George L. Farnsworth

A REMOVAL MODEL FOR ESTIMATING DETECTION PROBABILITIES FROM POINT-COUNT SURVEYS

Abstract Use of point-count surveys is a popular method for collecting data on abundance and distribution of birds. However, analyses of such data often ignore potential differences in detection probability. We adapted a removal model to directly estimate detection probability during point-count surveys. The model assumes that singing frequency is a major factor influencing probability of detection when birds are surveyed using point counts. This may be appropriate for surveys in which most detections are by sound. The model requires counts to be divided into several time intervals. Point counts are often conducted for 10 min, where the number of birds recorded is divided into those first observed in the first 3 min, the subsequent 2 min, and the last 5 min. We developed a maximum-likelihood estimator for the detectability of birds recorded during counts divided into those intervals. This technique can easily be adapted to point counts divided into intervals of any length. We applied this method to unlimited-radius counts conducted in Great Smoky Mountains National Park. We used model selection criteria to identify whether detection probabilities varied among species, throughout the morning, throughout the season, and among different observers. We found differences in detection probability among species. Species that sing frequently such as Winter Wren (Troglodytes troglodytes) and Acadian Flycatcher (Empidonax virescens) had high detection probabilities (∼90%) and species that call infrequently such as Pileated Woodpecker (Dryocopus pileatus) had low detection probability (36%). We also found detection probabilities varied with the time of day for some species (e.g. thrushes) and between observers for other species. We used the same approach to estimate detection probability and density for a subset of the observations with limited-radius point counts.

Factors Affecting Nesting Success of Wood Thrushes in Great Smoky Mountains National Park

Recent evidence suggests that the nesting success of forest-interior Neotropical migrants is lower in fragmented habitat. We examined the nesting success of Wood Thrushes (Hylocichla mustelina) in a large contiguous forest from 1993 to 1997. From a sample of 416 nests we tested for predictors of daily nest survival rates, including activity at the nest and vegetation parameters at the nest site. We tested whether disturbance during nest checks (as measured by the behavior of the adults) was related to subsequent nest predation. Females were more likely to vocalize when brooding chicks than when incubating eggs. However, we found no evidence that observer disturbance or Wood Thrush activity influenced daily nest survival rates. Wood Thrushes nested predominately in small hemlocks, generally surrounded by many other small hemlocks. However, survival rates of nests in hemlocks were not significantly different from those in other substrates. Overall, neither activity at the nest nor habitat in the vicinity of the nest was a good predictor of nesting success, and only one vegetation characteristic, a measure of concealment, was significantly correlated with successful nesting. Brood parasitism by Brown-headed Cowbirds (Molothrus ater) was extremely low (<2% of nests parasitized). However, nesting success was moderate (daily survival rate = 0.958) when compared with other published studies from more-fragmented landscapes. Our results suggest that daily nest survival rates do not increase monotonically from small to very large forest patches.

OBSERVATIONS OF WOOD THRUSH NEST PREDATORS IN A LARGE CONTIGUOUS FOREST

Abstract We used inexpensive (<

VALIDATING THE ASSUMPTIONS OF THE MAYFIELD METHOD

30) cameras to document predators at active Wood Thrush (Hylocichla mustelina) nests in Great Smoky Mountains National Park. We observed such predators as black rat snakes (Elaphe obsoleta), American Crows (Corvus brachyrhynchos), southern flying squirrels (Glaucomys volans), and black bears (Ursus americanus) remove the contents of nests. Camera installation had no measurable effect on nest survival; daily nest survival was approximately 0.96 for nests with and without cameras. However, placement of an artificial egg trigger in the nest appeared to reduce hatching success. The immobile egg trigger might have interfered with the female Wood Thrushs ability to incubate her eggs. The variety of nest predators observed and the moderate daily survival rates recorded suggest that predation is an important constraint on Wood Thrushes nesting in large contiguous forests.

Large scale wildlife monitoring studies: statistical methods for design and analysis

Abstract We tested the assumptions of the Mayfield method (Mayfield 1975) using 416 Wood Thrush (Hylocichla mustelina) nests monitored from 1993–1997 in the Great Smoky Mountains National Park. We tested for differences in nest survival throughout the breeding season and throughout the nesting cycle. We compared the Mayfield cumulative survival curve with a modified version of the Kaplan-Meier with staggered entry. In addition, we compared the group of nests found during nest initiation with those found later in the nesting cycle. There was no evidence nest survival varied through the breeding season or with different stages of the nesting cycle. Furthermore, the Kaplan-Meier survival curve agreed with the Mayfield calculation. We found no evidence of heterogeneity. Validation of Mayfield assumptions is important because the method is widely used, allows for statistical comparisons, and is critical for estimating seasonal fecundity in bird populations.