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;relationships between prey delivery rates, hatching date, the number of young fledgedper 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 hunting 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 (0.0) 136 (25.7) Total 156 (29.5) 176 (33.3) 162 (30.6) 26 (4.9) 9 (1.7) 529 (100.0) 1986 Male 97 (28.7) 105 (31.1) 57 (16.9) 22 (6.5) 1 (0.3) 282 (83.4) Female 27 (8.0) 10 (2.9) 17 (5.0) 1 (0.3) 1 (0.3) 56 (16.6) Total 124 (36.7) 115 (34.0) 74 (21.9) 23 (6.8) 2 (0.6) 338 (100.0) 1987 Male 90 (16.3) 84 (15.2) 217 (39.2) 7 (1.3) 1 (0.2) 399 (72.2) Female 38 (4.8) 35 (6.3) 77 (14.0) 4 (0.7) 0 (0.0) 154 (27.8) Total 128 (23.1) 119 (21.5) 294 (53.2) 11 (2.0) 1 (0.2) 553 (100.0) Total Male 405 (23.2) 376 (21.6) 482 (27.7) 60 (3.4) 11 (3.4) 1,334 (76.6) Female 138 (8.0) 96 (1.1) 163 (9.3) 10 (0.6) 1 (0.6) 408 (23.4) Total 543 (31.2) 472 (27.1) 645 (37.0) 70 (4.0) 12 (0.7) 1,742 (100.0)

8.15, df = 3, P = 0.04); females delivered fewer during brood rearing (0.37 f 0.16 items/hr, OD birds than males. = 23 1) and were significantly higher than during Prey categorieschanged over the years (Table the preincubation stage (0.10 f 0.08 items/hr 1). Prey items identified as Townsend’s ground OD = 91) (Table 2). The lowest delivery rates squirrelsdecreased from 35.7% in 1984 to 30.6% were observed during the incubation stage(0.08 in 1985, and further declined to 21.9% in 1986 f 0.07 items/hr, OD = 246) (DMR, P = 0.05). (Table 1). In 1987, however, 53.2% of the diet Males delivered 76.6% ofall prey to the nesting was composed of Townsend’s ground squirrels. territories at a rate of 0.15 f 0.14 items/hr or Interpretation of these data is limited, however, 2.30 f 2.11 items/day (OD = 568) over the because of the large percentage of unidentified entire nesting cycle (Table 2). Males delivered items (3 1.2%). an average0.10 f 0.08 items/hr (OD = 91; 97.8% For each nesting pair of falcons, the percentage of all delivered prey items) during preincubation, of Townsend’s ground squirrels delivered to the decreasingto 0.07 -t 0.07 items/hr (OD = 246; nesting territory was calculated for the brood- 95.3% of all delivered prey items) during incu- rearing stageand regressedon productivity. Pro- bation. They rapidly increased the number of ductivity increased significantly with the per- prey delivered immediately after hatching (Fig. centage of Townsend’s ground squirrels in the 2) from a rate of 0.18 + 0.12 items/hr when falcon’s diet (t-test slope, b = 0.08, t = 3.77, P young were O-3 days of age (OD = 14) to a peak = 0.0006) (Fig. 1). of 0.32 * 0.16 items/hr when young were 8-14 days of age. Prey delivery rates for males then PREY DELIVERIES decreasedand remained at about 0.24 items/hr Significant differencesin prey delivery rates per for the rest of the brood-rearing stage. pair/hour were found among years, stages,and Females delivered few prey items to the nest- nesting pairs (ANOVA, F = 26.21, df = 3 and ing territory during both preincubation and in- 518, P < 0.0001; F = 402.54, df = 2 and 518, cubation (Fig. 2). During brood rearing, how- P -c 0.0001; and F = 3.17, df = 44 and 518, P ever, prey delivered by the female rapidly < 0.0001, respectively). Prey delivery rates were increased from 0.02 f 0.03 items/hr (OD = 14) high in 1984, 1985, and 1987 (0.24 f 0.17,0.21 when young were O-3 days of age to a peak of f 0.19, and 0.22 & 0.2 1 items/hr, respectively); 0.19 f 0.11 items/hr (OD = 4 1 days)when young the lowest rate was recorded for 1986 (0.13 f were 21-27 days of age, about 2 weeks after the 0.12 items/hr) (Duncan’s multiple range test peak in prey deliveries by the male. Prey delivery [DMR], P = 0.05). Prey delivery rates peaked rates of female subsequently declined to 0.13 f ANTHONY M. A. HOLTHUIJZEN

0 I 2 3 4 5 Young / pair FIGURE 1. Townsend’sground squirrelas a percentageof the Prairie Falcon’s diet (X f SE) during broodrearing regressed on youngfledged/pair, 1984- 1987.Numbers above SE barsrefer to the number of nestingpairs.

