 approaches to summarizing patterns of population change for groups of species: (1) a summary of regional patterns of change, and (2) a region-free smoothed map of population change.

Both approaches use information about population trends for individual species, estimated using the estimating-equations approach (Link and Sauer 1994) on each survey route. However, they differ greatly in how information is combined among species and regions.

Approach 1: Regional summaries of percentage of species

with increasing populations

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, and variances of the percentage are estimated using bootstrapping. See Link and Sauer (In Press) for a detailed description of the procedure. We use a z-test to evaluate the null hypothesis that the percentage of species with increasing trends does not differ from 50.

We present the information for trends estimated for the entire survey area, for the interval 1966 - 1994. In this summary analyses, results are presented for species seen on >14 routes.

Approach 2: Smoothed maps of mean trends for groups

on individual survey routes

This approach involves estimating a measure of population change for each species in the guild for each route, summarizing the change for the guild at the route, and using mapping procedures to display regional patterns of change in the guild. This approach is extremely flexible, can incorporate any method of estimating and summarizing change within routes, and circumvents criticisms about inappropriate grouping of regional patterns.

To actually implement this approach, we estimate trends over the interval of interest for each species on each route (Link and Sauer 1994), incorporating necessary within-route covariables (Sauer et al. 1994). Then, we average the trends (transformed to log scale) by route for each species in the group of interest, to get a mean trend and its variance. These means are then geographically-smoothed in the same way that the trend maps are smoothed from route-specific data, and displayed on a background map of the survey area.

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