Ibis (2011), 153, 303–311 Western Snowy Plovers Charadrius alexandrinus nivosus select nesting substrates that enhance egg crypsis and improve nest survival MARK A. COLWELL,1* JASON J. MEYER,1 MICHAEL A. HARDY,1 SEAN E. MCALLISTER,2 AMBER N. TRANSOU,3 RON R. LEVALLEY2 & STEPHEN J. DINSMORE4 1Wildlife Department, Humboldt State University, Arcata, CA 95521, USA 2Mad River Biologists, 417 Second St., Suite 201, Eureka, CA 95501, USA 3California State Parks, PO Box 2006, Eureka, CA 95502, USA 4Department of Natural Resource Ecology and Management, 339 Science II, Iowa State University, Ames, IA 50011, USA Predation is an important cause of nest failure for many birds and has shaped the life- history characteristics of many species, especially ground-nesting shorebirds. We exam- ined nesting success, causes of clutch failure and nest survival in relation to variation in substrate characteristics in a colour-marked population of Western Snowy Plovers Charadrius alexandrinus nivosus breeding on riverine gravel bars in coastal northern California. Plovers experienced higher nesting success on gravel bars than on nearby beaches, which were characterized by more homogeneous, sandy substrates. On gravel bars, Plovers nested in habitats characterized by large, heterogeneous substrates, with more egg-sized stones, compared with random sites. Egg crypsis, as indexed by time required of a naïve observer to detect a nest, increased with number of egg-sized substrates. Nest survival correlated negatively with heterogeneity of substrates and positively with the number of egg-sized stones. Consistently high nesting success of Plovers on gravel bars indicates that this high-quality habitat deserves special manage- ment considerations given the species’ threatened status. Keywords: egg crypsis, habitat selection, nest survival, reproductive success, shorebird. Predation strongly affects the productivity and This general behavioural dichotomy exists within demography of birds (Lack 1968, Martin 1993). two principal shorebird lineages (Van Tuinen Consequently, it has shaped the evolution of et al. 2004), with sandpipers concealing nests in diverse life-history traits associated with breeding, vegetation, and plovers, avocets ⁄ stilts and thick- including nest-site selection behaviours (Cody knees nesting in open habitats (Colwell 2010). 1985), parental distraction displays (Gochfeld Females of both groups lay patterned eggs that 1984) and egg coloration (Collias & Collias enhance crypsis. 1984, Underwood & Sealy 2002, Kilner 2006). Visual crypsis of eggs is effective when egg Many ground-nesting birds build nests in vegeta- appearance (size, colour and pattern) resembles tion that offers concealment to incubating adults the background such that detection by a predator and eggs, which may increase nest survival. is impaired (Endler 1978). Shorebird eggs are Individuals of other species select nest-sites in usually cryptically patterned, with a dull base col- sparsely vegetated habitats and rely on early our and variable markings that disrupt the outline detection of predators, adult departure from the of the egg (Collias & Collias 1984). Egg coloration nest and egg crypsis to enhance nest survival. has been shown to influence crypsis and nest survival in at least one species of shorebird. Solis *Corresponding author. and de Lope (1995) showed that Stone-curlew Email: [email protected] Burhinus oedicnemis eggs survived better when No claim to original US Government works Journal compilation ª 2011 British Ornithologists’ Union 304 M. A. Colwell et al. their base colour matched background substrate. crypsis of river nests to a naïve human observer, Habitat heterogeneity also may influence prey and analyse nest survival in relation to variation in crypsis, as some backgrounds may be inherently substrate size and heterogeneity. more difficult for a predator to search. In support of this, search times of Great Tits Parus major METHODS increased when prey matched a larger, patterned background (Merilaita et al. 2001). Additionally, Study area and field methods within-clutch variation in egg colour was positively related to nest survival in Namaqua Sandgrouse We studied Plovers in Humboldt County, Califor- Pterocles namaqua (Lloyd et al. 2000), suggesting nia, on gravel bars of the lower 15 km of the Eel that varied egg coloration makes a nest more diffi- River, near its confluence with the Pacific Ocean. cult to detect. For species that nest in open, spar- Gravel bar substrates varied in size from fine sely vegetated habitats, the size and heterogeneity sediments of clay and silt to large stones. Breeding of substrates may confer added crypsis to eggs, but habitats were mostly unvegetated, although sparse this relationship has rarely been addressed. stands of willow Salix spp. and White Sweet The Western Snowy Plover Charadrius alexan- Clover Melilotus alba occurred throughout the drinus nivosus is a threatened shorebird that breeds study area. Colwell et al. (2010) provide a detailed along the Pacific Coast of North America in description of the study