Long-term trends of reproductive success of black grouse Lyrurus tetrix in the southern Swiss Alps in relation to changes in climate and habitat

Abstract

Breeding  success  of  an  Alpine  black  grouse  Lyrurus  tetrix  population  in  southern  Switzerland was monitored from 1981 to 2020. This long-term dataset allows explor-ing  relationships  of  reproductive  rates  with  climate  and  habitat,  which  have  shown  marked changes during this period. Over the 40 years, the average elevation of black grouse  breeding  sites  increased  by  around  100  m  in  central/southern  Ticino  but  showed only a slight increase in northern Ticino, where black grouse occur at higher elevations. Average reproductive rates in northern Ticino remained constant through-out the study period but declined in central/southern Ticino. Relationships between reproductive success and weather as well as habitat variables were analysed with a mul-tiple regression model. Temperature during the early chick-rearing phase and the time of  egg-laying  was  positively  correlated  with  reproductive  rate.  Correlations  between  reproductive  rates  and  precipitation  were  less  clear,  and  only  small  proportions  of  the  variance  in  reproductive  rates  could  be  explained  by  precipitation.  Brush  forest  explained the greatest amount of variation in reproductive rate (6.2%). Forest, alpine agricultural areas and unproductive vegetation all showed a positive relationship with reproductive rate, but the proportion of the variance explained was small. Year (5.1%) and its interaction with region (2.3%) explained considerable amounts of the variance. While  in  northern  Ticino  reproductive  success  did  not  show  a  negative  trend  when  correcting for weather and habitat changes, there remained a negative trend over the years in central/southern Ticino. Despite the positive correlations of reproductive rate with temperature, increasing temperatures do not appear to have improved reproduc-tive success, likely as a result of habitat changes that forced black grouse towards higher elevations. Changes in reproductive success were limited to the southern region, indi-cating deteriorating conditions at the edge of the distribution range.