DIVISION OF LABOR: INCUBATION AND BIPARENTAL CARE IN HOUSE SPARROWS (PASSER DOMESTICUS)

Auk, The, Jul 2005 by Bartlett, Terri L, Mock, Douglas W, Schwagmeyer, P L

ABSTRACT.-

In the great majority of animal taxa, males do not participate in parental care, but substantial paternal care is common across avian species. We examined male and female incubation contributions in House Sparrows (Passer domesticus), quantifying the incubation behavior of free-living, individually color-banded parents during 47 nesting cycles. We also measured the relative warmth of male and female incubation surfaces. Females spent more time incubating than their male partners, and female time incubating served as the best single predictor for hatching success. Considered alone, male time incubating correlated negatively with hatching success, but that effect was nullified when female incubation was taken into account. Females had warmer abdomens than males, a difference that may reflect greater development of brood patch and effectiveness of incubation in that sex. Here, male badge size was not demonstrably associated with either male or female incubation patterns or hatching success.

Key words: division of labor, House Sparrow, incubation, parental care, Passer domesticus.

División de Labores: Incubación y Cuidado por Ambos Progenitores en Passer domesticus

RESUMEN. - En la mayoría de los taxa animales los machos no participan en el cuidado parental; sin embargo, un cuidado paterno substancial es común en las aves. Examinamos las contribuciones de los machos y las hembras durante la incubación en la especie Passer domesticus, cuantificando el comportamiento de incubación de parejas en libertad y marcadas individualmente con anillos de colores durante 47 ciclos de nidificación. También medimos la température relative de las superficies de incubación de las hembras y de los machos. Las hembras permanecieron incubando por un periodo de tiempo mayor que sus parejas macho y el tiempo de incubación por la hembra fue la variable unica que mejor predijo el éxito de eclosión. Al considerar sólo el tiempo de incubación de los machos, este se correlacionó negativamente con el éxito de eclosión, pero este efecto se anuló cuando se tomó en cuenta la incubación por parte de la hembra. Las hembras tuvieron abdómenes más cálidos que los machos, una diferencia que puede reflejar un mayor desarrollo del parche de incubación y una mayor efectividad de la incubación por parte de las hembras. En este estudio, el tamaño del parche del macho no se asoció con los patrones de incubación del macho ni de la hembra, ni con el éxito de eclosión.

ABOUT 95% OF all birds are at least socially monogamous (Lack 1968, Black 1996), with both male and female partners typically providing substantial parental care in nidicolous species. Nevertheless, considerable variation exists in the type and magnitude of male contributions. Males may or may not incubate eggs, brood nestlings, deliver food, stand watch, actively deter predators, and assist the fledglings after they have left the nest (e.g. Ketterson and Nolan 1994, Liker and Szekely 1999, Currie et al. 2001). The diversity in paternal care presumably stems from balancing offspring care against other means of enhancing fitness, especially time and energy expenditure allocated to the incompatible activities of foraging for self-maintenance, seeking additional mating opportunities, or both (Maynard Smith 1977, Jönsson et al. 1998, Wright et al. 1998)

Most research on biparental care in birds has focused on nestling food-provisioning activities, rather than on contributions of each sex during alternative forms of care, such as incubation and brooding (Whittingham and Dunn 2001). Studies that have addressed the process of incubation, in turn, have focused primarily on its physiological aspects (heat transfer, energy costs, hormone variation, etc.; e.g. Biebach 1984, Jones 1987, Chaurand and Weimerskirsch 1994, Minguez 1998). Nevertheless, behavioral studies of species with biparental incubation (e.g. Drent 1970, Hatch 1990, Pinxten et al. 1993, Reid et al. 2002) have the potential to explore the specific role of males in this form of parental care and hence the relative importance of male investment at this early stage of the reproductive cycle.

Embryo survival, proper embryo development, and length of the developmental period are all affected by the necessity of keeping eggs within a given temperature range (Farmer 2000). Failure to maintain eggs within the temperature range for embryonic development can cause hatching failure (Webb 1987). In general, most avian embryos must be maintained between 30°C and 40°C, with the average temperature for passerines being ~32°C (Webb 1987). Because temperatures >40.5°C can kill embryos (Conway and Martin 2000), incubation also insulates eggs against excessively high temperatures.

Although no morphological specializations are required for transferring body warmth from parents to eggs, and only modest physical modifications have evolved (e.g. increased vascularization and feather loss to create a "brood patch"), incubation has been shown to carry a variety of costs (Visser and Lessells 2001). In males, decreased circulating levels of testosterone, in tandem with increased levels of prolactin, may curtail the number of extrapair copulations or additional social mates a male can obtain while incubating. Specifically, such fluctuations in those hormones may reduce sexual behavior in male birds (Ketterson and Nolan 1994, Moreno et al. 1999). Incubation is also likely to carry nontrivial energetic costs associated with heat generation and transfer to eggs (Siikamaki 1995, Heaney and Monaghan 1996). Finally, incubation is simply more timeconsuming and spatially confining than most other forms of parental care (Ketterson and Nolan 1994), automatically limiting time for foraging (Webb 1987, Siikamaki 1995) and other valuable activities, while simultaneously rendering the immobile parent more vulnerable to predators (Stoleson and Beissinger 2001, Amat and Masero 2004). When summed, those costs may explain why males do not incubate in many avian species (Ketterson and Nolan 1994).