DOI: 10.5061/dryad.mp7h92r
title: Data from: Effects of distance on detectability of Arctic waterfowl using double-observer sampling during helicopter surveys

description: Aerial survey is an important, widely employed approach for estimating
 free‐ranging wildlife over large or inaccessible study areas. We studied
 how a distance covariate influenced probability of double‐observer
 detections for birds counted during a helicopter survey in Canada’s
 central Arctic. Two observers, one behind the other but visually obscured
 from each other, counted birds in an incompletely shared field of view to
 a distance of 200 m. Each observer assigned detections to one of five 40‐m
 distance bins, guided by semi‐transparent marks on aircraft windows.
 Detections were recorded with distance bin, taxonomic group, wing‐flapping
 behavior, and group size. We compared two general model‐based estimation
 approaches pertinent to sampling wildlife under such situations. One was
 based on double‐observer methods without distance information, that
 provide sampling analogous to that required for mark–recapture (MR)
 estimation of detection probability,
 urn:x-wiley:20457758:media:ece34824:ece34824-math-0001, and group
 abundance, urn:x-wiley:20457758:media:ece34824:ece34824-math-0002, along a
 fixed‐width strip transect. The other method incorporated double‐observer
 MR with a categorical distance covariate (MRD). A priori, we were
 concerned that estimators from MR models were compromised by heterogeneity
 in urn:x-wiley:20457758:media:ece34824:ece34824-math-0003 due to
 un‐modeled distance information; that is, more distant birds are less
 likely to be detected by both observers, with the predicted effect that
 urn:x-wiley:20457758:media:ece34824:ece34824-math-0004 would be biased
 high, and urn:x-wiley:20457758:media:ece34824:ece34824-math-0005 biased
 low. We found that, despite increased complexity, MRD models (ΔAICc range:
 0–16) fit data far better than MR models (ΔAICc range: 204–258). However,
 contrary to expectation, the more naïve MR estimators of
 urn:x-wiley:20457758:media:ece34824:ece34824-math-0006 were biased low in
 all cases, but only by 2%–5% in most cases. We suspect that this
 apparently anomalous finding was the result of specific limitations to,
 and trade‐offs in, visibility by observers on the survey platform used.
 While MR models provided acceptable point estimates of group abundance,
 their far higher stranded errors (0%–40%) compared to MRD estimates would
 compromise ability to detect temporal or spatial differences in abundance.
 Given improved precision of MRD models relative to MR models, and the
 possibility of bias when using MR methods from other survey platforms, we
 recommend avian ecologists use MRD protocols and estimation procedures
 when surveying Arctic bird populations.
description: QMG 2014 Detections
 Distance Double-observerContains information
 about each animal group either observer from a helicopter in flight:
 Species code, 'capture' history, count, whether group was flying
 or not, distance bin of detection by front observer, distance bin of
 detection by back observeranalyze_waterfowl_mrds_FINALR-code for analyzing detections of Arctic waterfowl using double-observer sampling during helicopter surveys