SELECTED ASPECTS OF BURROWING OWL ECOLOGY AND BEHAVIOR

Transcription

1 SLCTD ASPCTS OF BRROWING OWL COLOGY AND BHAVIOR DNNIS J. MARTIN1 Departmet of Biology iversity of New Meico Albqerqe, New Meico INTRODCTION field. All owls were weighed, ispected for ectopara- Pblicatios dealig with the biology of the sites, eamied for molt, ad had their remiges ad retrices measred before release. Brrowig Owl (Speotyto ciclaria) ap- Two yog owls were captred drig their first peared as early as 1874 (Coes 1574). Most observed emergece from their est brrow ad maiof the may brief accots sice pblished taied i captivity for behavioral stdies. deal with the descriptio of food habits or COLOGY are casal observatios of behavior. Mch of Habitat descriptio. The stdy area cothe iterest i Brrowig Owls is de largely taied two cocetratios of estig holes 3 to their diral activity ad some of their odd miles S of Albqerqe at a elevatio of behaviorisms ft. Oe was a 1.8 mile-log sectio of Recetly, Thomse (1971) ad Colombe Tijeras Arroyo; the secod, a 0.5 mile-log (1971) provided the first major stdies of this sectio where a railway ct throgh a hillside. species. Thomse s ( 1971) stdy agai co- The two sites were 0.5 mile apart ad the sidered food habits of Brrowig Owls, bt iterveig area served as mtal foragig she also ivestigated behavior ad poplatio grod. The stdy area is characteristic desert dyamics. Colombe (1970, 1971) ivestigated grasslad showig effects of overgrazig (fig. some physiological aspects of Brrowig Owls, 1). Plat forms preset are assorted aal their food habits, behavior, ad seasoal movegrasses, sakeweed ( Gtierreia sp. ), formets. wig saltbrsh (Atriple caesces), rabbit- The prpose of this stdy is to compare the brsh ( Chrysothams aseoss), ad Rshabitat reqiremets, behavior, ad poplasia thistle (Salsol kali). Withi the arroyo tio dyamics of Brrowig Owls i Cetral area there were abot 68 brrows that ap- New Meico with those i Califoria. peared sitable for Brrowig Owls; the rail- MATRIALS AND MTHODS road track area cotaied 10. Brrow descriptio. All brrows ihabited I stdied a poplatio of 15 breedig pairs of Brrow- drig the 1970 ad 1971 breedig seasos ig Owls ad their offsprig from 5 May 1970 to 20 had bee sed by owls i previos years. May 1971 ear Albqerqe, New Meico. Observatio time totaled over 400 hr. Most observatios were Therefore, little brrow ecavatio was remade from a car or o foot ad 7 50 bioclars ad corded. However, estig material was a 20 spottig scope were sed. A portable blid added each year. was employed for close observatios ad for recordig Rock sqirrel (Spermophils variegats) vocalizatios. Lights distrbed the owls ad ths were brrows are sed eclsively by the owls. No ot sed. Vocalizatios were recorded with a her 4000 prairie dogs presetly live o the area. Br- Report-L Tape Recorder ad her M-514 microphoe rows may be fod at ay positio i the at 7r$ ips. A 24-ich parabolic reflector sometimes arroyo or railway ct. Brrow sites are deswas sed. A more complete aalysis ad discssio of igated as lip, wall, or bottom (fig. 2). Seve vocalizatios of Brrowig Owls ca be fod i Marti ( 1973 ). of the 15 breedig brrows were i the bottom, Of breedig pairs, ie females ad ie males were for were i the wall, ad for were i the lip. color-baded for idividal recogitio. At least oe The apparet preferece for bottom brrows adlt of each pair was baded at all bt two brrows. may reslt from freqet distrbace of lip Oe mated adlt ad 48 vog also were baded. brrows by hikers ad horseback riders; also, ach owl received a.s. Fish ad Wildlife bad ad a two-color combiatio of red, white, gree, or yellow wall brrows have few perch sites at the brcolor bads. Owls were captred by placig Havabart row etrace. trace sizes varied greatly traps withi the brrow moth as described by Marti (from cm to cm, B = 32 (1971a). All adlt owls were seed by presece or absece of a brood patch ad bv feather coloratio. 24 cm). This appeared to be a reslt of the This later was cofirmed by their behavior i the relative age of the brrow (mber of seasos sed). Ier tel size, which coformed 1 Preset Address: Departmet of Zoology, tah State iversity, Loga, tah closely to the physical dimesios of the adlt [4461 The Codor 75:446456, 1973

