Testosterone and Group Size in Cliff Swallows: Testing the “Challenge Hypothesis” in a Colonial Bird

The “challenge hypothesis” states that increases in testosterone levels of male animals during the breeding season are directly related to the extent of intrasexual competition for resources or mates that they experience. Although often tested in territorial species, the challenge hypothesis has not been evaluated for colonial animals that live in groups of different sizes and that thus experience different intensities of intrasexual competition. We measured circulating testosterone levels of male and female cliff swallows (Petrochelidon pyrrhonota) in southwestern Nebraska, where these birds nest in colonies of widely different sizes. Males had significantly higher testosterone levels than females, as expected. For males especially, there was a seasonal rise in testosterone levels early in the nesting cycle, corresponding to the period when birds were establishing nest ownership and egg laying, and then a fall as they switched to parental duties. Testosterone levels varied significantly with colony size; for both sexes, birds in larger colonies had higher levels of testosterone than those in smaller colonies when controlling for date. Age and body mass were not related to testosterone levels. Higher levels of testosterone for birds of both sexes in larger colonies probably reflect greater com-petition for matings, often extra pair, in the more social nesting situations. The results support the predictions of the challenge hypothesis

Testosterone and Group Size in Cliff Swallows: Testing the “Challenge Hypothesis” in a Colonial Bird

The “challenge hypothesis” states that increases in testosterone levels of male animals during the breeding season are directly related to the extent of intrasexual competition for resources or mates that they experience. Although often tested in territorial species, the challenge hypothesis has not been evaluated for colonial animals that live in groups of different sizes and that thus experience different intensities of intrasexual competition. We measured circulating testosterone levels of male and female cliff swallows (Petrochelidon pyrrhonota) in southwestern Nebraska, where these birds nest in colonies of widely different sizes. Males had significantly higher testosterone levels than females, as expected. For males especially, there was a seasonal rise in testosterone levels early in the nesting cycle, corresponding to the period when birds were establishing nest ownership and egg laying, and then a fall as they switched to parental duties. Testosterone levels varied significantly with colony size; for both sexes, birds in larger colonies had higher levels of testosterone than those in smaller colonies when controlling for date. Age and body mass were not related to testosterone levels. Higher levels of testosterone for birds of both sexes in larger colonies probably reflect greater com-petition for matings, often extra pair, in the more social nesting situations. The results support the predictions of the challenge hypothesis

Effects of Endogenous Steroid Hormone Levels on Annual Survival in Cliff Swallows

The hormone corticosterone is an important part of animals’ response to environmental stress, modulating short-term adaptive changes in behavior and physiology. The hormone testosterone is also critical, especially for males, in regulating the expression of sexual behavior and parental care. These hormones can have costly consequences, however, and within populations individuals show variation in endogenous levels of both corticosterone and testosterone. We studied how annual survival varied as a function of natural levels of these hormones in colonially breeding Cliff swallows (Petrochelidon pyrrhonota) in southwestern Nebraska, USA, in 2000–2003. We sampled hormone levels of birds caught at colonies and using mark–recapture, monitored their survival through subsequent years in the study area. Maximum-likelihood estimation and model fitting (with program MARK) revealed that birds sampled for corticosterone in colonies of all sizes late in the season had curvilinear survival; individuals with very low and very high levels of corticosterone had lower survival than those with intermediate levels. Annual survival of birds sampled earlier in the season, however, generally declined with increasing corticosterone level. More birds than expected, given the survival functions, had the very low corticosterone levels in nonfumigated colonies later in the year, suggesting perhaps a compensatory benefit unrelated to survival for very low corticosterone levels. In a more limited analysis, testosterone appeared to have little effect on annual survival, although some evidence suggested that females with endogenous testosterone levels below the mean for a given date might have survived better

