Nest Site Selection and Clutch Predation in the Snowy Plover

-We studied the relationship between clutch fate and nest site in Snowy Plovers (Charadrius alexandrinus) subject to a high level of clutch predation. Clutches beside objects had lower hatching success than those in the open or beneath objects. Those on sand-gravel substrate were less successful than those on alkali. Plovers nested more frequently on sandgravel early rather than later in the season. There were between-year differences in clutch placement with respect to substrate type and position related to an object. Predators, air temperature, and bias in our nest-finding abilities are hypothesized as factors influencing the results. Received 23 April 1984, accepted 16 October 1984. PLOVERS of the genus Charadrius typically nest in flat, unbroken, barren to sparsely vegetated habitats that include coastal beaches [Wilson's Plover (C. wilsonia) and C. melodus; Bergstrom 1982, Cairns 1982], river or lake shores (C. placidus, C. bicinctus, and C. dubius; Vaurie 1965, Johnsgard 1981, Cramp and Simmons 1983), dry grassland [Mountain Plover (C. montanus); Graul 1975], alpine tundra (C. mongolus), and arctic tundra (C. morinellus; Cramp and Simmons 1983). Nests are unlined to thinly lined, shallow depressions in hardened clay or silt or in loose cobble, pebble, or sand (Johnsgard 1981, Cramp and Simmons 1983). Being in the open, nests are exposed to weather and predators; adults and eggs are cryptically colored. Charadrius species sometimes place nests beside objects or clumps of vegetation, leading researchers to speculate about the advantages of this behavior; protection from weather, cue to nest location, and concealment have all been proposed. Tomkins (1944) suggested the first possibility when he described the objects next to Wilson's Plover nests as wind breaks, and Bergstrom's (1982) studies of the same species indicate that objects may have value as shields from the weather. Bergstrom found that nests in a hot Texas environment were oriented so that a clump of vegetation was usually on the south or southwest side, placing the nest in the lee of the normal wind direction and in shade during the afternoon, when temperatures could be very high. In contrast, Purdue (1976) felt that objects beside Snowy Plover (C. alexandrinus) nests on a salt pan in Oklahoma were too small for protection from either weather or predators. Maclean and Moran (1965) concluded that White-fronted Sandplovers (C. marginatus) did not need objects for finding the nest. They watched birds return as directly and as quickly to the nest when the object was next to it as when the object had been moved. Alternatively, they speculated that nesting next to an object may offer concealment to the bird and egg through the object's "disruptive effects." Bunni (1959) and Graul (1975) invoked a similar explanation for Killdeers' (C. vociferus) and Mountain Plovers' tendencies to nest beside objects. We found 68% of 136 Snowy Plover clutches were within 15 cm of an object during a survey of most of the California breeding population (unpubl. data). Boyd (1972) and Purdue (1976) also indicated this species frequently nests beside objects. While conducting field work at Mono Lake, California from 1978 to 1982 we found a population of Snowy Plovers afflicted with a high rate of clutch predation (Page et al. 1983). Predators were almost the sole cause of clutch loss; California Gulls (Larus californicus) were the major predators, followed in order of importance by Common Ravens (Corvus corax) and coyotes (Canus latrans). The plovers frequently nested under or beside driftwood or other objects scattered over two substrates, old beach ridges and alkali flats. This provided an excellent opportunity to determine if there was any correlation between clutch hatching suc347 The Auk 102: 347-353. April 1985 This content downloaded from 157.55.39.132 on Thu, 15 Sep 2016 05:53:36 UTC All use subject to http://about.jstor.org/terms 348 PAGE, STENZEL, AND RIBIC [Auk, Vol. 102 cess and placement. This paper reports our findings on this topic and provides additional information on variation in choice of nest site with season, year, and substrate type. STUDY AREA AND METHODS Study area.-The study was conducted at Mono Lake, an elliptical 15 x 20-km alkaline lake at the eastern base of the Sierra Nevada in Mono County, California. Our study area was a 4.7 x 1.5-km section of the northeast shore consisting of oldUbeach ridges and alkaline flats. The old beach ridges span a 300-m width and have a substrate of coarse sand sprinkled with dark-colored basalt pebbles, whereas the alkali flat spans 1,100 m between the beach ridges and the water and consists of alkali-encrusted sand overlaid in places by loose, drifted sand. The study area is essentially barren, has few nesting birds other than the Snowy Plover, and except for our activities was almost devoid of human disturbance. It is described in more detail in Page et al. (1983). Field methods.-We used similar nest-searching methods every year, but because of a greater effort in 1978, we undoubtedly found a greater proportion of nests than in subsequent years. We found nests by watching adults return to them; looking in likely places, such as along low ridges or beside objects; searching at a distance with 8x binoculars or 20x telescopes for incubating adults; following plover footprints in sand; and accidental discovery. From 1978 to 1980 we concentrated our searching effort within the 3.5-km western sector of the study area, but in 1981 and 1982 we searched the entire study area. Our analysis includes only nests found each year in the areas described above. In 1978 we marked nests with 0.3-m-high stakes placed 10 m to the south. Afterwards we marked nest locations by a variety of naturally occurring objects placed in various directions from 1 to 10 m from the nest. In all years, we described each nest by its distance and direction from the nearest reference stake, substrate type, and distance to objects and vegetation. Nests within 15 cm of an object were considered "beside objects"; those covered at least 5% from above were considered "under objects." Most nests with incomplete clutches were checked daily to determine the laying date of the last egg. Hatching dates were predicted for these nests based on the date the last egg was laid and the average 27day incubation period (Warriner et al. pers. comm.). For nests found after clutch completion, initiation and hatching dates were estimated using the eggflotation method (Hays and LeCroy 1971) with data we obtained from a coastal Snowy Plover population. This method was accurate to within ?3 days. Whenever possible, we checked clutches by walking within a few meters of them rather than straight to them. Our tracks may have cued predators to nests, but we saw nothing that suggested this. Coyotes did not follow our abundant tracks that criss-crossed the area. We checked most clutches at least once every 5 days until about 4 days before the predicted hatching date, when we normally began checking them 1 or 2 times daily. Nests were known to be destroyed when eggs disappeared well before the projected hatching date or when remains or predator tracks were found at the nest site. We obtained slightly different clutch sample sizes than for previously reported work (Page et al. 1983) by adding a few clutches whose outcome was uncertain (but could be guessed) and by subtracting a few because nest site had not been described. In the former cases we assumed that empty nests at the scheduled hatching time represented successful clutches when small eggshell fragments could be found in the scrape. Absence of the fragments indicated failure. The eggshell fragments are produced during egg pipping and could be found in the linings of known successful nests. The overall effect on the yearly measure of clutch fate was never greater than 3.5% between the two sample sets. We excluded all 1979 clutches because experiments with artificial clutches in the study area probably affected clutch fate (Page et al. 1983; unpubl. data). Clutch hatching rate is the proportion of clutches hatching