Divergent trends in migration timing of shorebirds along the Pacific flyway

Long distant migrants timing their arrival on the breeding grounds must make the tradeoff of optimal timing for breeding vs. optimal timing for survival. For many shorebird species, the flyway northward spans thousands of kilometers, and both conditions encountered en route and the priorities of individuals can affect the timing of migration. We used data from spring migration surveys of Western Sandpipers (Calidris mauri) and Pacific Dunlins (Calidris alpina pacifica) along the Pacific Flyway of North America to determine if the timing of their northward movements changed from 1985 to 2016. We examined 5 sites of varying size along the northern portion of the flyway from Washington, British Columbia, and Alaska, and estimated interannual trends in passage timing relative to each site’s distance to the breeding grounds. The peak passage dates at the sites closest to the species’ breeding grounds in Alaska shifted later by 1-2 days over the study period, while date of peak passage at sites further south shifted ~3 days earlier. A post-hoc analysis suggested local temperature also affected passage dates at most sites, with warmer temperatures being related to earlier passage. Discerning patterns of movement by Dunlins at southern sites was complicated by the presence of winter residents. Simulation analyses of sandpiper movement through a stopover site highlighted both length of stay and timing of arrival as important factors shaping peak passage estimates. We suggest Western Sandpipers appear to be arriving earlier at southern sites, and are spending longer at larger stopover sites such as Alaska’s Copper River Delta. Our analysis generates specific predictions for expected behavior on northward migration and we believe may be a useful indicator in other systems where historical count data are available. Ke

Species-environment associations and predicted distribution of Black Oystercatcher breeding pairs in Haida Gwaii, British Columbia, Canada

We present a species distribution model (SDM) for prediction of Black Oystercatcher (Haematopus bachmani) breeding pair occurrence in Haida Gwaii, British Columbia. Boosted regression trees, a machine learning algorithm, was used to fit the model. In total, 14 predictors were selected a priori through development of a conceptual model. Breeding pair occurrence data were compiled from two available surveys conducted in 2005 and 2010 (545 km of shoreline surveyed in total). All data were aggregated to common model units (vector polyline shoreline segments approximately 100 m in length), which approximate breeding territory size. The final model, which included eight predictors (distance to treeline, island area, wave exposure, shoreline type, intertidal area within 50 m, segment length, rat occurrence, and intertidal area within 1000 m), had excellent predictive ability assessed by 10-fold cross-validation (AUC = 0.89). Predictive ability was reduced when the model was trained and tested on spatially (AUC = 0.86) and temporally (AUC = 0.83) independent data. Distance to treeline and island area had greatest influence on the model (RI = 41.5% and RI = 36.7%, respectively); we hypothesized that these predictors are related to avoidance of predators. Partial dependence plots revealed that breeding pairs tended to occur: further from the treeline, on small islands, at high wave exposures, at moderate intertidal area, on bedrock or gravel shoreline types, and on islands without rats. However, breeding pairs tended not to occur on very small islands and at very high wave exposures, which we hypothesize to reflect avoidance of nest washout. Results may inform local conservation and management efforts, i.e., from predictive maps, and eventual development of a high-resolution (~100 m) model for prediction of Black Oystercatcher breeding pairs at a regional scale. Further, methods and GIS data sets developed may be used to model distribution of other coastal species in the region

Population trends of nesting Glaucous-winged Gulls, Larus glaucescens, in the southern Strait of Georgia, British Columbia

Between 1986 and 1999, the number of nesting Glaucous-winged Gulls (Larus glaucescens) at fourteen breeding colonies declined by 31% (1610 nests) in the southern Strait of Georgia, British Columbia. Declines in the number of nesting pairs at surveyed colonies ranged between 6% and 81% with the smallest change occurring on the largest breeding colony in the study area. We suggest that Glaucous-winged Gull breeding population declines in the southern Strait of Georgia may be due to increased frequency of colony disturbance, in particular increased disturbance by Bald Eagles (Haliaeetus leucocephalus)

Latitudinal temperature-dependent variation in timing of prey availability can impact Pacific seabird populations in Canada

We modelled how nestling growth rates of Cassinâ s Auklet (Ptychoramphus aleuticus (Pallas, 1811)) varied with timing of peak copepod prey availability at two breeding colonies in British Columbia: on Triangle Island, in the California Current Ecosystem, and Frederick Island, in the Gulf of Alaska Ecosystem. We used time series of nestling growth rates and estimated the seasonal timing of peak biomass of copepods (Neocalanus cristatus (KrĂśyer, 1845)) using a temperature-dependent phenology equation. We developed a single model to examine intercolony differences in the effect of the timing of regional peak prey biomass on seabird nestling growth rates. This model indicated nestling growth rates on Triangle Island varied widely and were positively associated with timing of peak zooplankton biomass, such that higher growth rates were observed when the peak biomass occurred later in the breeding season. In contrast, nestling growth rates were consistently high at Frederick Island, where peak copepod biomass always occurred relatively late. If ocean climate warming results in a poleward shift of Neocalanus abundance and induces earlier and more narrow timing of availability, episodes of poor nestling growth will increase in frequency on Triangle Island, and could eventually affect auklets on more northerly colonies.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author