area. sparsely vegetated habitats characterized by fine We monitored a population of colour-marked substrates and scattered debris (Page et al. 1995). Plovers from 2001 to 2009 under permit (United Clutch predation is an important factor limiting States Fish & Wildlife Service permit TE-823807- population recovery (United States Fish & Wildlife 3; California Department of Fish & Game collect- Service 2007). In coastal northern California, ing permit 801059-03; Humboldt State University Plovers nest in a unique habitat: large-grained, IACUC 00 ⁄ 01.W.83.A; USFWS Federal banding heterogeneous substrates on gravel bars of the permit 22971). Observers surveyed Plovers from lower Eel River. Since 2001, we have monitored mid-March to early September, recording colour reproductive success of a colour-marked popula- band combinations and searching for nests. We tion and shown that gravel bars are higher quality found most nests by observing courting and incu- breeding habitats as evidenced by significantly bating adults. Nest age was determined by observ- higher per-capita fledgling success compared with ing dates of clutch completion, back-dating ocean beaches (Colwell et al. 2005, 2010). We 31 days from the date of hatch (Page et al. 1995), hypothesize that differences in substrates influence or floating completed clutches (Alberico 1995). the ability of visual predators (principally Com- We monitored nests one to four times a week by mon Raven Corvus corax, and American Crow observing incubating adults or, if no adult bird was Corvus brachyrhynchos) to find eggs and chicks present, by approaching nests to verify the pres- (Colwell et al. 2007). Specifically, on sandy bea- ence of eggs. A nest was categorized as successful ches it may be easier for corvids to find nests for if at least one egg hatched. Failed nests were of two reasons. First, comparatively large eggs con- several types: those in which (1) eggs were left trast markedly with fine-grained substrates of unattended by adults for prolonged periods and beaches. Secondly, adult Plovers leave tracks in the parents were absent or observed breeding else- sand leading to nests, which are easily detected by where (abandoned), (2) damaged eggs, dried yolk corvids. By contrast, nests on gravel bars occur in or eggshell fragments were observed in the nest coarser substrates where Plovers rarely leave cup and recent vehicle tracks were present near tracks. However, gravel bar substrate size, hetero- the nest (vehicle), (3) eggs disappeared after high geneity, and patchiness vary greatly and nests water in the river washed over nest-sites (flood- occur in a wide range of substrates from homoge- ing), (4) eggs disappeared since a recent visit and neous sand or small pebbles (< 2 cm diameter) to prior to predicted hatch date, and direct observa- heterogeneous mixes of sand, pebbles, stones tion or tracks indicated a predator consumed the (2–10 cm) and cobble (> 10 cm). Here, we pres- eggs (predation), and (5) nest cups were empty ent data on nesting success of gravel bar-nesting but a long interval had transpired such that Plovers, compare substrates at nests with random we could not confidently assign the nest to any sites to evaluate nest-site selection, examine the category (unknown). No claim to original US Government works Journal compilation ª 2011 British Ornithologists’ Union Snowy Plover egg crypsis 305 Habitat measures Data summary and analyses In 2001–2004, we measured substrate characteris- We examined nest-site selection by comparing sub- tics at all nests that we found by centring over the strate characteristics between 100 nests and paired nest (or paired random site) a 1-m2 plastic random sites sampled over 4 years. For each nest sampling frame subdivided by cord at 20-cm inter- and random site, we used the sample of 16 vals, such that crossing lines created 16 sampling measures to characterize mean size (mm) and points. Using callipers, we measured (to the near- heterogeneity (sd) of substrates, and the number of est 0.1 mm) the greatest exposed axis of the sub- egg-sized substrates (20–40 mm), which brackets strate lying under each sampling point. In 2001, Plover egg size (31.3 ± 1.0 mm; Page et al. 1995). however, an observer collected data by categorizing We compared substrates between nests and ran- substrates into 10 pre-defined size classes (Meyer dom sites using two-tailed paired t-tests because 2005). We converted these ordinal measures to substrate size (r = 0.60, P < 0.01) and heterogene- continuous data using the middle value of each ity (r = 0.48, P < 0.01) were positively correlated category. The largest category had no upper limit, between nests and random sites across samples. hence we used the average rock size for substrates We omitted a small number of nests (10) from the larger than 100 mm from subsequent years. To nest-site selection and survival analyses (see below) minimize disturbance and reduce the risk of lead- because substrates had changed since a Plover’s ing predators to nests, we measured habitat after original selection of the site.