Gyrfalcon predation on ptarmigan: numerical and functional responses

Abstract

Summary1. Gyrfalcon predation on ptarmigan during the breeding season was studied in north-east Iceland 1981±97. The ptarmigan population went through a complete 10-year cycle of numbers with a 4´3-fold dierence in density between high and low years. The yearly number of occupied gyrfalcon territories was correlated with ptarmigan density with a 3-year time-lag. Total falcon numbers in late summer (territorial adults +¯edglings) showed a 2-year lag with ptarmigan numbers. Variability in falcon density was signi®cantly less than that of ptarmigan. It is suggested that the factors contributing to the time-lag between the two populations are the year-round residency of falcons on nesting territories, and late maturity (2-to 4-year-old). Mean brood size and the proportion of the territorial falcon population breeding successfully showed no relation to ptarmigan numbers. 2. Ptarmigan density in spring was an important factor determining composition and variability in the diet of gyrfalcons during the breeding season. Alternative prey for falcons included waterfowl, alcids and waders. Adult ptarmigan featured highly in gyrfalcon diet in all years. The functional response curve to changes in ptarmigan density was slightly convex or close to linear and showed relatively little change in the average number of ptarmigan killed per individual predator over the range of ptarmigan densities observed. 3. The impact of gyrfalcon predation on the ptarmigan population and ptarmigan density was in opposite phases, and the predation rate peaked during the decline and the low phase of the ptarmigan cycle. This suggests that gyrfalcons accelerate decline, accentuate the amplitude and aect the length of the low phase of the ptarmigan cycle. The following factors seem to promote the destabilizing eects of predation by gyrfalcons: they are resident specialist predators, they show a delayed numerical response to changes in ptarmigan density and a high utilization of this prey at all phases of the ptarmigan cycle.

Gyrfalcon Courtship - A Mechanism to Adjust Reproductive Effort to the Availability of Ptarmigan.

“…When ptarmigan were relatively scarce, non-laying, delayed nesting, and clutch desertions were common. These observations were consistent with those of other studies (see Nielsen 1999).…”

Diet of the Gyrfalcon (Falco rusticolus ) in Northern Fennoscandia.

“…In Iceland, for example, Nielsen (1999Nielsen ( , 2003 has shown that the proportion of pairs starting to breed depends on the density of the Rock Ptarmigan. The decline of Willow Ptarmigan and Rock Ptarmigan populations have been evaluated as the most worrying threats to the Gyrfalcon in northern Europe, together with lack of secure nest sites, according to the official Gyrfalcon Action Plan of the European Union (Koskimies 1999(Koskimies , 2006a.…”

“…In Europe, research on this topic has been most intensive so far in Iceland (Bengtson 1971, Nielsen and Cade 1990, Nielsen 1999, 2003, but there are important publications also from Norway (Hagen 1952, Langvatn 1977, Langvatn and Moksnes 1979, from Sweden (Lindberg 1983, Nyström et al 2005, and from the Kola Peninsula, Russia (Dementiev and Gortchakovskaya 1945, Kishchinskiy 1958, Giljazov et al 2008). In the New World, diet of the Gyrfalcon has been studied in Greenland (Burnham andMattox 1984, Booms andFuller 2003), Canada (Poole and Boag 1988) and Alaska (Cade 1960).…”

“…Diet composition varied annually and regionally on a wider geographical scale in Iceland, depending on the relative abundance of the suitable prey species from year to year and from region to region (Nielsen and Cade 1990, Nielsen 1999, 2003.…”

“…Later Langvatn and Moksnes (1979) proposed that the Gyrfalcon chooses as additional prey other birds of the same size class, and in Iceland the diet of the Gyrfalcon includes much "alternative" prey (e.g., waterfowl), if the Rock Ptarmigan density has markedly decreased (Bengtson 1971). Nielsen (1999Nielsen ( , 2003 has studied in detail the effect of Rock Ptarmigan population cycles on the population size and breeding success of the Gyrfalcon, as well as the numerical and functional response of the falcons to their prey. In spite of the potential flexibility of the falcons, Rock Ptarmigan cycles determine significantly the population performance of Gyrfalcons.…”