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Travis L. Booms

Gyrfalcon food habits in central West Greenland during the nestling period

I used time-lapse video, pellet, and prey remains analysis to study the food habits and feeding behavior of nesting gyrfalcons (Falco rusticolus) in central West Greenland during the 2000 and 2001 field seasons. I collected 2,677.25 hours of videotape from three nests, representing 93.7, 87.3, and 49.3% of the nestling period at each nest. The video recorded 921 deliveries of 832 prey items. I placed 95.3% of the items into prey categories. The image quality was very good, but did not reveal enough detail to identify most passerines to species. I found no evidence that gyrfalcons were negatively affected by the video system after the initial camera set-up. The video system experienced some mechanical problems, but was a reliable technique to document nesting gyrfalcon food habits in West Greenland.

Most gyrfalcons exhibited similar feeding behavior during the nestling period. Ptarmigan delivered to nests were usually plucked prior to delivery and included the breast and superior thoracic vertebrae. Arctic hare leverets were rarely plucked and often delivered in parts. The most common leveret part delivered was the hind legs attached to the lower back. Passerines were rarely plucked and usually delivered whole. After feeding, adults removed almost 20% of prey items including those with and without obvious meat still attached. Prey delivery rates were similar among nests and increased as nestlings aged. Prey delivery frequency peaked in the morning and evening, with a distinct lull in the late evening and early morning hours. Male and female adults delivered a similar number of prey, though males typically delivered smaller prey than females. Gyrfalcons cached and re-delivered at least 9.1 % of all items delivered and cached some items multiple times.

Gyrfalcons relied heavily on rock ptarmigan (Lagopus mutus) and arctic hare (Lepus arcticus). Combined, these species contributed 79.2 - 90.6% of the total diet, depending on the method used. Passerines contributed the third most percent biomass and percent pellet composition. All three methods documented at least one seasonal shift in diet composition and the video revealed two shifts. The first was a shift from ptarmigan to hare in mid-June. The second change in diet composition was an influx of passerine fledglings in late June. Assuming video results best approximated the actual diet, prey remains and pellets overestimated ptarmigan and underestimated arctic hare in the diet. Prey remains underestimated the proportion of passerines in the diet, while pellets overestimated passerines with the same severity. The video-monitored gyrfalcons consumed 94.0 - I 1 0.1 kg of food per nest during the nestling period, higher than previously estimated. Video and prey remains data gave 1.47 and 1.28 Shannon Wiener H-values, similar to the mean of other gyrfalcon diet studies. Using a combination of the three methods was important to accurately document gyrfalcon diet.