0.10 items/hr. The number of prey delivered through the combined activities of the male and the female falcon showed a steady increase fol- lowing hatching and peaked when the young stabilized in weight at 21-27 days of age. The frequency distribution of prey deliveries during an observation day showed a bimodal pattern (Fig. 3). The first peak was in late mom- ing (2-5 hr after sunrise) and the second in late afternoon (12-13 hr after sunrise). A stronger bimodal pattern was demonstratedfor males than for females, but this was probably a reflection of sample size, becausemales delivered 75% of all prey. RELATIONSHIPS BETWEEN PREY DELIVERY RATES, PRODUCTIVITY, HATCHING DATE, AND WEIGHT bF YOUNG

co-cor-hlr-bo\Dboooo No significantdifferences were found among years oooooo~---o- when the average number of prey delivered per ddd ddd ddd ddd pair/hour during brood rearing was regressedon productivity (ANCOVA, slopes F = 2.27, df = ooor--cow-r- mlno -o-oooh(~*-or-4 3 and 35, P > 0.10; intercepts F = 2.14, df = 3 dddddddddddd and 35, P > 0.10). Therefore, prey delivery data were combined for all years. The average prey delivery rate per pair/hour significantly increased with the number of young fledged per pair (Fig. 4). The average prey delivery rate per pair/hour during brood rearing decreasedsignificantly with progressively later hatching dates for 1985 and 1986 (b = -0.01, t = -2.83, df = 1, P = 0.02; and b = -0.004, t = -3.35, df = 1, P = 0.01) but not for 1984 and 1987 (t = -1.48, df = 1, P = 0.18; and t = -1.34, df = 1, P = 0.21). Differences among years were not significant PREY DELIVERY IN PRAIRIE FALCONS 419

0.5 r- C H total delwered delivered , I

male

0 I \

0 + L‘ 0.0 cached n 02 20 cached ; 5 _F ,o \ c 2 &6 0.1 IO \

6 a’

0.0 0 retrieved retrieved

0.2 30

0, I

-50 0 50 0 0 5 IO 15 Day of nesting cycle Hour after sunrise FIGURE 2. Number of prey items (3 + SE) delivered FIGURE 3. Diurnal frequency distribution of prey to the nestingterritory, cached,and retrieved per hour, items delivered, cached, and retrieved per half-hour by Prairie Falcon nesting pairs, for each sex, 1984- by Prairie Falcon nestingpairs (568 observationdays), 1987. C = clutch completion; H = hatching. and for each sex, 1984-1987.

(ANCOVA, slopes:F = 1.34, df = 3 and 33, P CACHING AND RETRIEVAL > 0.10; and intercepts: F = 1.46, df = 3 and 33, Prairie Falcons cached prey items in clumps of P > 0.10). When the data were pooled for all grass(Poa spp.), big sagebrush(Artemisia triden- years,again a decreasewas found in prey delivery tutu), or other brushy vegetation, on ledges,and rates with hatching taking place later in the nest- in small cavities in the nesting territory. Male ing season (Fig. 5). This coincided with a de- and female falcons used separate caching sites creasein productivity (f-test slope, b = -4.93, t within a general caching area. The falcons were = 46.67, P = 0.0001, r, = 0.33) (Fig. 6). very alert when caching. Just before storing a Weights of both male and female young showed prey item, they would stand upright with their a weak, but nonsignificant increasewith prey de- necks craned, observing their surroundings livery ratesper pair/hour (males: Pearson’s prod- sometimes for several minutes. The prey would uct moment correlation coefficient r, = 0.49, n then be transferred from the talons to the bill = 13, P = 0.09; and females: rs = 0.47, IZ = 17, and pushedinto the vegetation or a crevice. After P = 0.06, respectively). scanning the environment, the falcon would flv 480 ANTHONY M. A. HOLTHUIJZEN

