- University of Tennessee
We present and compare demographic data for cerulean warblers (Dendroica cerulea) from 5 study sites across the range of the species from 1992 to 2006. We conducted field studies to collect data on daily nest survival, nest success, and young fledged per successful nest, and we used data to estimate fecundity. Daily nest survival, nest success, young fledged, and fecundity varied widely across the cerulean range and among years. Study sites in agriculture-dominated landscapes (Mississippi Alluvial Valley, IN, and MI, USA) had negative growth rates in all years monitored because measured values of nest success and young produced per successful nest were incapable of offsetting apparent mortality. Ontario (Canada) and Tennessee (USA) populations had greater nest success and fecundity but still appeared to be incapable of producing stable populations (λ = 1) under field-measured and assumed conditions. We had survival data only for one site (Ontario); thus, additional survival data are greatly needed to enable more reliable estimates of population growth. Conservation strategies for cerulean warblers in agriculture-dominated landscapes (e.g., Mississippi Alluvial Valley, IN, and MI) may require major landscape-level habitat reconfiguration to change agriculture-dominated landscapes to forest-dominated landscapes to increase fecundity. Conservation strategies in predominantly forested landscapes in the core of the range (e.g., TN) require a focus on minimizing habitat loss and developing management prescriptions capable of improving fecundity. In both cases, based on sensitivity and elasticity analyses, efforts to improve survival during the nonbreeding season would have the greatest positive effect on population growth.
We located cerulean nests by observing parental behavior, including following females with nest-building material, locating females that chipped from the nest during incubation, and observing adults carrying food to nestlings. We monitored nests every 2-3 days to determine nest fate (failure or fledging) on the 5 study sites across a range of years from 1992 to 2006, depending on the site. We checked nests daily in the latter stages of the nestling cycle as fledging became imminent. We used the Mayfield (1975) method to calculate daily nest survival rates based on nest exposure days. We used Program Contrast (Hines and Sauer 2004) to compare cerulean daily nest survival rates (DSR) among study sites, for all years pooled within individual sites. We calculated Mayfield estimates of nest success by raising DSR to the 25th power, based on number of exposure days in the typical nesting cycle (4 days laying, 11 days incubation, and 10 days nestling stage). We determined number of young fledged from successful nests by using binoculars and spotting scopes to count young in the nest on the day prior to fledging. In Ontario, we also counted young postfledging to confirm nestling counts. For each study site, except the MAV, we calculated annual values and an overall mean and standard error (Johnson 1979) for Mayfield (1975) nest success and young fledged per successful nest. For the MAV site, we collected data from 3 areas (Meeman Shelby Forest, Chickasaw National Wildlife Refuge, and Desha Delta Hunt Club) from 1992 to 2005, but data for individual years were generally too limited to calculate annual parameter estimates. As a result, we simply calculated a total estimate of nest success for each MAV site alone with all data pooled. We limited number of nesting attempts in the analysis to 3 attempts to produce one brood. We followed the recommendation of Gryzbowski and Pease (2005) by basing this parameter estimate on observed nesting behavior, observed length of the breeding season (approx. 60 days), and time needed for renesting (5 days), constrained by nesting success (Ricklefs 1973).