Prey Delivery, Caching, and Retrieval Rates in Nesting Prairie Falcons

Abstract

From 1984-1987, 48 nesting attempts of Prairie Falcons (Falco mexicanus) were observed for 568 days (8,397 hr) in southwestern Idaho. Observations started 1-5 weeks prior to incubation until the young were 35 days old or a nesting attempt failed. I investigated prey delivery, caching, and retrieval rates during the nesting season; relationships between prey delivery rates, hatching date, the number of young fledged per pair, and the physical condition of the young were assessed. The main prey item was the Townsend's ground squirrel (Spermophilus townsendii), which accounted for 37% of all delivered prey items (n = 1,742 items). Males delivered 97.5, 95.3, and 70.2% of all prey items during the preincubation, incubation, and brood-rearing stages, respectively. Overall, males delivered 76.6% of all prey. Prey delivery rates by a pair were three to four times higher during brood rearing than during preincubation and incubation. The prey delivery through the day was bimodal in frequency and may have reflected the diurnal aboveground activity pattern of the falcon's main prey item, the Townsend's ground squirrel. Diurnal caching patterns followed delivery patterns. Prey delivery rates per pair/hour decreased with progressively later hatching dates, resulting in a decrease in productivity. Prey delivery rates per pair/ hour and the proportion of Townsend's ground squirrel in a falcon pair's diet increased with the number of young fledged per pair. Caching and retrieval rates peaked during the first 2 weeks of brood rearing and then declined. Retrieval rates did not differ from caching rates. Prey retrievals peaked in the early morning and late afternoon hours. Caching was considered an important behavioral mechanism to maximize food intake and to dampen fluctuations in prey availability.