JUVENILE AND ADULT SURVIVAL IN THE SOCIABLE WEAVER (PHILETAIRUS SOCIUS), A SOUTHERN-TEMPERATE COLONIAL COOPERATIVE BREEDER IN AFRICA

Auk, The, Oct 2004 by Covas, Rita, Brown, Charles R, Anderson, Mark D, Brown, Mary Bomberger

ABSTRACT.-

We used capture-recapture analyses to describe juvenile and adult survival from 1993 to 2001 in a population of Sociable Weavers (Philetairus socius), a colonial, cooperatively breeding passerine of southern Africa. We examined temporal variation in survival and the role that the breeding season's length and environmental factors play in determining survival patterns in the population. Annual survival probability (mean ± SE) was 0.66 ± 0.02. In contrast to most passerines, juveniles and adults had similar survival probabilities; survival rates did not vary significantly between years. We found no relationship among temperature, rainfall, and survival. Relatively high survival rates in Sociable Weavers probably result from a benign climate and easy access to food in winter. Juvenile survival may also be enhanced by prolonged parental care and delayed dispersal. Received 1 April 2003, accepted 24 June 2004.

RESUMEN. - Utilizamos análisis de captura y recaptura para describir la supervivencia de juveniles y adultos entre 1993 y 2001 en una población de Philetairus socius, una especie de ave pascrina colonial con cría cooperativa del sur de África. Examinamos la variación temporal en la supervivencia, y la importancia de la longitud del período de apareamiento y de los factores ambientales para la supervivencia de los individuos de la población. La probabilidad de supervivencia anual (media ± EE) fue de 0.66 ± 0.02. En contraste con la mayoría de los Passeriformes, los individuos juveniles y adultos mostraron probabilidades de supervivencia similares, y las tasas de supervivencia no variaron significativamente entre anos. No encontramos relación alguna entre la temperature, la cantidad de lluvia y la supervivencia. La relativamente alta probabilidad de supervivencia observada se encuentra probablemente relacionada con un clima benigno y fácil acceso al alimento durante el invierno. La supervivencia de los juveniles puede además estar influenciada positivamente por un período de cuidado parental prolongado y por la dispersión retardada de las crías.

COMPARATIVE STUDY OF life-history evolution requires knowing how survival varies with age and environmental conditions (Stearns 1992, Charlesworth 1994). Studying survival, however, is often difficult, because individuals must be marked and monitored for multiple years. In birds, we know especially little about annual survival rates of populations in the tropics and the Southern Hemisphere, where relatively few long-term population studies have been conducted. Because birds in southern regions generally have smaller clutches, longer developmental periods, and higher adult survival than their northern-temperate counterparts (Rowley a and Russel] 1991, Martin 1996, Ghalambor and Martin 2001), parents may invest more energy in fewer young (Williams 1966, Martin 1996, Russell 2000). As a result, juvenile (i.e. postfledging) survival may be higher at southern latitudes than in more temperate locations (Martin 1996, Martin et al. 2000, Russell 2000).

Recent work has shown that adult survival in some southern populations is higher than in similar taxa in northern-temperate regions (Johnston et al. 1997, Ghalambor and Martin 2001, Peach et al. 2001). How survival at southern latitudes varies between years and how it may be affected by environmental factors is largely unknown. Passerine survival can be affected by food availability (Jansson et al. 1981, Newton 1998) and winter severity (McNamara and Houston 1990, Cuthill and Houston 1997). However, most tropical and southern-temperate regions experience mild winters-which probably support relatively high food levels (Oatley 1982, Rowley and Russell 1991) -and less extreme weather conditions than more northerly areas. Knowing how survival of southern-temperate species is affected by food availability and weather conditions would help in understanding the mechanisms that regulate survival at southern latitudes.

Here, we investigate juvenile and adult survival in the Sociable Weaver (Philetairus SOCZHS), a colonial cooperative passerine endemic to the semi-arid savannas of southern Africa. The latitudinal trend in avian life histories predicts higher adult survival in Sociable Weavers than in most northern-temperate passerines. High juvenile survival should also occur because-besides benefiting from factors that might increase adult survival-young in this sedentary species receive extended parental care and remain in the natal colony for at least four months (Covas 2002, R. Covas and C. Doutrelant unpubl. data). Sociable Weavers are suitable for study of environmental factors and survival because they inhabit a highly fluctuating environment, where rainfall, which is variable and often low, is the main determinant of food availability and reproductive effort (Maclean 1973c, Lloyd 1999). In our study area, winters are usually sunny, with mild daytime temperatures. However, night-time temperatures can drop to several degrees below zero, representing a potential cost in terms of thermoregulation (White et al. 1975, du Plessis and Williams 1994). The present study aims to (1) estimate juvenile and adult survival in the Sociable Weaver and (2) investigate temporal variation in survival -in particular, how survival is affected by winter temperature, rainfall, and duration of the breeding season.