2 COLOGY AND BHAVIOR OF TH BRROWING OWL 447 FIGR 2. Nestig sites of Brrowig Owls. A = lip; B = wall; C = bottom. FIGR 1. Habitat of stdy area i cetral New Meico. A = wall brrow of Brrowig Owls. B = breedig brrow of Bar Owls. The distace betwee A ad B is abot 3 m. Railroad ct-throgh is ear poles i backgrod. birds, was more iform tha the etrace size(1013cmto5012cm,~=ll20 cm). Regardless of brrow positio, there was a slight aglar drop from the etrace, both iside ad otside. Wall brrows possessed the greatest drop to the otside, bottom brrows, the least. There appeared to be o preferece for directio of brrow opeig: three faced west; five, soth; ad seve, orth. Vacat brrows were commo. The average distace from a ihabited brrow to a vacat oe was 25 m. The distace to the closest breedig eighbor averaged 166 m. This is cosiderably greater tha that reported by Thomse (1971). I feel, thogh, it is a accrate idicatio of territory size, de to the presece of apparetly sitable sed brrows betwee est sites. Territory parameters appear to be differet i this poplatio from those of a typical Brrowig Owl coloy ad will be discssed more flly later. Three brrows were cosiderably earer, bt this deviatio probably reslted from the very close occrrece of 10 sitable breedig brrows preset i a 140-m sectio of arroyo. Reprodctive sccess. Brrows were ot ecavated to determie cltch size. Bet (1938) reports ormal cltch size as 7-9 eggs; ths, 15 breedig pairs, averagig 7 eggs per cltch, wold prodce 105 eggs. Oe cltch was laid per year i this poplatio. The total maimm mber of yog observed o my stdy area drig the smmer of 1970 was 78. There probably was estlig mortality before the yog bega to perch at the brrow etrace (at abot 2 weeks of age). Of the origial 78 yog observed, 74 or 94.9% fledged. Mea reprodctive sccess was 4.9 yog per pair. No correlatio betwee brrow positio ad mber of yog fledged was apparet. The mea yog fledged per lip brrow was 4.7; wall brrow, 5.5; ad bottom brrow, 4.7. The lowest possible mea cltch size for my poplatio was 5.2 eggs, based po the 78 yog see. Thomse (1971) states that prodctivity i her poplatio of Brrowig Owls i Califoria was 2.7 yog per pair i 1965 ad 1.9 yog per pair i Ths, reprodctive sccess, measred as yog fledged, was cosiderably less i her poplatio. Seal dimorphism. Males are larger i most species of birds. This patter is reversed i most of the Strigiformes ad Falcoiformes; the oly strigiform eceptio i North America is the Brrowig Owl (arhart ad Johso 1970). My data cofirm this dimorphism i size. Several weights were take 2-4 weeks after the eggs hatched, at a time whe the female s weight shold ot have bee affected by egg layig. Males otweighed females by a mea of 7.9 g (N = 7 ad 6, respectively), with the F test sigificat at the 0.05 level (Sokal ad Rohlf 1969:208). Feather coloratio also varies with se. Males appear lighter tha females throghot the smmer. After postptial molt, the sees are alike dorsally. Females geerally appear darker o their breast at this time de to more etesive barrig. Barrig varies idividally, as some females are very white o the breast ad some males qite dark; ths, this caot be sed as a reliable meas of seig idividals. By the first of Je most males appear lighter tha most females o the head, back, wigs, ad tail. This apparetly is a reslt of fadig, as the males remai otside the brrow drig the day, ad of wear, as they do most of the foragig throghot the smmer. Behavioral differeces are meros ad will be discssed later. The oly morphological character with behavioral implicatios is the female brood patch. Waderig ad migratio. The yog bega to wader eve before they cold fly. Freqetly, a family was divided betwee the breedig brrow ad oe or more accessory

