Behavioral components of detection underlying bias in population estimation of the golden-cheeked warbler, Dendroica chrysoparia.

Background/Question/Methods: The need for accurate and unbiased estimates of population density or abundance has spurred the development of occupancy and N–mixture models which estimate occupancy or abundance corrected for imperfect detection. An important source of bias for these models is spatial heterogeneity in the probability of detection due to variation in density itself. We tested the hypothesis that two components of singing behavior that affect detectability (song rate and song type) scale disproportionately with increasing population density in the endangered golden-cheeked warbler, Dendroica chrysoparia. In 2009 using autonomous recording units we estimated the average rate of singing and use of two song types at fixed detection stations within six study sites located within the Balcones Canyonlands Preserve, Austin, Texas (USA). Independent estimates were used to establish among-site variation in the density of territorial male warblers. Results/Conclusions: We found that singing rate per detection station increased disproportionately with increasing density of territorial males. Mechanisms underlying the relationship between density and average song rate per detection station include changes in the singing behavior and (or) territorial movements of individual males. In 2010 we tested the hypotheses that both the number of neighboring territorial males and the overall number of territorial males per study plot affected the singing behavior of male warblers by recording the song rate and song type of individual focal males at each study site. At the level of the individual we found no effect of either aspect of territorial male density on singing behavior. However, analysis of independently collected spot-mapping data from 2009 indicated that both territory size and degree of territorial overlap were significantly affected by density; territory size decreased and territory overlap increased with greater density. Thus, an alternative mechanism for the relationship between average song rate and density may be due to an increase in the probability that a male bird is present at a given survey station at higher density sites as a result of an increase in the average number of territories surrounding each station. Overall the results this study indicate that (1) a density-detectability correlation may be present in this system indicated by an effect of density on average song rate at the population level and (2) this effect may be due to a decrease in average territorial movements per male as density increases.