Causes and timing of low breeding success in capercaillie

We have been counting and recording the breeding success of capercaillie since 1990. © GWCT

BACKGROUND

Capercaillie in Scotland have seen a considerable decline in numbers and a range contraction since the 1970s. The latest national survey in 2015/16 concluded that they remain at a critically low level of around 1,000 birds, although many believe the number to be lower. Previous studies highlighted low breeding success as the proximate cause of declines. We have been counting capercaillie annually in Scotland since 1990, using pointing dogs in August to find well-grown broods. In most years, we record a high proportion of hens without broods, but it is unclear at which stage of the breeding attempt failure has occurred. In partnership with Scottish Natural Heritage, Scottish Forestry, Forestry and Land Scotland and the Cairngorms National Park Authority, we embarked on a project to radio-tag well-grown hen chicks and monitor their breeding attempts in the following years.

Between 2015 and 2017, we fitted six individuals with 13 gram radio transmitters with an expected battery life of 30 months. This allowed us to follow two hens over three breeding seasons and three hens over two breeding seasons (one hen died at

the start of its first breeding season). Of the 12 possible breeding attempts monitored, no evidence of breeding was found for three hens, all in their first springs. Average clutch size was 6.9 eggs (n = 9, range 5-9 eggs), with estimated first egg laying dates ranging from 23 April to 20 May (n = 6). Three clutches were predated by mammals (33%). The camera near one nest captured clear images of a pine marten removing eggs, the loss of the other two nests was assumed to be from a mustelid (probably pine marten) from blurred camera images or egg remains.

Brood size at hatching averaged 6.5 chicks (n = 6, range 5-9). Two hens had chicks that died before we first checked them at nine days old, one brood was lost when chicks were four to 11 days old and one when they were 15-19 days old. One successful brood was reared in 2018 by a three-year old hen and fledged one chick; another brood reared in 2019 by a two-year old hen fledged two chicks. Mean chick survival from hatching to last flush at seven to nine weeks old was estimated to be 8%. This means that the radio-tagged hens raised 0.3 young per hen with 17% of hens fledging broods.

Annual surveys with pointing dogs over the same years in broadly the same set of forests, resulted in similar productivity of 0.5 young per hen (range: 0.2-0.9; total of 210 hens recorded) and 24% of hens fledging broods (range: 14-40%).

Radio-tagging hens provided insights into causes of breeding failure, principally predation of clutches by pine martens and low chick survival, however, it was not possible to determine the causes of low chick survival. Previous studies have identified inclement post-hatch weather, low invertebrate availability and predation as being important. Sheep ticks are also known to reduce chick survival in other ground-nesting birds so tick parasitization of capercaillie chicks is considered a priority subject for further research.

KEY FINDINGS

We radio-tagged six juvenile capercaillie hens, which allowed us to monitor 12 subsequent breeding attempts. Only two breeding attempts produced fledged young (17% success). Low productivity was due to 60% of first-year hens not nesting, 33% predation of clutches and only 8% chick survival from the nests that hatched. Unless management is undertaken to increase productivity, a second extinction of capercaillie in Scotland looks increasingly likely.

Kathy Fletcher

Tick parasitization of capercaillie and their chicks is considered a priority subject for further research. © Frank Law