001. -5, 2 3 4 5 0 I 2 3 4 5 0 I Young / paw Young /pair FIGURE 4. Averagenumber of preyitems delivered FIGURE 6. Julian hatchingdate (X i SE) regressed per Prairie Falcon pair/hour during brood rearing(X on the number of Prairie Falcon youngfledged/pair, f SE) regressedon young fledged/pair,1984-l 987. 1984-1987. Numbersabove the SE bars refer to the Numbersabove the SE bars refer to the number of numberof pairsobserved. nestingpairs.

of prey items were cached during brood rearing off. A falcon would immediately retrieve a prey (0.10 f 0.11 items/hr, n = 23 1 days) than during item when watched by its mate, other raptors, either preincubation (0.03 -t 0.06 items/hr, n = or ravens (Corvuscorux). Potential cacherobbers 91 days) or incubation (0.03 f 0.05 items/hr, IZ were chased off before the falcon would cache = 246 days) (DMR, P = 0.05) (Table 2). Caching prey. rates were high during the first week of brood Caching rates per pair/hour showedsignificant rearing (0.14 & 0.13 items/hr, y1= 14 observa- differencesamong years, stages,and nesting pairs tion days, young O-3 days old), then decreased (ANOVA, F = 4.62, df = 3 and 5 18, P = 0.0003; to 0.07 +- 0.07 items/hr (n = 40 days) when 65.11, df = 2 and 518, 0.0001; and F = P > F young were 14-20 days and remained at this level 2.51, df = 44 and 518, 0.0001, respec- = P > for the rest of the brood-rearing stage (Fig. 2). tively). High cachingrates were observedin 1987 Female falcons cachedmost prey items (77.7%). (0.07 * 0.11 items/hr, II = 163 days), 1984 (0.06 Males only cachedprey when the female did not + 0.08 items/hr, n = 89 days), and 1985 (0.05 accept the prey, the female was absent, or the -t 0.07 items/hr, n = 162 days), which were all young did not consume the prey. The diurnal higher than in 1986 (0.04 f 0.06 items/hr, n = frequency distribution pattern of prey caching 154 days) (DMR, P = 0.05). A greater number closely followed the pattern described for delivered items with higher numbers of prey cached in the early morning and late afternoon (Fig. 3). Retrieval rates per pair/hour did not show differences among years, but differed significantly ; e among stagesand nesting pairs (ANOVA, F = \ Y = 1~113-0~005x .L_ 2.37, df = 3 and 5 18, P = 0.07; F = 64.25, df = :: \ 2 and 518, P > 0.0001; and F = 1.73, df = 44 D and 5 18, P = 0.003, respectively). The lowest $ .z retrieval rate was found for 1986 (0.05 ? 0.08 % items/hr, OD = 154); the retrieval rates for 1984, % e 1985, and 1987 (0.07 & 0.11, OD = 89; 0.06 f a 0.1 - 0.08, OD = 162; and 0.07 + 0.10 items/hr, OD = 163, respectively) were not significantly different, but all were higher than for 1986 (DMR,

110 120 I30 140 150 I60 170 P = 0.05). During brood rearing, retrieval rates/ Hatch (Julian day 1 hour were significantly higher (0. IO -t 0.11items/ hr, OD = 231) than during both the preincu- FIGURE 5. Averagenumber of prey deliveredper pair per hourduring brood rearing regressed on Julian bation and incubation stages(0.03 f 0.06, OD hatchingdate, 1984-l 987. = 91, and 0.03 ? 0.05 items/hr, OD = 246, PREY DELIVERY IN PRAIRIE FALCONS 481

TABLE 3. Pearson’sproduct moment correlationcoefficients between prey delivered,cached, and retrieved per hour by Prairie Falconpairs, 1984-1987.OD refersto observationdays.

