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Data from: Divergent estimates of herd-wide caribou calf survival: ecological factors and methodological biases


Ellington, Edward Hance; Lewis, Keith P.; Koen, Erin; Vander Wal, Eric (2021), Data from: Divergent estimates of herd-wide caribou calf survival: ecological factors and methodological biases, Dryad, Dataset,


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.


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.

Usage Notes

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.