M.L. Legare | USGS Science

USGS2012

Habitat fragmentation effects on annual survival of the federally protected eastern indigo snake

The eastern indigo snake (Drymarchon couperi) is a federally listed species, most recently threatened by habitat loss and habitat degradation. In an effort to estimate snake survival, a total of 103 individuals (59 males, 44 females) were followed using radio-tracking from January 1998 to March 2004 in three landscape types that had increasing levels of habitat fragmentation: (1) conservation cores; (2) conservation areas along highways; (3) suburbs. Because of a large number of radio-tracking locations underground for which the state of snakes (i.e. alive or dead) could not be assessed, we employed a multistate approach to model snake apparent survival and encounter probability of live and dead snakes. We predicted that male snakes in suburbs would have the lowest annual survival. We found a transmitter implantation effect on snake encounter probability, as snakes implanted on a given occasion had a lower encounter probability on the next visit compared with snakes not implanted on the previous occasion. Our results indicated that adult eastern indigo snakes have relatively high survival in conservation core areas, but greatly reduced survival in conservation areas along highways and in suburbs. These findings indicate that habitat fragmentation is likely to be the critical factor for species' persistence.

Animal Conservation↗

USGS1999

Vocalization behavior and response of black rails

We measured the vocal responses and movements of radio-tagged black rails (Laterallus jamaicensis) (n = 43, 26 males, 17 females) to playback of vocalizations at 2 sites in Florida during the breeding seasons of 1992-95. We used regression coefficients from logistic regression equations to model the probability of a response conditional to the birds' sex, nesting status, distance to playback source, and the time of survey. With a probability of 0.811, non-nesting male black rails were most likely to respond to playback, while nesting females were the least likely to respond (probability = 0.189). Linear regression was used to determine daily, monthly, and annual variation in response from weekly playback surveys along a fixed route during the breeding seasons of 1993-95. Significant sources of variation in the linear regression model were month (F = 3.89, df = 3, p = 0.0140), year (F = 9.37, df = 2, p = 0.0003), temperature (F = 5.44, df=1, p = 0.0236), and month*year (F = 2.69, df = 5, p = 0.0311). The model was highly significant (p < 0.0001) and explained 53% of the variation of mean response per survey period (R2 = 0.5353). Response probability data obtained from the radio-tagged black rails and data from the weekly playback survey route were combined to provide a density estimate of 0.25 birds/ha for the St. Johns National Wildlife Refuge. Density estimates for black rails may be obtained from playback surveys, and fixed radius circular plots. Circular plots should be considered as having a radius of 80 m and be located so the plot centers are 150 m apart. Playback tapes should contain one series of Kic-kic-kerr and Growl vocalizations recorded within the same geographic region as the study area. Surveys should be conducted from 0-2 hours after sunrise or 0-2 hours before sunset, during the pre-nesting season, and when wind velocity is < 20 kph. Observers should listen for 3-4 minutes after playing the survey tape and record responses heard during that time. Observers should be trained to identify black rail vocalizations and should have acceptable hearing ability. Given the number of variables that may have large effects on the response behavior of black rails to tape playback, we recommend that future studies using playback surveys should be cautious when presenting estimates of 'absolute' density. Though results did account for variation in response behavior, we believe that additional variation in vocal response between sites, with breeding status, and bird density remains in question. Playback surveys along fixed routes providing a simple index of abundance would be useful to monitor populations over large geographic areas, and over time. Considering the limitations of most agency resources for webless waterbirds, index surveys may be more appropriate. Future telemetry studies of this type on other species and at other sites would be useful to calibrate information obtained from playback surveys whether reporting an index of abundance or density estimate.

Book chapter↗