Companion studies of Willow Ptarmigan (Lagopus lagopus) and Gyrfalcons (Falco rusticolus) in the central Yukon from 1978 to 1983 allowed us to examine Gyrfalcon reproductive performance at 14 nest sites in relation to ptarmigan abundance and other potential effects, including weather variables, the previous year’s success, nest site characteristics, and Golden Eagle (Aquila chrysaetos) nesting density. Ptarmigan abundance declined six-fold and was mirrored by a significant decline in Gyrfalcon breeding success (breeding failure 58%, clutch desertion 33%). Clutch size showed little variation, although deserted nests held fewer eggs than did successful nests, and there were more four-egg clutches when ptarmigan were most abundant. An average of 2.26 young fledged per nest during abundant ptarmigan years, and 0.l8 when ptarmigan were declining. No other factors were correlated with Gyrfalcon reproductive success. Juvenile ptarmigan density had a compensatory effect: even when ptarmigan breeding numbers dipped, Gyrfalcons bred successfully if the proportion of juvenile ptarmigan was high. Clutch initiation date was a good predictor of Gyrfalcon breeding performance. Early clutches had more eggs (67% with 4 eggs compared to 27% in late nests), were less likely to be deserted (5% vs. 59%), and fledged more young (93% vs. 38%). Two Gyrfalcon pairs, supplemented with food in a poor ptarmigan year, fledged young at a rate and schedule comparable to pairs during a peak ptarmigan year. We discuss interesting observations about Gyrfalcon brood success, as well as Golden Eagle nesting density as a potentially important aspect of the reproductive ecology of Gyrfalcons in this population.
