DOI: 10.5061/dryad.xgxd254db
title: Data from: Divergent estimates of herd-wide caribou calf survival: ecological factors and methodological biases

description: Population monitoring is a critical part of effective wildlife management,
 but methods are prone to biases that can hinder our ability to accurately
 track changes in populations through time. Calf survival plays an
 important role in ungulate population dynamics and can be monitored using
 telemetry and herd composition surveys. These methods, however, are
 susceptible to unrepresentative sampling and violations of the assumption
 of equal detectability, respectively. Here we capitalized on 55 herd-wide
 estimates of woodland caribou (Rangifer tarandus caribou) calf survival in
 Newfoundland, Canada using telemetry (n = 1,175 calves) and 252 herd-wide
 estimates of calf:cow ratios (C:C) using herd composition surveys to
 investigate these potential biases. These data included 17 herd-wide
 estimates replicated from both methods concurrently (n = 448 calves and n
 =17 surveys) which we used to understand which processes and sampling
 biases contributed to disagreement between estimates of herd-wide calf
 survival. We used Cox proportional hazards models to determine if
 estimates of calf mortality risk were biased by the date a calf was
 collared. We also used linear mixed effects models to determine if
 estimates of C:C ratios were biased by survey date and herd size. We found
 that calves collared later in the calving season had a higher mortality
 risk and that C:C tended to be higher for surveys conducted later in the
 autumn. When we used these relationships to modify estimates of herd-wide
 calf survival derived from telemetry and herd composition surveys
 concurrently, we found that formerly disparate estimates of woodland
 caribou calf survival now overlapped (within a 95% confidence interval) in
 a majority of cases. Our case study highlights the potential of
 under-appreciated biases to impact our understanding of population
 dynamics and suggests ways that managers can limit the influence of these
 biases in the two widely applied methods for estimating herd-wide
 survival.
description: Staff from the government of Newfoundland and Labrador monitored
 540 radio-collared woodland caribou calves across eight herds during the
 population growth phase (1979–1997) and 635 radio-collared calves across
 five herds during the population decline phase (2002–2014). Capture
 methods are described in Rayl et al. (2014) and Mumma et al.
 (2019). Staff from the government of Newfoundland and
 Labrador conducted 249 herd composition surveys for 26 woodland caribou
 herds across Newfoundland from 1979–2014, except for the years 1998–2001.
 Herd composition survey methods varied both temporally and spatially from
 1979–2014 and we did not necessarily know the exact methodology used for a
 specific herd year. We summarize below a broad picture of the methods
 used. Between the months of September and December, observers (typically
 two plus the pilot) flew rotary-wing (and perhaps in earlier years,
 fixed-wing) aircraft over an area believed to be where herds were
 currently located. Occasionally, telemetry data were used to estimate
 where a woodland caribou herd was located and in other years, the historic
 position of the herd was used. Herd composition surveys were typically
 conducted over one day but occasionally surveys were conducted over
 multiple days if weather was poor or woodland caribou were difficult to
 locate. For larger herds, the herd composition survey was a sampling
 effort but for smaller herds total counts were occasionally conducted. The
 crew counted the number of woodland caribou within basic demographic
 groups: adult male, adult female, and calf, and if needed the pilot would
 use the aircraft to separate large groups into more manageable sub-groups
 for counting. Staff from the government of Newfoundland
 and Labrador also conducted herd composition surveys during the months of
 May and June to estimate the proportion of parous cows (an estimate of
 fecundity). Cows with calves close by or with visible signs of pregnancy
 were considered parous.
description: Within the Rdata file are three dataframes:  TELE_DATA - this contains caribou
 calf survival data HCS_DATA_FALL - this contains
 caribou herd composition survey data from the fall HCS_DATA_SPR - this contains
 caribou herd composition survey data from the spring
 Please feel free to contact me with any questions about these
 datasets.