Breeding probability, survival, and causes of mortality in translocated Black Grouse (Lyrurus tetrix) in the Hautes Fagnes, Belgium

The mortality process is a key factor in avian population dynamics and conservation strategies like translocation. Understanding mortality drivers, particularly in translocated individuals, is essential for Grouse conservation. Black Grouse (Lyrurus tetrix) populations have strongly declined across Western and Central Europe. We studied five reinforcement campaigns of the last Belgian population in the Hautes Fagnes, using wild individuals translocated from Sweden. Between 2018 and 2024, we monitored 65 released individuals and investigated 44 deaths. Predation accounted for 88.7%, maybe 93.2%, of all recorded deaths, largely attributable to mammalian predators (72.8%), whereas 15.9% of death were caused by raptors. However, in 36.4% of mammalian predation cases, we could not determine ante-mortem nor post-mortem injuries due to insufficient traces. Other causes of mortality were rare, with fence collisions accounting for 4.5%, and no evidence of disease-related deaths. Mortality causes and cumulative mortality risks were independent of sex. Survival analysis showed high mortality in the first two months (26.7% per month), followed by a lower but ongoing rate (7.5% per month). During the first year, overall survival was 24.6%. Survival varied greatly from year to year, ranging from 100 to 30% after 60 days post-release. Breeding probability was also assessed. Around 50% of first-year females are expected initiate incubation if not predated, though the observed rate dropped to 23.9% with predation. Among these, 33.3% of nests are lost, potentially predated, and half of the mothers were lost during chick dependence. In contrast, females tracked into their second year experienced no predation nor their eggs, though the small sample size warrants caution in interpretation. The data collected show that increasing predator control, particularly in the first two months post-release but also beyond, is essential to enhance the restoration success of this Black Grouse population by improving survival, nesting success, and chick recruitment rate.

Translocation · Survival curves · Predation · GPS-tracking · Brooding · Grouse conservation