j RaptorRes. 23(4):162-166 ¸ 1989 The Raptor ResearchFoundation, Inc. OBSERVATIONS ON THE EVENING DEPARTURE AND ACTIVITY OF WINTERING SHORT-EARED OWLS IN NEW JERSEY THOMAS BOSAKOWSKI ABSTR^CT.--WinteringShort-eared Owls (Asioflammeus) were primarily crepuscularand nocturnal. Roostdeparture occurred most frequently after sunset(83%) with mostexceptions occurring on heavily overcastdays. Owls usually departed singly or in tandemand engaged in a steadydirect flight, presumably to a predeterminedhunting area. Hunting was rarely initiatednear the roostsite. Night observations up to 5 hr after sunsetrevealed that owls huntedcontinuously into the night and were not merely crepuscular.Despite their reputationas an on-the-wingpredator, extended periods of perch-huntingwere oftenobserved after sunset,particularly on windlessnights. Active hunting from percheswas evidenced by a continualseries of pouncesand huntingflights that werelaunched from the sameor nearbyperches. Owls respondedon 3 of 5 trials to broadcastsof prerecordedShort-eared Owl calls with vocalizations and/or vigorouscircling flights over the calling station. The Short-earedOwl (Asioflarnrneus)is primarily and Central North America, Peterson Field Guide Series nocturnalduring the winter months,and hencerel- Record,Houghton Mifflin Co., Boston)at known Short- eared Owl locations for at least 8 min. Taped calls were ativelyfew attemptshave been made to studyactivity broadcastat full volume with a portable 7 watt-output patternsand behaviorof the specieson wintering cassettetape-recorder placed on the roofof a parkedvehicle grounds(e.g., Short and Drew 1962; Clark 1975; with observers inside. Marr and McWhirter 1982). In this paper I present information on evening roost departure, social in- RESULTS AND DISCUSSION teractionsand hunting activitiesof wintering Short- Evening Departure. Short-eared Owls were eared Owls in New Jersey. highly crepuscularand nocturnal in the Meadow- STUDY AREA AND METHODS lands study area. I observedevening departures on The primarystudy site was an inlandtidal marshknown 22 occasionswith departures ranging between 28 asthe HackensackMeadowlands (Lyndhurst, Bergen Co.) min before-24 min after sunset, although the ma- previouslydescribed in detail (Bosakowski1984). Briefly, jority of owls (45 of 54 = 83.3%) emergedafter the marshesare dominatedby commonreed (Phragmites sunset(Fig. 1) which is very similar to the 81.9% comrnunis)and are surroundedby denseurban develop- ment. Several active and inactive landfill mounds are pres- which Clark (1975) observed.On 3 eveningsowls ent in the marsh as well as many warehouses,rights-of- departedbefore sunset, but the skywas heavilyover- way, and occasionallight industry. Weather data were cast(total skycover rating 10 of 10, National Weath- obtained from the National Weather Service,Newark In- er Service). Hendrickson and Swan (1938:585) stat- ternationalAirport, located13 km southof the studyarea. ed "the birds were observed to hunt on several During the winters of 1982 and 1983, 25 hr of obser- vauon were made on 22 different evenings.I usually ar- occasionsas early as 3 p.m. on a cloudyday and just rived at least 0.5 hr before sunset to make observations at before sunseton a clear day." dusk at a known roostingarea (Bosakowski1986). Ob- Despite 8 winters of field work in the Meadow- servations were continued until at least 0.5 hr after sunset and occasionallyup to 1.5 hr. From a 3 m mound, I landsarea, I have only observed1 Short-earedOwl recordedthe number, location, activity, flight direction, flying during mid-day. However, the owl was being and time that owls emergedfor initial eveningflight. Dur- mobbedby an American Crow (Corvusbrachyryn- xnglate winter 1988, an additional22 hr of observation chos)and may have been flushed.The owl quickly were conductedon 7 eveningsfrom late afternoonup to dove for cover and was never observedhunting. At 5 hr after sunsetin this same general study area, plus 5 hr at the "Sod Farms" of Pine Island (Orange Co., NY), the Great Swamp site I observed2 owls emergejust 2 hr at Great Swamp National Wildlife Refuge(Morris after sunsetwhile waiting at a known roost site. At Co, N J), and 2 hr at a Hightstown roost(Monmouth the Hightstown roost1 owl emergedjust after sunset. Co, N J). Eveningroost arrival timesof coexistingNorth- At the Sod Farms site, I observedfrom 3-6 owls on ern Harriers (Circus cyaneus)in the Meadowlands site 3 evenings.On the first observationday, owls were were previouslyreported (Bosakowski 1983). On 5 differentnights, I playedprerecorded Short-eared activelyhunting and flying at about 90 min before Owl "barking" calls (A Guide to Bird Songsof Eastern sunseteven though the skywas partly sunny.How- 162 WINTER 1989 SHORT-EAREDOWLS IN NEW JERSEY 