Count trends for migratory Bald Eagles reveal differences between two populations at a spring site along the Lake Ontario shoreline.

Kyle R Wright

doi:10.7717/peerj.1986

Abstract

The recovery of Bald Eagles (Haliaeetus leucophalus), after DDT and other organochlorine insecticides were banned in the United States, can be regarded as one of the most iconic success stories resulting from the Endangered Species Act. Interest remains high in the recovery and growth of the Bald Eagle population. Common to evaluating growth and recovery rates are counts at nesting sites and analyses of individuals fledged per season. But this is merely one snapshot that ignores survival rates as eagles grow to maturity. By analyzing indices from migration counts, we get a different snapshot better reflecting the survival of young birds. Different populations of Bald Eagles breed at different sites at different times of the year. Typical migration count analyses do not separate the populations. A separation of two distinct populations can be achieved at spring count sites by taking advantage of the tendency for northern summer breeding birds to migrate north in spring earlier than southern winter breeding birds who disperse north later in spring. In this paper I analyze migratory indices at a spring site along Lake Ontario. The analysis shows that eagles considered to be primarily of the northern summer breeding population showed an estimated growth rate of 5.3 ± 0.85% (SE) per year with 49% of eagles tallied in adult plumage, whereas the migrants considered to be primarily of the southern breeding population had an estimated growth rate of 14.0 ± 1.79% with only 22% in adult plumage. Together these results argue that the populations of southern breeding Bald Eagles are growing at a substantially higher rate than northern breeding eagles. These findings suggest that aggregate population indices for a species at migration counting sites can sometimes obscure important differences among separate populations at any given site and that separating counts by time period can be a useful way to check for differences among sub-populations.

Highlights

Data from raptor migration counts can provide scientists and managers insight into a population’s well-being and dynamics (Kerlinger, 1989; Goodrich & Smith, 2008)
The results of this analysis show that, over the twenty-five year period from 1991 to 2015, the annual growth rate for Bald Eagles passing over Derby Hill Bird Observatory (DHBO) before the end of March is 5.3% while the growth rate for Bald Eagles passing over DHBO after May 10 is 14% and that the difference between these two rates is statistically significant (Table 1)
Bald Eagles on their way to north to breed during summer months are likely passing over DHBO in the months of February, March and April, and are unlikely to be making that passage at late as May 11 (Buehler, 2000; Wheeler, 2003)

Summary

Introduction

Data from raptor migration counts can provide scientists and managers insight into a population’s well-being and dynamics (Kerlinger, 1989; Goodrich & Smith, 2008). The landmark 2008 State of North America’s Birds of Prey (Bildstein et al, 2008) contains analyses of migration counts based largely on fall sites, and Farmer and Smith’s 2010 follow up uses similar statistics to analyze spring migration data and indices. These studies, like many others, while quite robust in some respects, fail to do much analysis on the two distinctive populations of Bald Eagles encountered at many migration count sites in the Mid-Atlantic and Northeastern regions of the United States. This paper contains an investigation into the consequences of separating the analysis of the two populations at one spring migration site

Methods

Results

Conclusion