Survival rates of adult and juvenile gyrfalcons in Iceland: estimates and drivers

Abstract

Knowledge of survival rates and their potential covariation with environmental drivers, for both adults and juveniles, is paramount to forecast the population dynamics of long-lived animals. Long-lived bird and mammal populations are indeed very sensitive to change in survival rates, especially that of adults. Here we report the first survival estimates for the Icelandic gyrfalcon (Falco rusticolus) obtained by capture-mark-recapture methods. We use a mark-recapture-recovery model combining live and dead encounters into a unified analysis, in a Bayesian framework. Annual survival was estimated at 0.83 for adults and 0.40 for juveniles. Positive effects of main prey density on juvenile survival (5% increase in survival from min to max density) were possible though not likely. Weather effects on juvenile survival were even less likely. The variability in observed lifespan suggests that adult birds could suffer from human-induced alteration of survival rates.

Keywords: Capture-recapture; Gyrfalcon; Iceland; Mark-recovery; Survival.

Figure 1. Observed capture histories of gyrfalcons (Falco rusticolus) ringed in Iceland (1973–2019), with the state sequence for individuals ringed (1-blue: live capture or resighting; 2-black: dead recovery; 3-white: neither seen nor recovered).

Figure 2. Logistic regression of juvenile gyrfalcon survival (s₁).

(A) s₁ as a function of ptarmigan abundance x₁. (B) Corresponding distribution of x₁. Standardized ptarmigan abundance of −1.43 (observed min) and 2.58 (observed max) were approximately 2.5 and 12.5 individuals.km⁻², respectively.