Mark–recapture and behavioral ecology: a case study of Cliff Swallows

Mark–recapture and behavioral ecology: a case study of Cliff Swallows.— Mark–recapture and the statistical analysis methods associated with it offer great potential for investigating fitness components associated with particular behavioral traits. However, few behavioral ecologists have used these techniques. We illustrate the insights that have come from a long–term mark–recapture study of social behavior in Cliff Swallows (Petrochelidon pyrrhonota). The number of transient swallows passing through a colony per hour increased with colony size and was responsible in part for increased rates of ectoparasite introduction from outside the group into the larger colonies. Annual survival probabilities of males engaging in extra–pair copulation attempts were lower than those of males not seen to commit extra–pair copulations, suggesting that males who engage in this behavior may be inferior individuals and that females do not benefit from copulating with them. Females engaging in intraspecific brood parasitism had higher annual survival probabilities than ones either parasitized by others or not known to be either hosts or parasites. This suggests that parasitic females are high–quality birds and that brood parasitism is an effective reproductive tactic for increasing their fitness. By estimating first–year survival of chicks, we found that a clutch size of 4 eggs is often the most productive, on average, as measured by recruitment of offspring as breeders, although birds laying the more uncommon clutch size of 5 fledge more young on average. This helps to explain the observed clutch–size distribution in which clutch size 4 is the most commonly produced. Marcaje–recaptura y ecología del comportamiento: el ejemplo de las golondrinas de frente canela.—El método de marcaje–recaptura y los métodos de análisis estadísticos asociados al mismo brindan un enorme potencial para investigar componentes del estado de salud asociados a determinados rasgos de comportamiento. Sin embargo, son pocos los ecólogos del comportamiento que han empleado dichas técnicas. En este artículo se presentan los resultados de un estudio a largo plazo de las golondrinas de frente canela (Petrochelidon pyrrhonota), en el que se empleó la técnica de marcaje–recaptura para analizar su comportamiento social. Observamos que cuanto mayor era el tamaño de la colonia, más elevado era el número de golondrinas que pasaba por la misma cada hora, siendo esto parcialmente responsable, en las colonias de mayor tamaño, de un aumento en las tasas de introducción de ectoparásitos desde fuera del grupo. Las probabilidades de supervivencia anual de los machos que intentaron llevar a cabo cópulas fuera de la pareja fueron inferiores a las de los machos a los que no se les había visto copular con una hembra distinta a la de su pareja, lo que sugiere que los machos que adoptan este comportamiento pueden ser individuos inferiores, y que las hembras no se benefician de copular con ellos. Las probabilidades de supervivencia anual de las hembras que participaron en parasitismo de nidada intraespecífico fueron más elevadas que las de aquellas que habían sido parasitadas por otras, o que las de aquellas que no eran conocidas como huéspedes ni como parásitas. Ello sugiere que las hembras parásitas son aves de alta calidad y que utilizan el parasitismo de nidada como una táctica reproductora eficaz para aumentar su buen estado de salud. La estimación de supervivencia de los polluelos durante el primer año de vida nos permitió determinar que una puesta de cuatro huevos suele ser, por término medio, la más productiva, medido según el reclutamiento de polluelos como aves reproductoras, aunque las aves que tienen una puesta poco usual de cinco huevos se desarrollan, por término medio, a una edad más temprana. Esto ayuda a explicar la distribución observada del tamaño de las puestas, siendo la de cuatro huevos la que se produce con mayor frecuencia

Glucocorticoid Hormone Levels Increase with Group Size and Parasite Load in Cliff Swallows

Animals often cope with adverse events by releasing glucocorticoid hormones, which in turn promote increased energy assimilation. In captive animals, crowding also leads to increased glucocorticoid activity, probably because of increased levels of social competition. We investigated how group size and ectoparasite infestations affected endogenous levels of the glucocorticoid hormone, corticosterone, in colonial cliff swallows, Petrochelidon pyrrhonota, in southwestern Nebraska, USA. Parasites were removed from some colonies by fumigating nests. Baseline levels of corticosterone in breeding adults varied significantly with whether parasites were present, colony size (measured by total number of active nests at a site), and nesting stage. Across all analyses, birds from fumigated colonies averaged significantly lower baseline levels of corticosterone. These levels in adults increased with colony size at nonfumigated sites, especially during the period when nestlings were being fed, but no relation or the opposite one was found for birds in fumigated colonies. Baseline corticosterone levels were unrelated to sex, age, body weight, or testosterone levels in adults. Corticosterone concentrations tended to increase during a bad-weather event when food was scarce. Patterns in nestling and recently fledged juveniles were consistent with those in adults. The increased baseline levels of corticosterone in birds of larger colonies appear related to the larger number of parasites there. Higher levels of corticosterone probably facilitate increased allocation of time and energy to foraging and greater energy assimilation during challenging events such as bad weather, parasitism by blood-feeding bugs in large colonies, and the period when young are becoming independent of their parents

Effects of Endogenous Steroid Hormone Levels on Annual Survival in Cliff Swallows

The hormone corticosterone is an important part of animals’ response to environmental stress, modulating short-term adaptive changes in behavior and physiology. The hormone testosterone is also critical, especially for males, in regulating the expression of sexual behavior and parental care. These hormones can have costly consequences, however, and within populations individuals show variation in endogenous levels of both corticosterone and testosterone. We studied how annual survival varied as a function of natural levels of these hormones in colonially breeding Cliff swallows (Petrochelidon pyrrhonota) in southwestern Nebraska, USA, in 2000–2003. We sampled hormone levels of birds caught at colonies and using mark–recapture, monitored their survival through subsequent years in the study area. Maximum-likelihood estimation and model fitting (with program MARK) revealed that birds sampled for corticosterone in colonies of all sizes late in the season had curvilinear survival; individuals with very low and very high levels of corticosterone had lower survival than those with intermediate levels. Annual survival of birds sampled earlier in the season, however, generally declined with increasing corticosterone level. More birds than expected, given the survival functions, had the very low corticosterone levels in nonfumigated colonies later in the year, suggesting perhaps a compensatory benefit unrelated to survival for very low corticosterone levels. In a more limited analysis, testosterone appeared to have little effect on annual survival, although some evidence suggested that females with endogenous testosterone levels below the mean for a given date might have survived better

Ursinus College Bulletin Vol. 8, No. 8

A digitized copy of the May 1892 Ursinus College Bulletin.https://digitalcommons.ursinus.edu/ucbulletin/1075/thumbnail.jp

Ursinus College Bulletin Vol. 8, No. 5

A digitized copy of the February 1892 Ursinus College Bulletin.https://digitalcommons.ursinus.edu/ucbulletin/1072/thumbnail.jp

Ursinus College Bulletin Vol. 8, No. 4

A digitized copy of the January 1892 Ursinus College Bulletin.https://digitalcommons.ursinus.edu/ucbulletin/1071/thumbnail.jp

Ursinus College Bulletin Vol. 8, No. 5

A digitized copy of the February 1892 Ursinus College Bulletin.https://digitalcommons.ursinus.edu/ucbulletin/1072/thumbnail.jp