DOI: 10.5061/dryad.4xgxd254v
title: Habitat-driven vulnerability to nest predation in Arctic-breeding plovers based on artificial nest experiment and real shorebird nest monitoring – Survival dataset

description: Lower vulnerability to predation should increase the capacity of prey
 populations to maintain positive population growth rate in
 regions characterized by high predation pressure. Some arctic nesting
 shorebirds nest almost exclusively in areas where predation pressure is
 regularly released. The few species that can breed within the
 entire distribution range of the Arctic fox (Vulpes lagopus), the
 main nest predator in the arctic tundra, are supposedly less sensitive
 to predation. However, empirical data supporting this hypothesis
 are scarce and mechanisms driving interspecific variation in
 vulnerability to nest predation are poorly documented.
 We monitored nest success of two shorebird species with
 contrasting breeding distribution and nesting habitat. We found
 that i) when co-existing at the same breeding site, the
 widely distributed Ringed Plover nesting along stony shores
 showed a higher nest survival rate than the Golden Plover nesting
 in mesic tundra, and ii) such differences in nest survival were at
 least partly driven by the nesting habitat type per se, with
 lower predation risk in stony shores than in adjacent mesic
 tundra. We suggest that the use of safer nesting habitat by some shorebird
 species can contribute to maintaining viable breeding populations
 over a broader distribution range.
description: Shorebirds nest
 monitoring This
 study was conducted over three years (2014-2016) on the southwest plain of
 Bylot Island, Nunavut, Canada (73° 08’ N, 80° 00’ W), located in Sirmilik
 National Park. The landscape is dominated by mesic tundra on the uplands
 and both mesic tundra and wetlands in the lowlands. The mesic tundra is
 covered with relatively lush vegetation for this latitude, mainly composed
 of low shrubs (Salix, Vaccinium) and forbs (e.g. Cassiope, Dryas) with
 grasses and mosses. The American Golden Plover (Pluvialis dominica) and
 the Ringed Plover (Charadrius hiaticula) are both common breeding
 shorebird species within the study area. The American Golden Plover
 prefers to breed in the low vegetation of mesic tundra, while the Ringed
 Plover breeds on stony and sandy shores and gravel bars with scarce
 vegetation along coasts and rivers. Each summer, nesting plovers were
 monitored along a 50-km-long coastline. We found nests either
 opportunistically or through systematic search (line transect surveys)
 from mid-June to mid-July. Nests were marked with a 10 cm wooden stick
 (medical tongue depressor) and a natural object (rock or feather) placed 5
 m and 7 m from the nest respectively. We typically visited nests every 3-5
 days or every 2-3 days when nearing the estimated hatching date. The
 incubation stage was estimated for each nest using the flotation method.
 Incubation lasts 26 days for the American Golden Plover and 24 days for
 the Ringed Plover. A nest was considered successful if at least one egg
 hatched or if one of the following criteria were met: 1) &lt; 5 mm of
 residual egg shell was found in the nest material close to the estimated
 hatching date 2), the nest was hatching (starred or pipped) on the last
 visit and was empty on the next visit, and 3) the nest was empty on the
 last visit and the banded adult was later seen with chicks. Shorebirds
 chicks generally leave the nest within 24 hours of hatching. Predation is
 the main cause of shorebird nest failure in our study area and other
 sources of mortality are marginal (nest abandonment or nest flooding was
 confirmed in only 3 out of 236 plover nests with known fate).
 Artificial nest
 experiments Artificial nest experiments were
 conducted during the plover incubation period in 2015 and 2016. Each year,
 a total of 60 paired artificial nests were deployed in suitable habitat
 used by the two selected plover species: 30 artificial nests were placed
 on stony shores (nesting habitat of the Ringed plover) and 30 nests in
 mesic tundra (nesting habitat of the American Golden Plover). The two
 paired nests were separated by 150-200 m and the distance between
 artificial nests positioned in each habitat was within the range of
 distances recorded for real nests on Bylot Island (average distance: 877 m
 and 547 m, range 341-1431 m, 52-1845 m, for artificial and real nests
 respectively). Artificial nests were deployed on 12 July 2015 and 10 July
 2016 and were visited after 1, 2, 4 and 6 days. Each artificial nest
 consisted of four quail eggs (Coturnix japonica) placed in a shallow
 depression in the ground; quail eggs are similar to plover eggs in colour
 and size. The artificial nests were marked as the real nests (see above),
 but a nail wrapped in fluorescent tape was hidden beneath the eggs so
 depredated nests could be located easily. To reduce human scent, all eggs
 and pieces of material used were handled with latex gloves, and
 researchers used the sole of their rubber boots to make the nest
 depression. The Arctic fox is the main predator of both real and
 artificial shorebird nests in our study area. However, artificial nests
 are more vulnerable to avian predators than real nests, which may result
 from the absence of parental nest defence and because uncovered artificial
 nests can be more easily detected by avian predators than nests covered by
 the incubating adult. To examine if habitat-driven patterns in predation
 risk are affected by the exclusion of avian predators, we conducted
 another experiment using artificial nests covered with a patch of lichen
 commonly found in the study area (genus Bryoria or Gowardia). The lichen
 covers were maintained on top of the eggs using a wooden stick inserted in
 the ground in the middle of the nest. We deployed 20 paired covered
 artificial nests on 28 June 2016 (20 nests in stony shores and 20 in mesic
 tundra), and visited them after 2, 6 and 15 days of exposure. Using
 motion-triggered cameras, we confirmed that covered artificial nests were
 only depredated by the Arctic fox in our study area (Bêty and
 Léandri-Breton, unpublished data, N = 82 depredated nests monitored with
 cameras in 2015 and 2016). Covered nests also allowed us to control for
 potential differences in quail egg crypsis associated with habitat
 types. Statistical analyses
 For real shorebirds nests,
 we compared the daily nest survival of the two plover species using the
 logistic-exposure method. Year was included as a random factor to control
 for inter-annual variations. The logistic-exposure method is a generalized
 linear model with a binomial response distribution (1 when the interval
 nest fate is a success and 0 when depredation occurred) using a logit link
 function to account for variations in the length of observation intervals.
 This method is advantageous as it allows the inclusion of random effects
 (e.g. “year”) and requires no assumptions about when nest losses occur. We
 assumed a constant daily nest survival during the nesting period. For
 graphical representation, daily nest survival was estimated through a
 logistic exposure model per species and per year. The nest success
 probability over the entire incubation period was obtained by raising the
 daily nest survival to the power of the mean incubation period of each
 species (26 days for the American Golden Plover and 24 for the Ringed
 Plover). We used Cox proportional hazards regression (R package “survival”
 2.40-167) to assess the effect of habitat types on the survival of
 artificial nests. Cox model tests for a relationship between Kaplan-Meier
 survival estimates, a nonparametric statistic commonly used to estimate
 survival over time, and explanatory variables. Cox model is very
 appropriate for artificial nests because it allows for right-censoring
 data when nests survive past the end of the experiment. Year was treated
 as a random variable. The proportion hazards assumption was verified by
 calculating the correlation between scaled Schoenfeld residuals and
 survival time. For visualization purposes, predation risk was estimated by
 fitting a Kaplan-Meier survival probability curve for each species and
 "year" without random effect. All analyses were carried out in R
 version 3.2.270.