3 448 DNNIS J. MARTIN TABL 1. Seasoal flctatios i a Brrowig Owl poplatio i cetral New Meico. Moths BW_V May J J A 0 N D.I F M A May 02 1 M 2: 3M F 4M 5; F 7 $I SF 9 10 ; 11 M 12!!! M 18 g? J??? J? J #9? J? Ll J#lS? M = male, F = female, derlied idicates bird was baded. Plai = same bird preset as idicated by brrow mber o left margi.? = se ad age kow. = idetifiable adlt. = ew bird at brrow. J = baded jveile. Nmbered symbol o right desigates brrow sed the previos year. Birds idicated for a specific moth were ot ecessarily all see o the same day. brrows. Parets cotied to feed yog eve whe they moved as far as 100 m from the breedig brrow. My freqet distrbace of certai brrows dobtedly was resposible for some waderig. Thomse ( 1971) also describes this behavior, althogh she was carefl ot to distrb the brrows with traps. Oce the yog cold fly, they might be fod at ay vacat brrow withi 300 m of the breedig brrow. As they became icreasigly idepedet, they became more solitary. Few cases of family strctre remaiig itact throghot September were observed. I these cases, the yog were completely idepedet. A few baded owls were observed drig the witer (table 1). They sally were see for a few days, disappear for a few weeks, ad the reappear for a few days. This occrred with five idividals. Ths, some owls appear to wader sporadically i witer. The migratory stats of the Brrowig Owl i the West ad Sothwest is clear. Breckle ( 1936)) workig with data from 481 baded Brrowig Owls, demostrated a defiite migratio from Soth Dakota to Teas. He frther idicates that Brrowig Owls i Califoria are omigratory. This view is shared by Thomse (1971) for Brrowig Owls i the Oaklad viciity of Califoria, ad by Best (1969) ad J. S. Ligo (1961) for Brrowig Owls i New Meico. They sggest the owls may remai iside their brrows drig the day, becomig strictly octral drig the witer moths. Colombe ( 1971) assmes that part of the Califoria poplatio is migratory, with immigratio ito Sother Califoria occrrig from the orth. Owls bega leavig my stdy area as early as 2 Agst 1970 (table 1). I oe case a etire family of eight disappeared simltaeosly ad was ot see agai. sally, jveiles bega to disperse drig the first

4 COLOGY AND BHAVIOR OF TH BRROWING OWL 449 TABL 2. Dates of arrival for part of a poplatio of Brrowig Owls i cetral New Meico, Sprig Arrival dates Males of brrows Females of brrows - 19 March 0, 1, ad 11 lad11 19 March-20 March 5ad9 5ad9 21 March-26 March 7 27 March-l April 2 ad 20 0,2, ad 20 2 April-3 April 15 ad half of Agst. May adlt females also left at this time. Approimately 10 baded owls remaied o the area by the ed of Agst. The poplatio remaied this size til mid- October. Oly for owls cold be located at ay time throghot November; oe, i December; three, i Jaary; ad two, i Febrary (table 1). All owls recorded drig the period from November to Febrary sally were see sig ear a brrow etrace. The baded birds observed, as previosly stated, were ot always the same, idicatig that they wadered over a larger area tha my stdy plot or remaied withi the brrows drig the day. Betwee March 1971, poplatio size icreased from its Febrary level of 2 to 17. Males sag almost immediately after their arrival. This led to the collectio of precise data for the arrival of 18 owls (table 2). If a male was paired, the female was earby while he sag. Males ad females appeared