Nest-site selection by mallards and blue-winged teal in relation to microclimate

-It is unclear whether female ducks select nest sites on the basis of microclimatic features. Therefore, we tested the nest microclimate selection hypothesis by concurrently monitoring temperature (T), solar insolation, and relative humidity (RH) at Mallard (Anas platyrhynchos) and Blue-winged Teal (A. discors) nest sites and nearby randomly selected "nonnest" sites (hereafter "adjacent" sites). Operative temperatures (Te) were calculated for nest sites and adjacent sites. Mallard and teal nest sites received consistently less insolation than adjacent sites, but did not differ in T or RH. Te at Mallard nest sites was more moderate than at adjacent sites; nest sites were cooler between 0800 and 1730 CST. This pattern did not exist for teal nest sites, which had consistently lower Te throughout daylight hours. Coefficients of variation for all microclimatic variables did not differ between Mallard and teal nest sites and adjacent sites. We examined interspecific differences in nest-site microclimate by matching data by date and stage of incubation. Teal nest sites received more insolation and experienced greater Te than concurrently monitored Mallard nest sites, but there were no interspecific differences in T or RH. Our results provide only limited support for the microclimatic selection hypothesis, suggesting that other factors are more important determinants of nest-site selection in ducks. Received 20 June 1996, accepted 24 January 1997. SELECTION OF A NEST SITE is a critical component of avian reproduction because it determines the environment to which the parent and eggs will be exposed for a relatively long period, and it presumably has evolved in relation to predation, local availability of resources, and microclimate. In this paper, we focus on the influence of microclimate because maintenance metabolism accounts for 40 to 60% of total daily energy expended (Walsberg 1983a), and thermal regimes of an animal's microhabitat potentially can have a strong influence on energetic costs (Gabrielsen and Unander 1987, Novoa et al. 1990, Warkentin and West 1990, Gabrielsen et al. 1991, Swain 1991). Therefore, energy saved from appropriate nest-site choices can be channeled to other activities such as reproduction. Ducks typically experience low nesting success (Greenwood et al. 1995, Sovada et al. 1995, Beauchamp et al. 1996) and generally select nest sites that provide some degree of conceal3Present address: Stanley Consulting Group Ltd., 108-3502 Taylor Street East, Saskatoon, Saskatchewan S7H 5H9, Canada. E-mail: stanley.sktn@stantech.com ment (Klett et al. 1988, Greenwood et al. 1995). However, Clark and Nudds (1991) demonstrated that the fate of duck nests could not be determined solely on the basis of nest concealment. Although it is clearly beneficial to nest in areas with some cover, benefits in terms of increased nesting success within areas of "suitable" cover are not apparent. Therefore, ducks may use other cues in combination with concealment when selecting nest sites. One of the most common examples of microhabitat selection in birds is nonrandom distribution of nests in dense vegetation. Although nests located in dense vegetation are thought to be less vulnerable to predation (Patterson 1980, Cody 1985, Rands 1988, Filliater et al. 1994), they also benefit from protection from wind, nocturnal radiative heat loss, and diurnal heat gain (Walsberg 1981, 1985; Colwell 1992). Nesting birds can respond to climatic extremes by: (1) selecting appropriate microclimates in which to nest, (2) adapting nest structure to prevailing climatic factors, and (3) using behaviors such as shading to protect eggs (Horvath 1964). Site selection for thermal advantages has been proposed to explain nonrandom nest-site placement in a variety of species (e.g.