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

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Prey Delivery, Caching, and Retrieval Raes in Nesting Prairie Falcons The Condor92415-484 0 The Cooper Ornithological Society 1990 PREY DELIVERY, CACHING, AND RETRIEVAL RATES IN NESTING PRAIRIE FALCONS ANTHONIE M. A. HOLTHUIJZEN Environmental Affairs Department, Idaho Power Company, Box 70, Boise,ID 83707 Abstract. From 1984-1987, 48 nesting attempts of Prairie Falcons (F&o mexicanus) were observed for 568 days (8,397 hr) in southwesternIdaho. Observations started l-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;relation- ships between prey delivery rates, hatching date, the number of young fledged per pair, and the physicalcondition of the young were assessed.The main prey item was the Townsend’s ground squirrel (Spermophilzstownsendii), which accountedfor 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 abovegroundactivity 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 decreasedwith progressively later hatching dates, resulting in a decreasein productivity. Prey delivery rates per pair/ hour and the proportion of Townsend’s ground squirrel in a falcon pair’s diet increasedwith 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 cachingrates. Prey retrievalspeaked in the early morning and late afternoon hours. Cachingwas considered an important behavioral mechanism to maximize food intake and to dampen fluctuations in prey availability. Key words: Caching;diet: diurnal pattern; Falco mexicanus;Idaho; nesting;Prairie Fal- con;prey delivery;productivity. INTRODUCTION ety rates in raptorial birds during the nesting Snyder and Wiley (1976) hypothesized three dis- stage (Newton 1979, p. 168-l 7 l), but few studies tinct stages in the food needs of nesting raptors: have examined food requirements over the whole egg laying, incubation, and brood rearing. Fe- nesting season (Tinbergen 1940, Green 1976, males require additional food prior to and during Masman 1986). Prey deliveries may be associ- egg laying, as has been demonstrated with the ated with caching behavior, i.e., the deliberate Eurasian Kestrel, F&o tinnuncufus (Cave 1968, hiding of food items for later use (Oliphant and Dijkstra et al. 1982) Eurasian Sparrowhawk, Ac- Thompson 1976). Caching behavior has been re- cipiter nisus(Newton et al. 1983, Newton and ported in a number of raptors, notably the Amer- Marquiss 1984, Newton 1986) and Cooper’s ican Kestrel, F. sparverius (Mueller 1974, Nunn Hawk, A. cooperii (Snyder and Snyder 1973). et al. 1976, Collopy 1977), Eurasian Kestrel Therefore, food intake in raptors is likely to be (Rijnsdorp et al. 198 l), Merlin F. columbarius higher during egg laying than during incubation. (Oliphant and Thompson 1976) Gyrfalcon F. During incubation, food intake is hypothesized rusticolus (Poole and Boag 1988) Peregrine Fal- to remain stable. During the nestling stage, food con F. peregrinus (Palmer 1988), Eleonora’s fal- intake is expected to rapidly increase over the con F. eleonorae (Walter 1979) and Prairie Fal- period of rapid growth of the young (6-25 days con F. mexicanus (Oliphant and Thompson 1976, for most birds, Ricklefs 1968) and to decline Sitter 1983, Palmer 1988). Raptors may cache when asymptotic growth has been reached. Con- food to dampen changes in supply due to fluc- siderable information is available on prey deliv- tuating prey availability (Rijnsdorp et al. 198 1, Sitter 1983, Palmer 1988) or unfavorable hunt- ing conditions (Nunn et al. 1976, Palmer 1988, I Received 4 October 1989. Final acceptance15 De- Poole and Boag 1988). Thus, food caching is a cember 1989. means to reconcile prey availability and food [4751 416 ANTHONY M. A. HOLTHUIJZEN requirements. Removing a storedfood item from egorized observation days (OD) in 6-day inter- a cache is referred to as food retrieval (Collopy vals labelled by midpoints (preincubation, OD 1977). Here I report on prey delivery, caching, = 91: days -51, -45, -39, -33, -27, -21, and retrieval rates of the Prairie Falcon through - 15, -9, - 3; incubation, OD = 246: days 3, 9, the entire nesting season.Specifically, I examine 15, 21, 27, 33; and brood rearing, OD = 231: whether prey delivery rates, i.e., the frequency days 34, 39,45, 51,57, 63, and 69). Day 34 was with which prey items are delivered to the nest- included when falconshad recently hatchedyoung ing territory by an adult falcon, follow the pre- (i.e., ~4 days old). dictions made by Snyder and Wiley (1976). Also, Prey delivered to