Incubation temperatures and behavior of Crowned, Black-winged, and Lesser Black-winged Plovers

-Crowned Plovers (Vanellus coronatus), Black-winged Plovers (V. melanopterus), and Lesser Black-winged Plovers (V. lugubris) incubate their eggs during early spring in short unshaded grasslands and open woodlands. I studied incubation behavior in hot environments and the ability to control brood-patch temperatures. Crowned and Black-winged plovers kept brood-patch temperatures almost constant, while the temperature of Lesser Black-winged Plovers was more poorly controlled, probably as a result of a large body surface area-tovolume ratio. At high ambient temperatures, Crowned and Black-winged plovers shaded their eggs more than did Lesser Black-winged Plovers. Lesser Black-winged Plovers controlled nest-air temperatures at high ambient temperatures by frequent shifts in nest attendance by male and female, which allowed the nonsitting bird to cool in the shade. Received 30 January 1989, accepted 15 June 1989. LIKE all Vanellus species (Charadriiformes: Vanellinae), Crowned Plovers (V. coronatus), Blackwinged Plovers (V. melanopterus), and Lesser Black-winged Plovers (V. lugubris) are groundnesters. The three species are widely distributed (Maclean 1985). They nest in areas of short or burnt grass in open grasslands or open woodlands far from bodies of water, usually during late winter or early spring. Nests are scrapes on the ground and have no cover to shield incubating birds from the elements. Virtually all previous studies of ground-nesting birds in hot environments (except Maclean 1967, 1976) have dealt with species that nest near water and have described the bird's use of water to decrease egg temperatures during hot periods (e.g. Jayakar and Spurway 1965, Howell 1979, Grant 1982). I examined the responses of the incubating adults, and their eggs, to insolation. The effects of body size on incubation could be included because two species were of similar size (Crowned Plover: mass = 160-200 g, x = 167 g; Black-winged Plover: mass = 160-175 g, x = 167 g) and larger than the Lesser Black-winged Plover (mass = 107-130 g; x = 121 g) (Ward 1987). 1 Present address: Mitrani Centre for Desert Ecology, Blaustein Institute for Desert Research, Ben Gurion University of the Negev, Sede Boqer Campus 84993, Israel. MATERIALS AND METHODS I compared the incubation temperatures and associated behavior of the three plover species by detailed observation of nests over the entire incubation period, at the expense of studying many nests of each species. Incubation temperatures were recorded in one pair of Crowned Plovers in Pietermaritzburg, Natal, South Africa (29? 30'S, 30? 30'E), and three pairs at Ndumu, Natal, South Africa (28? 30'S, 32? 45'E), three pairs of Black-winged Plovers in Pietermaritzburg, and three pairs of Lesser Black-winged Plovers at Ndumu. All nests studied were successful. Plovers were nesttrapped (Sordahl 1980) and were marked individually with color bands. In 1985 and 1986, incubation temperatures at each nest were measured mostly during the day (sunrise to sunset) and for a single night (1800-0600). I measured temperatures with a Yellow Springs Instruments telethermometer placed in a blind 10 m from the nest and equipped with three thermistor probes that were calibrated to ? 0.1? C against a standard thermometer. Nest-air temperature was measured between the eggs on the floor of the nest. To measure brood-patch temperature, I placed a second probe on top of a plaster-of-paris model painted to resemble a plover egg. I secured the model in place to prevent the birds from turning it. The thermal conductivity (watt per meter Kelvin) of plaster-of-paris (0.432 W m-1 K-1, Weast 1986) is the same as that of an authentic egg (0.43 W -m-1 -K-1, Henderson 1963), so the buffering effect on the recorded brood-patch temperatures is likely to be similar to a natural egg. The third probe, to measure ambient temperature, was placed 1 m from the nest and 5 cm above ground. The external probe was painted with a white reflective paint and was not shaded. This produced the most accurate measurement of air temperature in the field, 10 The Auk 107: 10-17. January 1990 This content downloaded from 207.46. 13.101 on Sat, 08 Oct 2016 05:56:57 UTC All use subject to http://about.jstor. org/terms January 1990] Plover Incubation Behavior 11 with small error due to radiation and without the problem of inadequate ventilation experienced by shaded probes (Christian and Tracy 1985). I measured wind speed with an anemometer outside the blind, hand-held 5 cm above ground. Temperature and windspeed readings were taken every 15 min. Observations at night were made with a light-intensifying telescope. The time that the nest was occupied by either partner was nest attendance. I compared the amount of time that the nest was attended among 10? C ambienttemperature ranges. The amount of time that the nest was occupied by a particular individual was nest attentiveness. I compared the changes in an individual's contribution to incubation relative to the total time that the nest was attended among 10? C ambient-temperature ranges. I scored the length of incubation shifts by each individual within each 10? C ambient-temperature range. The number of observations equaled the number of incubation shifts per individual within a particular 10? C range. Comparisons were made of the relative contributions of males and females to incubation in different 10? C ambient-temperature ranges. A Wilcoxon test (Zar 1974) was used because the presence of the male and female on the nest is negatively correlated, which precludes the use of a parametric test. I analyzed shading behavior using the proportion of time that the birds shaded the egg relative to the total time that ambient temperatures were in a particular 5? C range. All data for nest attendance, attentiveness (except comparisons between male and female), and shading behavior were arcsine transformed (Zar 1974), with the number of observations equal to the number of hours or parts thereof (minimum of 30 min) that ambient temperature fell within a particular temperature range. The amount of time that a particular individual remained on a nest before being relieved by its partner was incubation-bout duration. The number of observations equals the number of incubation bouts within a particular 10? C temperature range.