The effect of human-modified landscape structure on forest grouse broods in two landscape types

Abstract

The population sizes and the breeding success of Finnish tetraonids have been decreasing for decades. In this study, the presence of a grouse hen with a brood in a landscape was used to indicate habitat-related breeding success. We combined the locations of 938 black grouse (Tetrao tetrix), 388 capercaillie (T. urogallus), and 917 hazel grouse (Tetrastes bonasia) broods after the breeding season in mid-August with landscape data by employing Geographic Information Systems (GIS) and grouse data derived from the Finnish wildlife triangle censuses conducted during 1997–2004. Two large study areas with different landscape structures; northern forest-mire area and southern cultivated area, were selected for the investigation. The presence of grouse broods was positively related to the amount of old coniferous mixed forest. Grouse broods may prefer this forest habitat because of a rich understorey cover and a rich bilberry field layer offering a diversity of insects as food. Broods had a strong positive response to the amount of forest habitat. The effect of forest fragmentation on the broods’ distribution did not increase even with decreasing forest habitat. We suggest that there are several ecological causes for the observed spatial correlations. Predation on nests and broods by generalist predators is presumably high in human modified open and semi-open landscapes. Against our expectations, the effect of landscape composition on grouse broods was more marked in the northern than in the southern study area, most likely because predator populations are more food-regulated in the north. This finding supports the alternative-prey hypothesis. Further, large drained and reforested peatland mire areas had a negative impact on grouse broods in the north. In the drainage areas, decreased availability of vegetation cover and insect food, increased predation risk, and drowning of chicks in ditches may increase brood mortality.