the nesting territory was clas- I investigate relationships between prey delivery sified as mammal, bird, reptile, or unidentified. rates, hatching date, the number of young fledged Mammalian prey items were further subdivided per pair (productivity), and their physical con- if possible. Items that were positively identified dition (i.e., weight). as Townsend’s ground squirrels (Spermophilus townsendii)were assignedto a separatecategory. METHODS All other mammals were lumped into the small From 1984-l 987, I studied 48 nesting attempts mammal category. Biomass estimates for deliv- of Prairie Falcons in the Snake River Birds of ered prey items were not attempted, becausethe Prey Area (SRBOPA) in southwesternIdaho for observer distance to the aeries was too great. In 568 days (8,397 hr). The study area is part of the 1986 and 1987, 65 young were weighed to pro- Western Intermountain SagebrushSteppe, char- vide an index of physical condition of the young, acterized by cold winters and hot, dry summers using a 1,000-g Pesola spring balance. Crop full- (U.S. Department of Interior 1979, West 1983). ness was estimated in increments of 25% and Each nesting falcon pair was observed, on av- weight of young adjusted accordingly (U.S. Bu- erage, once every 6 days. A nesting territory was reau of Land Management, unpubl. data). Young a confined locality where aeries were found, usu- were an average 31 + 2 (range = 25-35) days ally in successiveyears, and where no more than old when weighed. Their sex was determined by one pair bred at one time (Newton 1979). An morphological measurements and weight (Holt- observation day started 30 min before sunrise huijzen 1988). The weights of young were not and was terminated 30 min after sunset. Obser- adjusted for age, because Prairie Falcons reach vations were usually made from blinds placed at their asymptotic weight when they are about 25 distances ranging from 82-250 m (X = 150 m) days old (Fowler 193 1, Sitter 1983). Frequency from an aerie by two observers, each on a half- data were square root transformed (Sokal and day shift. They used lo-45 x Bushnell telescopes Rohlf 198 1). Statistical analyses were evaluated and 7 x 35 or 10 x 50 binoculars. The observers at P = 0.05. Variation is expressedin standard were systematically rotated through all observed deviations, unless otherwise noted. falcon pairs to minimize bias. The falcons were not individually marked. Sex of the focal bird RESULTS was determined by its relative size (female’s are about one-third heavier than males [Palmer DIET 19881) and sex-specificbehavior (e.g., position Mammals comprised at least 64.1% (range = of the falcon during copulation, food begging). 54.9-74.7%) of all freshly killed prey items (n = Observations usually started l-5 weeks prior to 1,742) delivered to the nesting territory (Table incubation and continued until the young were 1). Townsend’s ground squirrels occurred most 29-35 days old (just before their first flight; En- frequently in the Prairie Falcon’s diet (37.0% of derson 1964) or the nesting attempt failed. Pro- all delivered prey), followed by small mammals ductivity (young fledged per pair) was deter- not identified as Townsend’s ground squirrels mined by counting the number of young per (27.1%), birds (4.0%) and lizards (0.7%). Dif- occupied territory that reached 30 days of age ferencesin prey categoriesbetween the sexeswere (sensu Steenhof 1987). Young were aged with a found for 1986 (x2 = 12.37, df = 3, P = 0.006), photographic aging key (Moritsch 1983). Hatch- but not for other years (1984: x2 = 6.36, df = 3, ing dates were calculated by using the estimated P = 0.09; 1985: x2 = 6.40, df = 3, P = 0.09; and age of the young. Laying dates were based on a 1987: x2 = 1.15, df = 3, P = 0.76). Overall, 34-day incubation period (Burnham 1983). I cat- differences between the sexeswere slight (x2 = PREY DELIVERY IN PRAIRIE FALCONS 477 TABLE 1. Diets of nesting Prairie Falcons, 1984-l 987. Numbers in parentheses are percentages of total number of prey items delivered to the nesting territories each year. F=Y =ewY TOWllWld’S Year sex Unidentified Small mammal groundsquirrel Bird Lid Total 1984 Male 110 (34.2) 56 (17.4) 87 (27.0) 7 (2.2) 0 (0.0) 260 (80.7) Female 25 (7.8) 6 (1.8) 28 (8.7) 3 (0.9) 0 (0.0) 62 (19.3) Total 135 (42.0) 62 (19.2) 115 (35.7) 10 (3.1) 0 (0.0) 322 (100.0) 1985 Male 108 (20.4) 131 (24.8) 121 (24.8) 24 (4.5) 9 (1.7) 393 (74.3) Female 48 (9.1) 45 (8.5) 41 (5.8) 2 (0.4) 0
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