A Horvitz-Thompson-type estimator is introduced to estimate total abundance of the Bering-Chukchi-Beaufort Seas population of bowhead whales using combined visual and acoustic location data. The estimator divides sightings counts by three correction factors that are themselves estimated from various portions of the data. The first correction models how detection probabilities depend on covariates like offshore distance and visibility. The second correction adjusts for availability using the acoustic location data to estimate a time-varying smooth function of the probability that animals pass within visual range of the observation stations. The third correction accounts for whales passing during periods when one or both sighting stations were temporarily closed down. We derive an asymptotically unbiased estimator of abundance incorporating all these components and a corresponding variance estimate. Correcting the count of 4011 observed whales yields a 2011 abundance estimate of 16,820 with a 95% confidence interval of (15,176, 18,643) and an estimated annual rate of population increase of 3.7% (2.9%, 4.6%). These results are indicative of very low conservation risk for this population under the current low levels of aboriginal hunting permitted by the International Whaling Commission. Although few other capture-recapture surveys will confront exactly the same set of challenges addressed here, many studies face one or more issues that could be resolved by adapting portions of our approach or relevant underlying concepts thereof. Moreover, the generic estimator we derive represents an improved way to handle random correction factors rather than assuming fixed values.
Estimating visual detection probabilities of migrating western Arctic bowhead whales from independent observers on two ice‐based perches presents challenges that cannot be resolved using standard capture‐recapture methods. Distant visual sighting and resighting of intermittent whale surfacings is very unlike recapturing banded birds or tagged fish. We develop several bias correction strategies that are essential to confront difficulties introduced by sighting ambiguities, match uncertainty, group size inconsistencies, and the survey protocol. These are implemented using a weighted likelihood adaptation of a standard capture‐recapture model. This model estimates the relationship between detection probabilities and covariates associated with the sighting. We present a complete analysis of the bowhead example and discuss how the data configuration, weighting, and estimation methods presented here highlight issues that are confronted in the analysis of many other complex capture‐recapture datasets. Copyright © 2014 John Wiley & Sons, Ltd.
The diets of Steller (Eumetopias jubatus) and California (Zalophus californianus) sea lions in northwest Washington are poorly documented. We hypothesized that these species exploit the same prey in Washington because they are both generalist predators that utilize the same haul-out sites and are similar in behavior and body size. We analyzed 776 samples of scat from Steller sea lions and 263 samples of scat from California sea lions collected throughout each year during 2010-2013. The aim of this analysis was to characterize seasonal and annual diets, estimate biomass of prey consumed, and evaluate dietary niche overlap. Steller and California sea lions ate diverse diets that varied seasonally and annually. Primary prey groups for both sea lion species were Clupeidae, Salmonidae, Sebastidae, Rajidae, Pleuronectiformes, Squalidae, and Merlucciidae. We estimated that Steller sea lions ate 11,327 metric tons (t) (standard deviation [SD] 1600) and that California sea lions ate 9063 t (SD 4098) of prey per year during our study. We found significant dietary niche overlap between California and Steller sea lions that feed in northwest Washington.
Dickcissel (Spiza americana) males occupying territories in cropland sites produced songs that were less similar on average to other Dickcissel songs in their neighborhood than did Dickcissels living in grasslands, where conformity to the local vocal culture was higher. Further, Dickcissel vocal culture changed more quickly over time in cropland sites relative to grassland sites. These differences may have resulted from the lower site fidelity we observed in Dickcissel males in cropland sites relative to grassland sites. We expected this link with site fidelity because we hypothesized that conformity to local culture in Oscine songbirds and the persistence of culture over time and space are promoted by habitats that facilitate stable populations. In contrast, sites in which habitat features cause rapid population turnover provide more territory vacancies and so more opportunities for colonization. Colonization should drive cultural change, either through adult colonists importing foreign cultural variants or young colonists making errors as they learn the local song. This potential link between population turnover and cultural stability may apply to animal cultures more broadly and so may be a fruitful area for further research. Besides the link between site fidelity and cultural change over time, we also investigated the possibility that habitats with different levels of site fidelity might show differences in the spatial scale of song similarity. However, we found no evidence of such a difference. Finally, although our conclusions regarding conformity and change in vocal culture were based on many recorded songs, automated assessments of song similarity imprecisely estimated the overall degree of song similarity. Thus, we may have underestimated the strength of the effects of time and distance on song similarity.
Animal culture often shows geographic structure, with individuals in close proximity sharing more cultural features than individuals further apart. However, spatial extent of cultural features, along with the degree of conformity to local cultures, vary within and among species. Further, rates of cultural change presumably also vary, though documentation of temporal variability lags behind documentation of spatial variability. Understanding both spatial and temporal variation is essential to understanding cultural evolution, but mechanisms likely to be driving this variation have not been sufficiently explored. We hypothesized that conformity to local culture in Oscine songbirds and the persistence of culture over time and space are promoted by habitats that facilitate stable populations in which individuals show relatively high site fidelity. In contrast, sites in which habitat features cause rapid population turnover provide more vacant territories and so more opportunities for colonization. Colonization should drive more rapid cultural change, either through adult colonists importing foreign cultural variants or young colonists making errors as they learn the local song. To test this set of hypotheses, we examined temporal and spatial variation in vocal culture in a songbird (dickcissel, Spiza americana) in two distinct habitat types. As predicted, we found high site fidelity in relatively stable native grasslands and much lower site fidelity in nearby cropland sites which were disturbed by farming practices during the breeding season. We also found evidence of higher levels of song sharing and slower changes in vocal culture in our grasslands relative to croplands, though we found no evidence of different spatial scales of song sharing between these habitats. This is the first study we know of correlating the temporal rate of cultural change to differences in a demographic factor between habitats. Although our conclusions are based on many recorded songs, automated assessments of song similarity were imprecise and so our results here underestimate the overall degree of song sharing and thus possibly the strength of the effects of time and distance on this sharing. Further, because we examined song sharing at only seven sites, firm conclusions about site fidelity and song sharing will require larger samples in the future.
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