Nest attendance by male California gnatcatchers

-Changes in paternal relative to maternal care either seasonally or over the nest cycle may reflect conflicting reproductive strategies between the sexes or may enhance reproductive success in the presence of sex-specific, physiological constraints. To determine whether nest attendance by males relative to females changed seasonally or with stage of the nest cycle, 107 nests of the California Gnatcatcher (Polioptila californica) were monitored over the 1994 and 1995 breeding seasons, and the sex of the attending adult was noted during each visit. Relative nest attendance by the male (RNAM) was calculated by dividing the frequency of the male observed on the nest by the number of times either adult was seen on the nest. Although variability in RNAM was high, RNAM showed a decline from approximately 0.5 for first nests of the season to approximately 0.3 for late-season nests. RNAM was less during the nestling stage than during incubation or laying. During the nestling period, RNAM was less than relative nest attendance by the female, whereas prior to hatching, attendance patterns did not differ between the sexes. Because the cause of RNAM variation is unknown, future studies should aim at determining the whereabouts of gnatcatchers while off the nest. ATENCI6N DE NIDOS POR MACHOS DE POLIOPTILA CALIFORNICA Sinopsis.-Los cambios en el cuidado paternal relativo al cuidado maternal (ya sea temporal o a traves del ciclo de anidaje) pueden reflejar estrategias reproductivas conflictivas entre los sexos o pueden aumentar el 6xito reproductivo en presencia de limitaciones fisiol6gicas asociadas a un sexo. Se siguieron 107 nidos de Polioptila californica durante las temporadas reproductivas de 1994 y 1995 notando el sexo del adulto interviniente durante cada visita para determinar si la atenci6n masculina del nido (en comparaci6n con la de las hembras) cambia estacionalmente o con las etapas del ciclo de anidamiento. La atenci6n relativa del nido (RNAM) se calcul6 dividiendo la frecuencia del macho observado en el nido por el numero de veces que cualquier adulto se detect6 en el nido. Aunque la variabilidad en RNAM fue alta, se evidenci6 una reducci6n en RNAM de cerca de 0.5 para los primeros nidos a cerca de 0.3 para nidos tarde en la temporada. La RNAM fue menor durante el periodo de pichones que en los periodos de incubaci6n o de poner. Durante el periodo de pichones la RNAM fue menor que la atenci6n relativa del nido de la hembra, mientras que previo a la eclosi6n los patrones de atenci6n no diferian entre los sexos. Como se desconoce la causa de la variaci6n en RNAM, los estudios futuros debian enfocar en determinar la localizaci6n de los individuos cuando no esten en el nido. Few studies have examined variability in avian parental division of labor with respect to date of clutch initiation or different stages of the nest cycle (but see Buitron 1988, Fasola and Saino 1995, Guerra and Drummond 1995, and Rytk6nen et al. 1995). In some species, significant inequalities between male and female parental effort have evolved (Silver et al. 1985) and may reflect conflicting reproductive strategies between the sexes. For example, paternal care of the young may decrease (relative Current address: Department of Zoology and Center for Reproductive Biology, Washington State University, Pullman, Washington 99164 USA.