The GTNP Breeding Bird Monitoring Project: Covariation Among Sites in Breeding Densities

Abstract

The breeding densities of migrant birds are subject to a wide range of influences that may cause their variation, within a site from one year to another, within years but among habitats and sites from one location to another, and also among species with different migration strategies in terms of travel distances and wintering habitats. First measuring, and then understanding, this variation and its drivers is a substantial challenge for breeding bird monitors and population ecologists. Variation in breeding bird densities in Grand Teton National Park have been monitored since the early 1990's, following protocols instigated by M. Cody & S. Cain (1995 NPS Report). Of the thirty monitoring sites established by this report, one half to two-thirds have been censused yearly up to the present time, and a subset of the sites has been monitored yearly without discontinuities. Thus for many sites there is a census history of a decade or more, forming a data base that now approaches statistical adequacy for testing hypotheses about patterns of variation in breeding bird densities.This report presents preliminary data on the covariation of breeding densities, within and between species, over the various monitoring sites. Given that many of the breeding birds leave GTNP in the non­breeding season and overwinter elsewhere, early summer GTNP breeding densities are likely a consequence both of off-site conditions (winter survival and migration success) and on-site resources in the breeding habitat, likely in part weather-related. We ask questions such as: a) are there years when breeding densities are substantially higher than in other years? Are high-density years typical of many migrant species simultaneously, or do they occur independently over time among species? For a given species in a high-density year, are all breeding/monitoring sites occupied at higher density, with positive correlations among sites, or are some sites negatively correlated? Is the range of sites occupied by breeding birds greater in years when densities reach higher values in the most favored sites?