Flehavior stage Behavior Cached Retrieved Preincubation Delivered 0.30* 0.002 (ns) (OD = 91) Cached 0.19 (ns) Incubation Delivered 0.47** 0.17* (OD = 246) Cached 0.45** Broodrearing Delivered 0.18* -0.08 (ns) (OD = 231) Cached 0.61** All stages Delivered 0.46** 0.30** (OD = 5681 Cached 0.63**

respectively; DMR, P = 0.05) (Table 2). Like riod and with the number of young. Also, the caching rates, retrieval rates were highest during female did not hunt before the young had fledged. the first week of brood rearing (0.17 f 0.17 items/ In the second group, which included several ac- hr, OD = 14, young O-3 days old) then declined cipiters, food consumption did not increasewith to 0.09 +- 0.11 items/hr (OD = 4 1) when young age and the number of young; the female started were 14-20 days of age and stayed at this level hunting before the young had fledged. In Prairie for the remainder of the brood-rearing stage(Fig. Falcons, total prey deliveries increased during 2). Females retrieved most prey (73.7%) espe- the nestling stageand with the number of young. cially early in the brood-rearing stage.Retrieval Females procured prey throughout incubation, of prey items during the day was high in the early although at a very low rate, and continued to morning hours (O-l hr after sunrise), then rapidly provide prey in increasing numbers during the declined, reaching its lowest level when prey de- brood-rearing stage.Apparently, Prairie Falcons liveries peakedin late morning (Fig. 3). Prey items do not belong to either group, but occupy a po- were steadily retrieved during the afternoon. A sition along a continuum with the two groups at secondpeak occurredin late afternoon-early eve- the extremes. ning (11.5-13.5 hr after sunrise) followed by a The Prairie Falcon’s diet in the SRBOPA, as rapid decline. Both sexesshowed a similar diur- determined by direct observation, generally nal pattern in prey retrieval. agreed with earlier findings based on prey col- When prey delivery rates were high, more prey lections (U.S. Department of Interior 1979, was cached during each stage, as well as for all Steenhofand Kochert 1988). For instance,I found stagescombined (Table 3). Correlations between that at least 64% of the prey items were mam- prey delivery rates and retrieval rates per day malian, whereas 67.3% mammalian prey (per- were weak or nonsignificant. High caching rates centageof individuals) was estimated using prey were strongly associatedwith high retrieval rates collections (U.S. Department of Interior 1979). for the incubation and brood-rearing stagesand The Townsend’s ground squirrel representedthe for all stagescombined (Table 3). Prairie Falcons single most important prey item of Prairie Fal- retrieved on average as many prey items as they cons and comprised at least 22-53% of all deliv- cachedduring each of the stagesand for all stages ered prey. Similar findings were reported in the combined (t-tests, P’s < 0.10). same area by Steenhof and Kochert (1988). Prey delivery rates of Prairie Falcons closely DISCUSSION followed the hypothetical food intake curve as Newton (1979) divided studiesof diurnal raptors proposed by Snyder and Wiley (1976). Prey de- into two groups based on pattern of food con- liveries during preincubation were higher than sumption. For the first group, that included the during incubation, particularly during egglaying PeregrineFalcon, Osprey(Pundim haliaetus), and (O-l 2 days prior to clutch completion) when the several harrier species(Circus spp.), total food female needs additional food. Female Prairie consumption increased during the nestling pe- Falcons lay four to five eggs(Palmer 1988) which 482 ANTHONY M. A. HOLTHULIZEN

