Renee Robinette Ha

Effects of ecology and prey characteristics on the use of alternative social foraging tactics in crows, Corvus caurinus

The producer–scrounger model assumes that producers are animals that search for food, and scroungers are animals that steal food found by producers. By following foraging decisions made by beach-foraging crows, we assessed individual variability in strategy use, variables influencing the finder’s share (when defined as the probability of retaining a prey item in the presence of food theft behaviour) and the benefits and costs of producing and scrounging. Data were collected using focal and all-occurrences scan sampling. Results indicated that more individuals were likely to use both producing and scrounging tactics than to be ‘pure’ producers. No pure scroungers were documented. When producers obtained scroungeable items, scroungers attempted to steal the item 46% of the time. Forty-one per cent of attempted scrounges were successful. Thus, producers suffered a 19% loss rate of scroungeable items to scroungers, although producers had access to large numbers of small (nonscroungeable) items. Although there were significantly more scroungeable prey items available at lower tides, there was no significant relationship between prey density and scrounge attempts or actual thefts. No ecological factors (group size, temperature, time of day, season) or individual characteristics (age, sex) influenced the probability of retaining prey. Prey retention was determined only by characteristics of the prey items: larger prey items or those with longer handling times were most likely to be stolen.  2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.

Cost–Benefit Analysis

Cost–benefit analysis predicts that if a behavior is adaptive, the benefits of a behavior must exceed the costs of that behavior. Fundamentally, the benefits and costs relate to fitness, though currencies such as time and energy are often used as proxies of fitness. The cost–benefit approach has been extended to model when this benefit-to-cost ratio is optimal and states that an individual should maximize the benefit of the behavior while simultaneously minimizing any costs associated with the behavior. Because specific predictions can be made from these models, this technique has widespread application in the adaptive study of behavior.

Population status and nest success of the Critically Endangered Mariana Crow Corvus kubaryi on Rota, Northern Mariana Islands

The Mariana Crow Corvus kubaryi is a Critically Endangered species found only on the island of Rota, Northern Mariana Islands. It was extirpated from the neighbouring island of Guam by the introduced brown tree snake Boiga irregularis and the Rota population has been in decline since at least 1995. We identified only 60 pairs present on Rota in 2007 compared with an estimated 117 pairs in 1998, a decline of nearly 50% in nine years. The decline may be linked to proximity to human activities, though more data are needed. We monitored 204 crow nests between the 1996 and 2009 breeding seasons. Crows initiate clutches between August and April. The overall estimate of nest success was 25.7% ( n = 204). On average 49% of pairs produced at least one fledgling per season. The mean number of fledglings per pair per year is 0.66. Mean clutch size was 2.6 ( n = 82), mean number of nestlings was 1.4 ( n = 106), and mean number of fledglings per nest was 1.3 ( n = 68). Daily survival rates declined in later years, and increased during the nest cycle. The number of pairs with successful nests did not change during the study period, nor did the number of fledglings per pair. Predation appeared to be the primary cause of nest failure. The breeding season lasted around nine months and pairs re-nested after failure. Predation of adults and juveniles by cats, combined with possible inbreeding depression, habitat disturbance and human persecution appear to be the cause of decline of the Mariana Crow. We strongly recommend a programme of invasive predator control, habitat maintenance, and captive rearing to ensure the species’ survival.

NEST SITE SELECTION AND CONSEQUENCES FOR REPRODUCTIVE SUCCESS OF THE ENDANGERED MARIANA CROW (CORVUS KUBARYI)

Abstract Reasons for the decline of the Mariana Crow (Corvus kubaryi) on the Western Pacific island of Rota are currently unknown, but a need to protect nesting habitat has been suggested. We examined 55 actual nest sites and 60 random sites from 1997 to 1999 to investigate habitat characteristics specific to crow nest sites. Both nests and random plots were predominantly in limestone forest habitat. Discriminant function analyses indicate actual nest sites were differentiated from random sites based on a higher percentage of canopy cover and mean DBH of papaya (Carica papaya) and woody vines, as well as a higher stem count of species associated with limestone forests. This resulted in correct classification of a potential site as nesting versus random in 92% of the cases. Actual nests were >300 m from buildings, while random sites averaged (± SE) 226.7 ± 71.6 m from a building. Actual nest sites were about twice as far from a road as random nest sites. Twenty-eight of the 55 active nests fledged young. Nests in native forests were associated with higher reproductive success than nests in more disturbed areas. These findings suggest that damage to habitat from anthropogenic or natural causes may be limiting nesting success.

Home Range Estimates of Feral Cats (Felis catus) on Rota Island and Determining Asymptotic Convergence

Abstract: Feral cats (Felis catus) have been shown to be a main contributor to species decline throughout the world and are especially threatening to insular species that lack appropriate defense characteristics. To mitigate the impact of feral cats on threatened species, space-use data are commonly used to design control strategies. In this article we report on the performance of GPS datalogging collars and provide baseline information on daily space use and home ranges of feral cats that threaten an endangered species on Rota Island in the Commonwealth of the Northern Mariana Islands. Using 100% Minimum Convex Polygon (MCP), average adult male home range was 1.32 km2 (n = 2) and average adult female home range was 0.22 km2 (n = 3). Home ranges were deemed fully revealed if asymptotes were approached using incremental analysis. Currently, there is no objective method for assessing where an asymptote is approached. Here, we describe a methodology to do so with the application of a Michaelis-Menten model to incremental data. We conclude that GPS datalogging collars are a viable tool for feral cat location data collection on Rota Island and that the Michaelis-Menten model is useful for determining asymptotic convergence of incremental location data.

Modeling Impacts of Hunting on Control of an Insular Feral Cat Population

Abstract: When introduced to exotic ecosystems, feral cats can inflict irreversible harm on native fauna. This is especially true in insular ecosystems because endemic vertebrate species often lack predator defenses. Feral cat control programs have been implemented on islands throughout the world with varied success. Effective and responsible management of pest populations requires knowledge of the impact of control actions. Here, we examine a feral cat control program created for the protection of the critically endangered Mariana crow on Rota Island in the Commonwealth of the Northern Mariana Islands. We apply a discrete form of the Schaefer model to a 29-month time series of removal data. We use a negative log likelihood framework to determine maximum likelihood parameter estimates and Akaikes Information Criterion for small sample size (AICc) analysis to determine the best-fitting model. The model indicated that the removal program on Rota initially reduced cat abundance from an estimated 1,218 to 952 individuals within the first 18 months and then maintained the population near 1,000 individuals for the following 11 months. Given the current level of available funds, we suggest that application of uniform island-wide hunting effort may not be the optimal strategy to maximize crow protection; rather, we suggest a multifaceted, targeted approach focused on areas of high crow activity.