163 30_ r -- .20 P -- .15 20_ N -- 54 z 10_ DAY 8 o m- 0, 8 NTGHT 0 [] • ::3 -10. z -20_ o -3O JAN 1 FEB 9 MAR 21 APR 30 O 1 owl A 2 owl s [] 3 owl s Y -- 0.068X - 15.9 Figure 1. Short-eared Owl roost departure times relative to sunset.Data are combinedfrom the winters 1981-2 and 1982-3. For computation,12 Decemberwas consideredday 0 with tic marksdisplayed at 20-day intervals ever,on 2 subsequentobservation days, owls emerged ranginganimal speciesdepending on importanteco- just at or shortly after sunset. logical/ethologicalevents (e.g., care of young,court- I observeda slight tendencyfor roostdepartures ship, hut building). While the Short-eared Owl is to occur closerto sunsetand greater variability as frequently diurnal during the nesting season(Bent the seasonprogressed toward spring(Fig. 1). This 1938; Pitelka et al. 1955), my studieshave shown slight changemay be due to increasingday length the speciesto be primarily nocturnalin winter which which shortensthe available time period for noc- generallyagrees with the observationsof Clark (1975) turnal activities. Clark (1975) also found a similar and the Craigheads(1956). Although winter roost tendency,but owlsin his studyarea emergedslightly departuretimes are often closelyallied with sunset, earlier relative to sunset. In either study, evening Short-earedOwls occasionallyshowed some plas- departure from winter roostswas usually quite close ticityby departingearlier on overcast days (this study) to sunset.These results are in closeagreement with and later during inclementweather (Clark 1975). studies in other owl specieswhich show that the Interspecific Interactions. Avoidanceof inter- initial activityor roostdeparture is closelycorrelated ference interactionswith diurnal raptors (Jaksic with time of sunset (Glass and Nielsen 1967; Smith 1982) could be another factor affecting the emer- and Murphy 1973; Fuller 1979; Wijnandts 1984). gencetime of owls in the Meadowlands study area. Light cycle has been clearly demonstratedas the The owls I observedwere roostingwithin 100 m of primary synchronizerof circadianrhythms and diel a communal roost of 6-9 Northern Harriers (Bo- cycles (reviewed by Marler and Hamilton 1966) sakowski1983) and usually emergedjust after the although Tester (1987) has shown some marked last harrier had enteredthe roostfor the night (Fig. seasonalplasticity in activityperiods of severalfree- 2). It is temptingto speculatethat this nearly com- 164 THOMAS BOSAKOWSKI VOL. 23, NO. 4 JAN 1 FEB 9 MAR 21 APR 30 .... ß•- ..... lost Northern Harrier arriving ......0 ...... first Short-cored Owl leaving F•gure 2. Minimal temporaloverlap betweenShort-eared Owls and Northern Harriers at nearby winter roostsin 1983. plete lack of temporal overlap was intentional on the hunting begin immediately at the roost departure part of the owls. Glark (1975) and Watson (1977) site. have notedfrequent agonisticencounters and piracy Nocturnal Activity. Kemp (1982) reportedthat between harriers and $hort-eared Owls. The min- Short-earedOwls have been seenby car headlights imal temporal overlapobserved in the Meadowlands hunting in total darkness(no data or sourcegiven). area permitted only 2 interspecificagonistic encoun- Glark (1975) suggestedthat somehunting takesplace ters [1 Northern Harrier, 1 Rough-legged Hawk at night for he was able to 'squeakthem in' at various (Buteo[agopus)] and suggestsa benefit of temporal timesof the night in areaswhere he knew birds often avoidance.In contrast,at least 3 agonisticencounters hunted. In my study, observationsof nocturnal ac- with neighboringharriers were observedwhen owls tivitieswere aidedby the useof car headlights,street- hunted in the late afternoon at the Sod Farms site. lights, flashlights,or scanningfor owl silhouetteson Initial Activity. On 22 eveningsat the Mead- moonlit nights or againstthe glow of city lights on owlands site, I observed a total of 54 instances of the horizon. I made night observationsup to 5 hr owls emerging from their ground roosting sites in after sunset (2230 H) and observed that the owls the marsh. In most casesthe initial flight was di- hunted throughoutthe period on at least 7 nights. agnosticas owls would suddenlyemerge from the These observationshelp support the notion that the reedsand immediatelyascend to an altitude of 15- speciesis not merely crepuscular,but nocturnal as 20 m. At this point, owls would normally engagein well. a steadydirect flight with unchangingaltitude, some- $hort-eared Owls used a combinationof flying times in tandem (11 times), presumablyen route to and perch-sittingas hunting methods.On-the-wing a predeterminedhunting site. Only occasionallydid huntingwas used conspicuously more on windy nights WINTER 1989 SHORT-EAREDOWLS IN NEw JERSEY 165 aswould be expectedon