representsan estimated22-27% ofthe bird’s body for other raptor species,e.g., accipiters (Snyder weight, assuming that the fresh egg weight of a and Snyder 1973, Newton 1978). Prairie Falcon is about the same as that of a The bimodal pattern in prey deliveries of Prai- Peregrine Falcon (45 g; Ratcliffe 1980). During rie Falcons may to some extent reflect the diurnal these stagesmost of the prey is delivered by the availability of the Townsend’s ground squirrel. male (97.8% and 95.3%) so that the female can From late winter through early spring, Town- accumulate large body reserves for use in incu- send’s ground squirrels restrict their activities bation and nestling periods, conserve energy to aboveground to the warmer midday hours after produce eggs,and reduce the risk of damage to emergencein late February (Johnsonet al. 1977). the eggs through hunting activities (Newton As the season progresses,aboveground activity 1979). Occasionally, females would procure prey becomes more bimodal with a morning and late that was taken to the nesting territory and was afternoon peak. Thus, Prairie Falcons may syn- either consumed or cached. In most instances, chronize their hunting activities with the activity however, prey was not taken to the nesting ter- pattern of their main prey species.Similar rela- ritory, but was apparently consumed away from tionships were found for other raptors in relation the canyon, because females returned with ex- to small mammal activity during the day (Rijns- tended crops and blood on their talons or lower dorp et al. 198 1, Raptor Group RUG/RIJP 1982). abdomen. Males usually perched in the nesting The ability of the male to provide prey to the territory or incubated during the females’ hunt- female during the preincubation stage,and prey ing trips. During brood rearing a rapid increase abundance, are known to affect laying date and in prey delivery rates occurred. During the first therefore hatching date and clutch size of a num- 2 weeks male Prairie Falcons still delivered most ber of raptors (Cave 1968; Newton 1979, 1986; prey items. As the young required lessbrooding, Dijkstra et al. 1982; Newton et al. 1983; Newton females increased their contribution to the food and Marquiss 1984; Simmons et al. 1986; Bar- supply, which peaked when young stabilized in nard et al. 1987). In 1986, prey delivery rates by weight at about 24 days of age (Fowler 193 1, male Prairie Falcons during the preincubation Sitter 1983). Thus, prey delivery rates closely stage regressedon hatching date showed a sig- paralleled the growth rate of the nestlings, as was nificant negative relationship, but this relation- found for other avian species (Ricklefs 1968, ship was not found for other years. Possibly, in Newton 1979, Poole and Boag 1988). 1986, early nesting only took place when the male Prairie Falcons demonstrated higher prey de- provided more than the averagenumbers of prey livery rates with increasing brood size. However, during preincubation, i.e., the more proficient this does not mean that Prairie Falcons adjust males were associated with early nesting. This their delivery rate to the number of young. The has been observed in the Eurasian Sparrowhawk prey delivery rate is more likely to be determined (Newton 1986) and in nonraptorial birds (New- by the availability of prey, especially the Town- ton 1979, 1980). Also, low Townsend’s ground send’s ground squirrel. This is illustrated by data squirrel numbers in 1986 may have induced late for the 1986 and 1987 nesting seasons.In 1986, nesting, resulting in later average hatching dates Townsend’s ground squirrel density was low for 1986 as compared to other study years. In compared to 1987 (Peterson and Yensen 1986, addition, late-nesting Prairie Falcons were con- Quinney et al. 1987). The falcons spent two to fronted with decreasingprey availability due to three times as much time procuring prey than in the gradual estivation of Townsend’s ground 1987. Prey delivery rates for 1986 were signifi- squirrels from May through July (U.S. Depart- cantly lower than for 1987, as was productivity ment of Interior 1979, Smith and Johnson 1985). (Holthuijzen, unpubl. data). Also, productivity The net result was a decreasein productivity of was positively associatedwith the percentageof progressively later-nesting falcons. Townsend’s ground squirrel in the falcon’s diet As can be expected,caching and retrieval rates (Ogden and Homocker 1977, U.S. Department were high when peak numbers of prey were de- of Interior 1979, Steenhof and Kochert 1988) livered, particularly during early brood rearing, again emphasizing the importance of the avail- when the small falcon young need to be fed fre- ability of this species to nesting falcons. More quently and at regular intervals, but cannot con- likely, pairs provided prey at the maximum pos- sume much food (Ratcliffe 1980). The diurnal sible rate after their young hatch, as was found retrieval pattern again showedthe importance of PREY DELIVERY IN PRAIRIE FALCONS 483 cachingas a mechanism to maximize