Age-specific survival of male Golden-cheeked Warblers on the Fort Hood Military Reservation, Texas

Abstract

ABSTRACT. Population models are essential components of large-scale conservation and management plans for the federally endangered Golden-cheeked Warbler (Setophaga chrysoparia; hereafter GCWA). However, existing models are based on vital rate estimates calculated using relatively small data sets that are now more than a decade old. We estimated more current, precise adult and juvenile apparent survival (Φ) probabilities and their associated variances for male GCWAs. In addition to providing estimates for use in population modeling, we tested hypotheses about spatial and temporal variation in Φ. We assessed whether a linear trend in Φ or a change in the overall mean Φ corresponded to an observed increase in GCWA abundance during 1992-2000 and if Φ varied among study plots. To accomplish these objectives, we analyzed long-term GCWA capture-resight data from 1992 through 2011, collected across seven study plots on the Fort Hood Military Reservation using a Cormack-Jolly-Seber model structure within program MARK. We also estimated Φ process and sampling variances using a variance-components approach. Our results did not provide evidence of site-specific variation in adult Φ on the installation. Because of a lack of data, we could not assess whether juvenile Φ varied spatially. We did not detect a strong temporal association between GCWA abundance and Φ. Mean estimates of Φ for adult and juvenile male GCWAs for all years analyzed were 0.47 with a process variance of 0.0120 and a sampling variance of 0.0113 and 0.28 with a process variance of 0.0076 and a sampling variance of 0.0149, respectively. Although juvenile Φ did not differ greatly from previous estimates, our adult Φ estimate suggests previous GCWA population models were overly optimistic with respect to adult survival. These updated Φ probabilities and their associated variances will be incorporated into new population models to assist with GCWA conservation decision making. Survie en fonction de l'âge des Parulines à dos noir mâles sur la réserve militaire de Fort Hood, Texas RÉSUMÉ. Les modèles de population sont des éléments essentiels des plans de gestion et de conservation à grande échelle pour la Paruline à dos noir (Setophaga chrysoparia; abrégée PADN ci-dessous), désignée « menacée » par le gouvernement fédéral étatsunien. Toutefois, les modèles existants sont fondés sur des estimations de taux vitaux calculées à partir d'échantillons relativement petits qui datent maintenant de plus d'une décennie. Nous avons actualisé et précisé la probabilité de survie apparente (Φ) adulte et juvénile et calculé les variances associées pour les mâles PADN. En plus de ces estimations destinées à la modélisation des populations, nous avons testé les hypothèses de variations spatiale et temporelle de Φ. Nous avons évalué si une tendance linéaire de Φ ou un changement de Φ moyen correspondait à une augmentation réelle du nombre de PADN de 1992 à 2000, et si Φ avait varié entre les parcelles échantillonnées. Pour atteindre nos objectifs, nous avons analysé les données...

Population Viability of Golden‐cheeked Warblers in an Urbanizing Landscape

“…Previous population models have used juvenile survival estimates ranging from 0.28 to 0.50 (USFWS 1996, Alldredge et al 2004, Horne et al 2011, Duarte et al 2016 a ). When we used a value of 0.28 for juvenile survival, which represented approximately one‐half of our adult survival, and was the recent mean juvenile survival estimate for Fort Hood (Duarte et al 2014), the golden‐cheeked warbler population declined precipitously toward local extinction. We determined that a juvenile survival rate of approximately 0.40 was required to achieve a realized λ of 1 when adult survival and productivity were at baseline levels.…”

“…We determined that a juvenile survival rate of approximately 0.40 was required to achieve a realized λ of 1 when adult survival and productivity were at baseline levels. Apparent juvenile survival (which is confounded with potentially high natal dispersal) ranged from 0.17 to 0.46 annually, and confidence intervals included 0.40 for several years, on Fort Hood (Duarte et al 2014). Therefore, we believe a juvenile survival rate of 0.40 is biologically reasonable and feasible, at least in some years.…”

“…The BCP population was fairly stable when adult survival, juvenile survival, and productivity were high, but declined toward local extinction at low values. The ranges we used spanned the range of vital rates we estimated for the population from 2009 to 2015, as well as those estimated for other populations (Duarte et al 2014, Peak and Thompson 2014, Reidy et al 2016 b ). Importantly, the BCP population was predicted to decline using similar vital rates as those estimated for Fort Hood and elsewhere (Marshall et al 2013; Peak and Thompson 2014; Duarte et al 2016 a , b ).…”

Estimating adult sex ratios in nature

“…Conversely, bias in ASR estimates towards males may exist in many animal populations where males are easier to detect or catch than females [33]. In most songbird populations, for example, males may be more easily detected (and counted) than females due to their brighter plumage colours and their noticeable visual and vocal displays associated with territorial defence [62,63]. Male songbirds can also be surveyed or caught more frequently because they have larger home ranges or are more prone to prospect new territories than females, or because females often take care of all incubation duties, causing them to be on nests and unavailable for detection [34].…”

“…Therefore, counts obtained from stopover sites risk producing estimates that may not reflect the sex ratio of a population as a whole, but instead reflect sex differences in migratory patterns [71]. Furthermore, when species exhibit sexual segregation with regard to habitat, site-specific detection and capture probabilities can be biased towards the sex concentrated in the habitat where detection or capture is favoured, even if encounter probabilities are otherwise equal between the sexes [63].…”