LAYING PLASTICITY IN AN AVIAN BROOD PARASITE

ABSTRACT

Females of some bird species "steal" the parental care of other breeding individuals by laying eggs parasitically in their nests. In most populations, conspecific brood parasites lay parasitically before laying in their own nests. By contrast, in a young and expanding Common Moorhen (Gallinula chloropus) population, 17.4% (n = 8) of brood parasites laid parasitically when their own clutches were currently being incubated, and 13.1% (n = 6) laid parasitically when they were tending to their young. This result differs from previous data on conspecific brood parasitism in this (and other) avian species and suggests that site-specific ecological conditions exert differing selective influences on the behavioral development of populations. The behavior adopted by individual vertebrate species may not, therefore, be rigidly fixed throughout their geographic range. Received 26 April 2004, accepted 10 November 2004.

Key words: behavioral plasticity, Common Moorhen, conspecific brood parasitism, Gallinula chloropus.

Plasticidad en la Puesta de Huevos en un Ave Parásita de Cría

RESUMEN. - Las hembras de algunas especies de aves "roban" el cuidado parental de otros individuos al parasitarles los nidos con sus huevos. En la mayoria de las poblaciones, las aves parásitas coespecíficas realizan una puesta parásita antes de poner huevos en sus propios nidos. En contraste, en una población joven y en expansion de Gallinula chloropus, el 17.4% (n = 8) de las aves parasitaron nidos cuando sus propias nidadas estaban siendo incubadas,. y el 13.1% (n = 6) parasitaron nidos cuando estaban atendiendo a sus pichones. Este resultado difiere de los datos anteriores de parasitismo de cría para aves coespecíficas en esta y otras especies, y sugiere que las condiciones ecológicas específicas de los sitios ejercen influencias selectivas diferentes en el desarrollo del comportamiento de las poblaciones. Por lo tanto, el comportamiento adoptado por cada especie de vertebrado podría no estar fijado de modo rígido a través de su rango geográfico.

CONSPECIFC BROOD PARASITISM (CBP), in which a female lays eggs in conspecifics' nests, occurs in more than 230 avian species and attracts considerable interest from behavioral ecologists (Brown 1984, Davies 1988, Petrie and Møller 1991, Yom-Tov 2001). This additional reproductive output can increase the lifetime fitness of CBP strategists as they exploit the parental care of other breeding groups, leaving more time and resources for the care of their own broods (Brown and Brown 1989, 1998). Conspecific brood parasitism is frequently reported in avian species (namely the Anseriformes) in which females predominantly provide parental care and young do not require parental provisioning (Andersson 1984, Lank et al. 1989). Conspecific brood parasitism also occurs in populations of some semi-precocial (with semi-dependant young) arid altricial (with fully dependent young) species in which parental care is provided by both males and females (Yom-Tov 1980, Petrie and Møller 1991). Detailed studies of the laying behavior of brood parasites, amassed from a wide diversity of avian families, have revealed that females lay parasitically before laying in their own nests (Brown and Brown 1989, Alhund and Andersson 1991, Wiegmann and Lamprecht 1991, Sorenson 1993). An exception to that CBP laying trend occurs following nest prédation in some species, when females respond to the loss of their own clutches by laying "replacement eggs" in the nests of conspecific females (Feare 1991, Lyon 1993, McRae 1997b). Given the consensus of empirical evidence on the laying behavior of brood parasites, several evolutionary advantages to females laying eggs parasitically before attempting to lay in their own nests have been postulated (Petrie and Møller 1991). Locating and laying in a conspecific's nest after laying their own eggs may expose brood parasites' own clutches to an increased risk of parasitism and predation. Also, females may more effectively locate and monitor potential host nests if they do not have to balance the demands of laying eggs in their nests with those of pursuing a parasitic strategy (Brown and Brown 1989, Petrie and M011er 1991). To test those hypotheses, the CBP laying behavior of Common Moorhens (Gallinula Moropus; hereafter "moorhens") was studied in a rapidly expanding but high-density population (Forman 2001, Forman and Brain 2004).

METHODS

The study was conducted on a 10-year-old, color-ringed population of 200 moorhens at the National Wetlands Centre Wales, in Llanelli, South Wales, between 1997 and 1999 (Forman 2001). Daily nest searches were conducted in suitable habitat, and eggs were individually marked using an indelible pen on the day they were laid. The weight (in grams), length (in millimeters), and width (in millimeters) of individual eggs were also measured at that time, using a 50-g Pesola spring balance and a vernier caliper. Once located, nests were revisited to determine the fates of all eggs. Moorhens usually lay eggs at the rate of one per day (Gibbons 1986, McRae and Burke 1996). To accurately identify CBP events, two criteria were used (after McRae 1994). Conspecific brood parasitism was deemed to have occurred if (1) two or more eggs appeared in a nest within a 24-h period and (2) if an egg was laid in a nest two or more days after the host female had stopped laying. Those criteria have been positively validated in this species through multilocus DNA fingerprinting (McRae 1994, McRae and Burke 1996). A total of 588 nests (86%) was found either before laying had commenced or within one day of clutch initiation. Nests in which the exact laying dates of individual eggs were unknown were excluded from the analysis.