Survival Estimates for Snowy Plovers Breeding at Great Salt Lake, Utah

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Current knowledge of the breeding biology of Snowy Plovers (Charadrius alexandrinus) in
). Jolly-Seber models use resightings to estimate survival probabilities for birds that were not resighted but were actually still alive.The purpose of this paper is to determine annual survival estimates for Snowy Plovers at Great Salt Lake, Utah, using a hierarchical modeling approach recently developed by Lebreton et al. (1992).2).This model postulated that survival rates were time-dependent and resighting rates were sex-specific.Based on this model, survival rates tended to be relatively high during the winters 1990-1991 and 1992-1993, and low in the winter of 1991-1992 (Table 3).The best estimate for the average annual survival probability for an adult plover, 0.687, was determined using the Jolly-Seber model (S, P,; i.e., a model with constant survival and sex-specific resighting probabilities).

METHODS
Males had a significantly higher probability of being resighted than females (Table 3).Given this mean overall survival estimate, mean life expectancy for an adult plover (i.e., >-year old; Warriner et al. 1986) was 2.7 years.Few resightings of immature plovers resulted in imprecise survival estimates, resulting in large confidence intervals around the mean survival estimate of 0.385 (Table 3).

DISCUSSION
Snowy Plovers breeding at Great Salt Lake are migratory and winter around the Gulf of California and the west coast of Baja California (G.Page, M. Stern, and P. Paton, unpubl.data).The mean survival estimate I calculated for adult plovers in northern Utah (0.687) was lower than a minimum survival estimate for a population of Snowy Plovers consisting of both migratory and resident individuals in coastal California (0.752; Warriner et al. 1986).However, annual variation in survival estimates at Great Salt Lake (0.578-0.880;Table 3) encompassed the coastal California estimate.The average survival estimate for Snowy Plovers at Great Salk Lake was similar to a Jolly-Seber survival estimate for a migratory population of Piping Plovers (Charadrius melodus) nesting in North Dakota (0.664; Root et al. 1992).Root et al. (1992) found that annual survival probabilities for Piping Plovers fluctuated somewhat dramatically between years, although there was not significant annual variation due to large confidence intervals around yearly estimates.I also found that survival probabilities varied annually, but I could not differentiate between permanent emigration and increased mortality.It is possible that the low survival estimates for the winter of 1991-1992 were due to permanent emigration from Great Salt Lake, rather than higher mortality rates.Therefore, if permanent emigration was a relatively common event for Snowy Plovers at Great Salt Lake, then the survival estimates presented here underestimate true survival rates.Given the long-distance intraand inter-year movements patterns documented for this species (Stenzel et al., in press), it is quite possible that permanent emigration is a likely event for plovers breeding at Great Salt Lake.
There were not enough resightings of immature plovers (both nestlings and fledglings) at Great Salt Lake to accurately estimate survival for that cohort.
one observer in 1990 and two observers from 1991 to 1993.The other six sites were visited 1-4 times per month.Incubating birds were trapped with a circular funnel trap (Lessells 1984); others were captured 0.1-1 km ' Received 5 April 1994.Accepted 26 July 1994.
interaction on S and P a Model notation: S = survival probability, P = resighting probability, s = sex, t = time, x = interaction term.b Number of estimable parameters.c L = likelihood function.d Akaike's Information Criterion = 2np + -2 In L. This content downloaded on Wed, 6 Mar 2013 15:00:45 PM All use subject to JSTOR Terms and Conditions The only other published juvenile survival estimate comes from Page et al. (1983), who subsequently resighted 9 of 14 (64.3%)fledglings banded in coastal California.The estimate by Page et al. (1983) was based on fledglings; therefore, it was higher than the survival estimate that I calculated for immature plovers (i.e., both chicks and fledglings combined) in Utah.However, survival estimates for immature plovers were lower in California than Utah.Page et al. (1983) estimated a fledging rate of 0.546 chicks per successful brood at Mono Lakethis study.Because variations in immature survival estimates can have dramatic impacts on estimates of the finite rate of population change, it would be helpful to gather more data to obtain a better estimate of immature Snowy Plovers survival rates in western North America.As with Piping Plovers in the Great Plains (Root et al. 1992, Ryan et al. 1993), little is known about mortality factors for immature and adult Snowy Plovers at Great Salt Lake.I only observed a source of juvenile mortality once, when an American Kestrel (Falco sparverius) captured a two-week old chick.Red foxes (Vulpes vulpes) and Northern Harriers (Circus cyaneus) were other common, potential predators for juveniles in northern Utah (Paton, pers.observ.).In addition, Peregrine Falcons (Falco peregrinus) have been re-introduced to northern Utah by the Division of Wildlife, using a series of 10 hack towers around Great Salt Lake.The remains of several adult Snowy Plovers have been found at the base of these towers (Don Paul, Utah Division of Wildlife Resources, pers.comm.).Obviously, much more needs to be learned about the factors affecting Snowy Plover survival rates, especially in light of their recent listing as a threatened species.Financial support for this project was provided by the Native Wildlife Section of the Utah Division of Wildlife Resources.Jennifer Levy, Jack Dalton, and Craig Kneedy helped with the fieldwork.Thomas C. Edwards Jr. helped with statistical and editorial advice.I thank Frank Messina, Mark Ritchie, Kim Sullivan, John Kadlec, and two anonymous reviewers for provided useful suggestions to help improve the manuscript.Finally, I thank David R. Anderson for providing invaluable help with the survival analyses using program or indeterminate egg layer were conducted during 1990.One egg per nest was added or removed after the second, third, or fourth egg was laid in a nest, either on the day of laying or on the next day.The added eggs were taken from other nests in the vicinity.Although Kennedy (1991) and Haywood ' Received 12 April 1994.Accepted 28 July 1994.

assumed that resightings were equivalent to recaptures. These two tests examined the assumptions that birds released on different occasions had equivalent survival and re- sighting probabilities and whether or not sparse data could be pooled for further analyses (see Burnham et al. 1987, Lebreton et al. 1992 for further details). I then used Program SURGE (Lebreton et al. 1992) for model fitting and calculating likelihood ratio tests. Model se- lection was based on Akaike's Information Criterion (AIC), which was calculated from the maximum log- likelihood plus the model's number of estimable pa- rameters. The model with the lowest AIC was then selected as the best fitting CJS model, because AIC provides an unbiased criterion for model selection (Le- TABLE 2. Potential annual survival rate models for adult Snowy Plovers at Great Salt Lake.
Ssx., PS,x