food intake Company) for their financial and logistical support of and dampen fluctuations in prey availability. Re- the fieldwork. I gratefully acknowledge all research trieval rates peaked in the early morning hours assistantsinvolved in this study. L. Young provided organizationaland administrative support and partic- just after sunrise, when the young had been with- ipated in data collection. L. Oosterhuis, K. Steenhof, out food during the night. In the afternoon, re- and R. Williams made valuable comments on earlier trieval ratesremained moderately high, when prey versionsof the manuscript. L. Oosterhuisprepared the deliveries declined, and peaked again during late figures. Idaho Power Company kindly allowed us to use its facilities at Swan Falls Dam. This paper is a afternoon to early evening. joint contribution of the Idaho Power Company, the Poole and Boag (1988) reported a similar pat- U.S. Bureau of Land Management Snake River Birds tern in Gryfalcons, although caching was ob- of Prey ResearchProject, and the Pacific Gas and Elec- served only after the eggshad hatched. The cach- tric Company. ing rates were high when the young were small, then declined, and caching was not observed af- LITERATURE CITED ter the nestlings were 29 days of age. Prairie Fal- BARNARD,P., B. MACWHIRTER,R. SIMMONS,G. L. cons, however, continued to cacheprey through- HANSEN.AND P. C. SMITH. 1987. Time of breed- out the brood-rearing stage, possibly due to the ing and the seasonalimportance of passerineprey activity pattern of the Townsend’s ground squir- to northern harriers (Circuscyaneus). Can. J. Zool. rel. Rijnsdorp et al. (198 1) found that the Eur- 651942-1946. asian Kestrel also exploited prey with a distinct BURNHAM,W. 1983. Artificial incubation of falcon eggs.J. Wildl. Manage. 47:158-168. availability pattern, which in turn induced a CAM& A. J. 1968. The breeding of the kestrel. Falco diurnal rhythm in caching behavior. t&run&us L., in the reclaimed area Oostelijk Fle- The adaptive value of caching is determined voland. Neth. J. Zool. 18:313-407. by the ability of the bird to successfullystore the COLLOPY,M. W. 1977. Food cachingby female Amer- ican Kestrels in winter. Condor 79:63-68. prey in a suitable site without losing it to poten- DUKSTRA,C., L. VWRSTEEN,S. DAAN,AND D. MASMAN. tial cache robbers and to identify the cache site 1982. Clutch size and laying date in the kestrel at a later time to retrieve the item. Collopy (1977) Falco tinnunculus:effect of supplementary food. calculated a 70% successrate for free-ranging Ibis 124:210-213. American Kestrels retrieving prey, and Mueller ENDERSON,J. H. 1964. A study of the Prairie Falcon in the central Rocky Mountain region. Auk. 81: (1974) reported a successrate of 65% for captive 332-352. birds. Prairie Falcons appear to have a higher FOWLER,F. H. 1931. Studies of food and growth of successrate, becausethe number of cachesand the Prairie Falcon. Condor 33: 193-20 1. retrievals on a daily basis were not significantly GREEN,R. 1976. Breedingbehaviour ofospreys Pan- different. However, these calculations are based dion haliaetusin Scotland. Ibis 118:475-490. HOLTHUUZEN,A.M.A. 1988. Behavior and produc- on the number of cachesand retrievals, not in- tivity of nestingPrairie Falcons in relation to con- dividual prey items, so that the actual success struction activities at Swan Falls Dam, p. 56-93. rate may be lower. Prairie Falcons usually cached In K. Steenhof [ed.], Snake River Birds of Prey in restrictedlocations and systematicallychecked ResearchProject Annu. Rep. 1988. U.S. Dep. In- ter., Bur. Land Manage., Boise, ID. all preferred sitesfor cachedprey. This may sug- JOHNSON,D. R., G. W. SMITH,E. M. COBB,AND C. E. gestthat the falcons did not remember the exact WOODLEY.1977. Population ecologyand habitat caching location. Free-living American Kestrels requirementsof Townsendground squirrel, p. 143- approached caching sites more directly than 163. In Snake River Birds of Prey ResearchProj- ect Annu. Rep. 1977. U.S. Dep. of Inter., Bur. Prairie Falcons (Collopy 1977), and captive Land Manage., Boise, ID. American Kestrels could accurately locate a MASMAN,D. 1986. The annual cycle of the kestrel. caching site (Mueller 1974). Merlins (Oliphant Ph.D.diss., Univ. Groningen, Netherlands. and Thompson 1976) however, had to search MORITSCH,M. Q. 1983. Photographicguide for aging for the location of a cache, much like Prairie nestlineorairie falcons.U.S. Den Inter.. Bur. Land Manag;, Boise, ID. Falcons. MUELLER,H. C. 1974. Food cachingbehavior in the American kestrel (Falco sparverius).Z. Tierpsy- chol. 34: 105-l 14. ACKNOWLEDGMENTS NEWTON,I. 1978. Feeding and development of sparrowhawk Accipiternisus nestlings. J. Zool. (Lond.) Especially acknowledgedare A. Ansell (Idaho Power 184:465-487. Company), M. Kochert (U.S. Bureau of Land Man- NEWTON,_I. 1979. Population ecologyof raptors.Poy- agement), and R. Williams (Pacific Gas and Electric ser, Berkhamsted, Umted Kmgdom. 484 ANTHONY M. A. HOLTHUIJZEN

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