Habitat and diet of young grouse broods: resource partitioning between Capercaillie (Tetrao urogallus) and Black Grouse (Tetrao tetrix) in boreal forests

Abstract

We compared habitat use and diets of young Capercaillie and Black Grouse broods in a boreal forest in southeast Norway. We used pointing dogs to search for broods (N = 83) in mature ''natural'' forest types and examined the crop content of 66 chicks 1-9 weeks old. We also measured the abundance of insects in the habitats where broods were found. Although overlapping substantially in both habitat and diets, there were notable differences: Capercaillie broods were more frequently recorded in bilberry-dominated forest types, whereas Black Grouse preferentially used pine bog forest, a more open habitat with little bilberry. Capercaillie chicks ate proportionally more insects, particularly lepidopteran larvae, and insects dominated their diet for a longer period of time (until age 28-29 days) than in Black Grouse (14-15 days). After reaching their peaks, the quantity of insects in the crops declined rapidly especially in Capercaillie, and in one of 2 years this occurred at a time when insects, including larvae, were still abundant in the habitats. Among plant foods, both species ate large amounts of Bilberry (Vaccinium myrtillus) and Bog Whortleberry (V. uliginosum). The main difference between species was a large proportion of both over-wintered and new, not yet ripe, berries of Cranberry (Oxycoccus quadripetalus) in Black Grouse, and a higher proportion of the forb Melampyrum sylvaticum in Capercaillie. The difference in diets reflected their differential use of habitats; the Vaccinium-preferred habitats of Capercaillie were richer in insects, particularly larvae, than the pine bog habitat preferred by Black Grouse. Because insects, especially larvae, comprised a larger proportion of the diet of Capercaillie chicks and chicks of this species need more food to sustain their rapid growth, Capercaillie is likely to be more sensitive to variation in insect food than Black Grouse. Also, by reducing the abundance of bilberry, the main host plant of larvae chick food, clearcutting forestry has negative effects on the brood habitat quality of both species.

Declining reproductive output in capercaillie and black grouse – 16 countries and 80 years

“…Storaas et al (1999) hypothesized that during the development of clear-cut stands into thickets, good quality brood habitat becomes scarce and its limited availability could be regarded as an ecological trap. However, this probably has a finite temporal effect in the boreal forests as broods have recently been reported to forage in re-developing stands as they transition into older forest (Wegge & Kastdalen, 2008;Wegge & Rolstad, 2011). A similar mechanism was reported from Finland and Scotland, where broods survived better with increasing bilberry (Vaccinium myrtillus) cover Lakka & Kouki, 2009).…”

“…Climate is probably a more important driver of chick survival than nest success as chicks are reliant on insectivorous foods for the first couple of weeks post-hatching (Rajala, 1959;Atlegrim & Sjöberg, 2008;Wegge & Kastdalen, 2008). Furthermore, chick survival is positively related to temperature during the first two weeks post hatching and chick mortality directly related to adverse weather is rare, but cold and wet weather can predispose chicks to higher predation (Wegge & Kastdalen, 2007).…”

“…Hens with broods are more constrained in their habitat selection than brood-less hens (Wegge & Kastdalen, 2008). Storaas et al (1999) hypothesized that during the development of clear-cut stands into thickets, good quality brood habitat becomes scarce and its limited availability could be regarded as an ecological trap.…”

“…Capercaillie and black grouse broods select for certain habitat types (Wegge & Kastdalen, 2008), and a reduction in availability of these habitats may create ecological traps by increasing the search efficiency of predators (Storaas et al, 1999). In continental Europe and Britain, more than in the Fennoscandian countries, habitat fragmentation has isolated and separated populations from one another by non-habitat and woodland grouse habitats have continuously contracted over the past century.…”

“…However, chicks of both species may be vulnerable to low insect abundance because invertebrates are an important source of protein during the first couple of weeks after hatching (Rajala, 1959;Atlegrim & Sjöberg, 2008;Wegge & Kastdalen, 2008) and food limitation may predispose chicks to predation. Predation is the dominant proximate cause of losses of woodland grouse eggs, chicks and adults (Hagen, 1952;Hörnfeldt, 1978;Angelstam, 1984;Storaas & Wegge, 1987;Kurki et al, 1997;Borchtchevski et al, 2003;Wegge & Kastdalen, 2007;Ludwig et al, 2010;Wegge & Rolstad, 2011;Åhlen et al, 2013).…”