Does Shading Behavior of Incubating Shorebirds in Hot Environments Cool the Eggs or the Adults?

Abstract

-Birds that nest on the ground in hot environments employ behavioral and physiological mechanisms to prevent the overheating of their eggs and themselves (e.g. egg covering, belly soaking, orientation on the nest, ptiloerection, panting, and gular fluttering). One of the least energetically expensive behaviors employed during incubation at high temperatures is nest shading. During shading, the birds raise themselves onto their tibiotarsi so that the incubation patch is about 2 to 3 cm above the eggs. It has frequently been assumed that birds adopt shading behavior to allow for direct convective cooling of the eggs. However, an alternative hypothesis is that birds rise up onto their tibiotarsi in order to place themselves out of the boundary layer. In doing so, adult birds cool themselves directly by convection. The lowering of the adult's body temperature during nest shading may, of course, cool the eggs indirectly when the bird resumes incubation. We tested these hypotheses using a taxidermic model of a Crowned Plover (Vanellus coronatus), which nests in hot environments in South Africa. Field observations have shown that this species spends 20 to 50% of its time shading when incubating at high environmental temperatures. When we alternated the bird between incubating and shading positions, we found that bird temperature, but not egg temperature, was lowered by shading behavior. Furthermore, wind speed had a significant negative effect on bird temperature but not egg temperature. Therefore, our results support the hypothesis that shading behavior functions to lower the temperature of incubating adults rather than of the eggs. Received 24 September 1996, accepted 14 May 1997. INCUBATION IN BIRDS usually is considered to be a warming process (e.g. Vleck 1981, Whittow 1986). However, bird species that nest on the ground in environments characterized by high daytime temperatures have to employ behavioral and physiological mechanisms to prevent the overheating of their eggs and themselves (Schmidt-Nielsen 1965, Grant 1982, Ward 1990). These mechanisms include egg covering (Maclean 1972), belly soaking (Jayakar and Spurway 1965, Howell 1979, Maclean 1975, Grant 1982, Ward 1988), orientation on the nest, ptiloerection, panting, and gular fluttering (Maclean 1967, 1975, 1976; Grant 1982; Ward 1989a,b). Eggshells of birds nesting in semiarid to arid regions also have reduced water-vapor conductance, increased thickness, and reduced effective pore area (Ar et al. 1974, Vleck et al. 1979, Ward 1987, Arad et al. 1988). Attentive behavior at the nest is thought to be important for maintenance of egg tempera1 E-mail: downs@zoology.unp.ac.za 2 Present address: Mitrani Center for Desert Ecology and Ramon Science Center, Blaustein Institute for Desert Research, Ben Gurion University of the Negev, Sede Boqer 84990, Israel. tures below lethal levels in harsh, hot environments (Grant 1982, Ward 1989a,b). During the heat of the day, incubating shorebirds nesting near the Salton Sea in California spent most of their time shading or loosely sitting over their eggs (Grant 1982). Nest relief occurred about every 10 to 15 min during the heat of the day (Grant 1982). Indeed, many shorebirds that nest in areas of high ambient temperatures shade their eggs at moderate to high temperatures (Maclean 1976, Ward 1989a,b). The Crowned Plover (Vanellus coronatus) is a medium-sized shorebird (average body mass = 167 g; Maclean 1993) that commonly occurs in open grasslands and lightly wooded savannas in sub-Saharan Africa (Ward and Maclean 1989). The nest is a shallow scrape (1 to 2 cm deep) in the ground (Ward 1987). These ground nests may get cold at night, requiring the parent to incubate the eggs to keep them warm. A more stressful period for the incubating birds occurs during the main breeding season, when daytime surface temperatures usually exceed 50?C. During these periods, the eggs require cooling (Ward 1990). Typical egg temperatures in birds vary between 30 and 35?C, and hyperthermia evidently is more injurious to the de-