DOI: 10.5061/dryad.bk3j9kdfr
title: Data from: Evaluation of breeding distribution and chronology of North American scoters

description: Aim: North America’s scoter species are poorly monitored relative to other
 waterfowl. Black (Melanitta americana), surf (M. perspicillata), and
 white-winged (M. deglandi) scoter abundance and trend estimates are thus
 uncertain in many parts of these species’ ranges. The most extensive
 source of waterfowl abundance and distribution data in North America is
 the Waterfowl Breeding Population and Habitat Survey (WBPHS). Although the
 WBPHS effectively monitors most species, both its timing and geographic
 coverage may not allow for accurate scoter monitoring. Therefore, our goal
 was to better define when and where scoters breed to help interpret survey
 results and optimize survey methods for scoters. Location: Canadian boreal
 shield, taiga shield, and low-Arctic tundra; Alaska. Taxon: Scoters
 (Genus: Melanitta) Methods: We integrated satellite telemetry tracking
 data from scoters marked at multiple molting, staging, breeding, and
 wintering areas along the Atlantic and Pacific coasts to quantify
 continent-wide breeding chronology and distribution. We also examined
 possible drivers of variation in timing of arrival, length of stay, and
 departure at nesting locations. Results: We documented a
 northwest-to-southeast distribution of breeding sites across Alaska and
 Canada. On average, scoters arrived at nest sites on June 1. Surf scoters
 arrived earliest, stayed for shorter periods, and departed earliest.
 Pacific-wintering scoters began breeding earlier than Atlantic-wintering
 birds. Additionally, birds arrived at nesting locations earlier in years
 with earlier snowmelt, and later snowmelt reduced lengths of stay for
 males. Breeding chronology also varied by age group, with adults arriving
 earlier than subadults. Main conclusions: Our study is the first to
 comprehensively describe spatial variation in timing of breeding of both
 Atlantic and Pacific populations of all three scoter species across North
 America. Our results increase our understanding of how current surveys
 enumerate scoters and will inform possible supplemental efforts to improve
 continental monitoring of scoter populations.
description: Dataset 1. Scoter satellite telemetry breeding location and
 chronology data We compiled data for a total of 94
 black scoters, 124 surf scoters, and 95 white-winged scoters captured
 between 1999 and 2019 at sites in the Great Lakes and along the Atlantic
 and Pacific coasts. All captures were made during scoter molting, staging,
 and wintering periods (August - March). Birds were implanted with Platform
 Transmitter Terminals (PTTs), programmed to collect location information
 once every three to five days. Detection data were filtered to only
 include detections from 20 April to 1 July of each year (i.e., the
 suspected nesting period of North American scoters). We identified
 clusters of repeated detections where individuals settled for at least
 nine days within the potential breeding range. We identified nesting
 locations by drawing a 20 km buffer around the centroid of each cluster,
 and all detections within each 20 km buffer were considered movements
 within the nesting location. We then classified arrival date as an
 individual's first detection at the nesting location, departure date
 as the last detection at the nesting location, and length of stay as the
 number of days between an individual's last and first
 detection.  Dataset 2. Spatial predictions of scoter
 nesting location arrivals We used linear mixed-effects
 models to evaluate whether variations in scoter arrival dates at nesting
 locations differed by species, wintering origin, age class, sex, and/or
 snow-free date. Here, snow-free date was estimated at the nesting location
 of each PTT-marked individual using the National Oceanic and Atmospheric
 Administration (NOAA) National Snow & Ice Data Center’s Ice
 Mapping System (IMS) snow cover maps. We downloaded daily snow cover data
 for Canada and Alaska for all years with detection data (1999–2019) at a
 spatial resolution of 24 km. Snow-free date was identified for each 24 km
 square as the third consecutive day of the year (DOY) with no snow. We
 then calculated the snow-free date for each year at each nesting location
 as the mean DOY of snowmelt within a 24 km square buffer around each
 nesting location centroid. As a whole, estimated scoter
 breeding locations closely followed the delineation of the Taiga
 ecoregion, with additional detections in the Hudson Plains ecoregion and
 along the northwest coast of Alaska. We predicted each species’  arrival
 timing across this area using the best-supported model for scoter arrival
 dates, based on the lowest Akaike’s information criterion corrected for
 small sample size (AICc), and annual snowmelt data. First, we created a
 polygon for prediction (hereafter the ‘prediction polygon’). We downloaded
 Level 1 ecoregion shapefiles from the U.S. Environmental Protection
 Agency. We drew a 100 km buffer around the Taiga and Hudson Plains
 ecoregions to account for scoters potentially nesting in ecoregion
 transition zones. We then extended this polygon to include PTT-estimated
 breeding locations along the northwest Alaskan coast. We used the same
 polygon for all three species. Next, we estimated arrival dates for each
 species across the prediction polygon. To do this, we overlaid the
 prediction polygon with a hexagonal grid. Snow cover data had a spatial
 resolution of 24 km; we thus generated grids where each hexagon had an
 apothem (i.e., distance from the hexagon centre to side) of 24 km. Using
 annual snowmelt rasters from 2010 to 2020, we generated a new raster
 showing mean snowmelt date for this period (roughly the last 10 years of
 this study), which was used to determine mean snow-free date within each
 hexagonal grid cell. We then used each hexagon’s estimated mean snow-free
 date in our best-supported model to estimate arrival dates in each
 hexagon. For prediction, we held wintering origin at Atlantic, age at
 adult, sex at male, and year and animal identity at 0 (i.e., reference
 levels). All spatial analyses were completed using the “sf” and “raster”
 packages.   Dataset 3. Brood observations
 We collected brood observation data to evaluate whether PTT
 implantation delayed PTT-marked birds. Brood observation data were
 collected from various sources. We collected Labrador brood data from
 unpublished reports (Barrow, 1982; LGL Limited, 2008) and surveys (SGG).
 Québec data came from various reports and publications (Benoit, 2005;
 Benoit et al., 1994, 1996; Consortium Gauthier & Guillemette -
 G.R.E.B.E., 1990; Morneau, 1998; Morrier et al., 2008) and unpublished
 survey data (J-FP and CL). Information on northern Ontario broods came
 from Brook et al. (2012). J. Fisher (USFWS) provided a large dataset of
 brood observations collected in Alaska from 1990–1993. We estimated nest
 initiation dates (i.e., date the first egg was laid) by back-dating from
 the date of brood observation, using the median age of each duckling
 plumage age class (from Gollop & Marshall, 1954; adjusted for surf
 scoters according to Lesage et al., 1997), a laying rate of one egg per
 day, and a mean clutch size of 7 eggs. In total, nest initiation dates
 were estimated for 452 black, surf, and white-wing scoter broods observed
 between 1989 and 2012.
description: Data files can be opened with Microsoft Excel.