Sexual Dimorphism, Detection Probability, Home Range, and Parental Care in the Yellow-billed Cuckoo.

Abstract

The biology of many species of conservation interest is poorly understood. Often, little is known apart from population estimates. Yellow-billed Cuckoos (Coccyzus americanus) are a neotropical migrant, and due to loss of riparian breeding habitat, are of great conservation concern in the western United States. Little is known about their basic biology. In this study I looked at sexual dimorphism, responsiveness to call playback, home range size, and parental care. Determining sex of a study organism is fundamental to understanding almost every aspect of their biology and management. Although there has been speculation about methods of sexing adult cuckoos using measurements, vocalizations, and extent of white in the tail, we found the only reliable way to sex cuckoos utilized genetic markers. Females gave the ―coo‖ call, considered an advertisement call, significantly more than males. Although there have been extensive cuckoo surveys done in the western United States, there are no data on responsiveness or detectability using call playback surveys. We tested the standard call playback methodology with 18 radio marked adult cuckoos. Response rate during call playback tests averaged 59.5%, and was higher for males (72.7%, n=10) than females (40%, n=8). Detection rates were lower than response rates, averaging 32.4% overall, and were higher for males (43.2%, n=10) than females (16.7%, n=8). The low responsiveness and detectability of cuckoos may be influenced by their large home ranges. We monitored 28 cuckoos equipped with radio transmitters on the San Pedro Riparian National Conservation Area during 2001-2005. Average home range estimates were 95% KDE - 39 ha, 75% KDE - 17 ha, and 50% KDE 7.5 ha. There were large variances for all home range estimates, and females had significantly smaller home ranges than males. This may partially account for lower detectability of females. Cuckoo researchers have observed small clutch sizes, rapid development of young, and a third adult helping to raise young, but there are no previous studies of parental care with banded, known sex Yellow-billed Cuckoos. We followed 28 adult cuckoos with transmitters and placed video cameras on four nests. Although both parents constructed nests, incubated eggs, and cared for young, males did all nighttime incubation, provided the majority of food to nestlings, and all care to fledglings. Additionally, we confirmed the presence of a third adult providing care to nestlings. Occasionally during the nestling period females appeared to abandon a viable nest and initiate a nest with another male. Males may have larger home ranges in order to locate females, who may call to attract second males. This male may assist with the first nest effort, then leave with the female for a subsequent nest. These observations present a pattern of male-dominated parental care, and Yellow-billed Cuckoos appear to be facultatively serially polyandrous.