DOI: 10.5061/dryad.cjsxksn85
title: Impact of human disturbance on the abundance of non-breeding shorebirds in a subtropical wetland

description: Shorebird populations have declined due to several threats throughout
 their annual cycle. Anthropogenic disturbance is one of the most
 ubiquitous threats to shorebird conservation in North America. Here, we
 studied the influence of human disturbance on shorebird community dynamics
 during migration and winter in Ensenada de La Paz, a subtropical coastal
 wetland in Mexico. We used negative binomial generalized linear mixed
 models to investigate the associations between spatial, biological, and
 anthropogenic variation and local shorebird abundance that accounted for
 shorebird body size (small, medium, and large) and foraging strategy
 (visual and tactile) of 21 shorebird species. After controlling for these
 different correlates of abundance, human disturbance (people, vehicles,
 and dogs) was negatively associated with shorebird abundance. During
 winter, all shorebird species were negatively related to human disturbance
 but positively associated with presence of raptors. However, small,
 tactile foraging birds exhibited a proportionally larger negative response
 to human disturbance than other shorebird types, indicative of guild-level
 sensitivities to human disturbance regimes. The positive association
 between shorebird abundance and disturbance from predators was unexpected.
 Shorebirds likely concentrate in large groups to reduce predation risk,
 resulting in higher densities of shorebirds occurring in areas with high
 predation risk. Understanding factors influencing the abundance and
 habitat use of shorebirds on their non-breeding grounds is paramount to
 support management and conservation policies for shorebirds and their
 habitats.
description: We conducted an observational
 cross-sectional study with repeated surveys to determine effects of
 potential raptor and human disturbance on abundance of shorebirds. Area
 search surveys at 12 sampling units followed a monitoring protocol
 described in detail by Reiter et al., (2020), including documentation of
 environmental and habitat conditions in each sampling unit during each
 survey. The area flooded, vegetated, and bare ground was visually
 estimated at each sampling unit during field surveys. In addition, number
 of potential human disturbances (e.g., number of humans and number of
 dogs) and avian predators were recorded as they occurred during the survey
 period at each sampling unit (see details below). Twelve sampling units
 were surveyed twice a month, during neap rising tides, from September 2014
 to April 2015; each sampling unit was visited 15 times. Sampling units
 were fixed areas of homogeneous habitat delimited by a polygon and
 included a mosaic of natural and human-made habitats within Ensenada de La
 Paz. Intertidal mudflat was the dominant habitat type among sampling units
 and are mud-sandy areas exposed during low tides, with less than 10%
 vegetation cover, and was found in Marina Sur (6.1 ha), Zacatal (30.6 ha),
 Chametla 1 and 2 (7.8 and 23.2 ha, respectively), Centenario 1 and 2 (22.7
 and 9.2 ha, respectively), and Granjas Centenario (27.8 ha) sampling
 units. In addition, sandy beach habitat occurred at units Comitán 1, 2,
 and 3 (8.7, 9.3, and 12.6 ha, respectively). Human-made habitats included
 an abandoned shrimp farm (Granja Abandonada, 10.2 ha) and sewage ponds in
 the wastewater treatment plant of La Paz (Lagunas de Oxidación, 9.9
 ha). During each survey, a team of one observer and
 one annotator counted all shorebirds using the sampling unit, including
 shorebirds arriving or leaving the sampling unit, but not those flying
 over the sampling unit. Small flocks (˂300 birds) were counted, and we
 used the “blocks” method (Howes & Bakewell, 1989) to estimate
 numbers in flocks with ˃300 birds. Distance from the observation point to
 the birds in each count was generally less than 200 m. We sought a quick
 survey to assess bird estimates and behavior, so we tried to limit the
 time at each sampling unit while counting all birds accurately. Across all
 surveys, time spent at each sampling unit was between 8 and 44 min (mean =
 26 min, SD = 20); duration depended on species richness, number of
 individuals, and size of the sampling unit. Long-billed (Limnodromus
 scolopaceus) and Short-billed (L. griseus) dowitchers were grouped as
 dowitchers (Limnodromus spp.) because their identification in the field
 was difficult. All surveys were conducted during daylight hours, in the
 same tidal condition (neap rising tides), when wind was &lt; 25 km/hr,
 and when there was no rain to control for any effects on bird abundance
 behavior. We counted avian predators during surveys and
 considered them as the baseline of potential natural disturbance as they
 can disrupt normal activities of shorebirds. Avian predators included
 Peregrine Falcons (Falco peregrinus), Cooper’s Hawks (Accipiter cooperi),
 Crested Caracaras (Caracara cheriway), Zone-tailed Hawks (Buteo
 albonotatus), Merlins (Falco columbarius), and American Kestrels (Falco
 sparverius). In addition, avian predators in, perched adjacent to, or
 flying over the sampling unit were counted during each
 survey. We considered a potential cause of human
 disturbance to be “any human-induced activity that likely constitutes a
 sufficient stimulus to disrupt normal activities or the distribution of
 shorebirds compared to a situation without such activity” (page 2, Fox
 & Madsen, 1997). We counted number of potential causes of
 disturbance, as they occurred, during the survey period in the sampling
 unit (Sutherland & Green, 2004). We categorized potential human
 disturbances as dogs (without a leash, either alone or accompanied by
 people), people (running or walking, in a group or alone, or dogs with a
 leash), vehicles (cars, motorcycles, and ATVs), and other (horses, drones,
 and fishing boats).
description: We
 used generalized linear mixed models (GLMM; Zuur et al., 2009) to quantify
 associations of the count of shorebirds in different groups in each
 sampling unit with raptor and potential anthropogenic disturbance
 (events/minute), type of habitat, and percentage of the sampling unit that
 was flooded. Area (ha) of the sampling unit was also used to control for
 different sizes of sampling units. All models included a random effect of
 the sampling unit and a random intercept for species to account for
 different abundances among species and random slopes of the effect of
 human disturbance by species to control for variation in the effect of
 human disturbance across species. In addition, we consider random slopes
 for other covariates (e.g., raptor disturbance); however, the models would
 not converge, and since we were most interested in the effect of potential
 human disturbance, we considered random slopes for that covariate only.
 Because the distribution of shorebird counts had excess zeros and was
 overdispersed, we assumed a zero-inflated negative binomial distribution.
 Shorebird abundance was compared between migration (fall/spring) and
 winter, between three size classes (small, medium, and large shorebirds,
 sensu Page et al., 1997), and between two feeding strategies (tactile and
 visual, sensu Barbosa & Moreno, 1999). Species with &lt;0.02
 probability of occurrence were excluded from the analysis. Western
 Sandpipers (Calidris mauri) represented 67% of all counted birds.
 Therefore, similar analyses were performed for this single species during
 winter and migration. In total, we evaluated the model for 14 response
 variables, each representing a different combination of counts by
 shorebird species groups and/or seasons, but all with the same covariates
 considered. We considered migration (September-October and March-April)
 and wintering (November-February) seasons separately because we
 hypothesized that the responses of shorebirds to disturbance might differ
 during these different periods of the annual cycle.