o the stdy area apparetly paired or males arrived aloe. I coclde that migratio occrred i my stdy poplatio as evideced by the strikig redctio i poplatio size i late October, whe food was abdat, ad the sharp ad sdde icrease i poplatio size i mid- March. Where these owls witer is kow. BRDING BHAVIOR Pair formatio ad brrow selectio. Owls appear to arrive o the breedig area both paired ad sigly (table 2) drig March ad April. Of the ie breedig males ad ie females baded i 1970, si males ad two females retred (table 1). All retrig males i 1971 selected the same brrow they had ihabited i 1970 (table l), less the brrow had bee destroyed, i which case they assmed the same relative positio they held i 1970 with respect to the other owls ad brrows. No pairs i which both male ad female were baded i 1970 were paired i I oe case, a pair baded i 1970 both reap FIGR 3. Bowig display of Brrowig Owls ~;!JJ;$ ad display sed while givig primary sog peared o the area i The male had a ew mate ad the female mated with aother baded male (table 1). This sggests that Brrowig Owls do ot pair for life. Whether the low retr rate for females is attribtable to a higher mortality tha males or to a weaker bod to a previos area is ot kow. po arrival o the area, paired males immediately bega to prepare their chose brrow for habitatio ad to display for pair formatio. At sset they bega sigig ad this cotied throghot the ight. Descriptio of sog ad all other vocalizatios discssed here ca be fod i Marti (1973). The behavior occrred while the owl stood at the moth of his brrow or o the arroyo rim facig the arroyo. He beds forward so that his body early parallels the grod, with primaries ad secodaries held derside together over his back ad the white patches of the throat ad brow are displayed to their fllest (fig. 3). This postre ad behavior appeared to be stereotyped. Nest preparatio. Nest brrows ihabited drig 1970 ad 1971 had bee occpied i previos years. Ths, little ecavatig was apparet. The oly ecavatio observed was whe male owls scratched dirt ot of the brrow etrace. The owls faced ito the brrows ad scratched backwards with their feet. Best (1969) reports that the owls also walk throgh the brrow tel with otstretched wigs, dislodgig dirt from the walls. Thomse ( 1971) idicates Brrowig Owls i

5 450 DNNIS J. MARTIN FIGR 4. Breedig brrow of Brrowig Owls with dry feces shredded at etrace. Califoria may dig with their bills. I did ot observe either of these behaviors i my stdy poplatio bt I ackowledge that bill-diggig occrs i other New Meica Brrowig Owls ( Marti 1971b). Soo after ecavatio ceased, liig material was broght to the brrow etrace. This material cosisted of dry horse, cow, ad dog feces. Oce a adeqate pile had bee accmlated, the owls shredded the material ad carried it ito the brrow. The est chamber floor was lied approimately 2.5 cm deep with the shredded feces. Throgh this process the tel ad etrace also are lied (fig. 4). The Brrowig Owl is oe of the few North America owls which copiosly lie their ests. It is also the oly small owl whose est is readily accessible to grod-dwellig predators. The liig possibly provides islatio, bt may also serve to camoflage the owls scet, ad possibly that of its prey, from predators sch as badgers ( Taidea tams), coyotes ( Cais l&rs), sakes, domestic dogs, cats, etc. This view is also shared by J. D. Ligo (pers. comm.). If oe removes feces from the etrace ad the first meter of the tel, they are replaced withi oe day. Cortship behavior. Cortship behavior, apparetly had beg by 17 March whe some mated Brrowig Owls arrived o the stdy area. It cotied til mid-april. The behavior sally bega shortly before dsk ad cotied for l-2 hr. Cortship behavior ivolves assorted displays, vocalizatios, ad postres by both males ad females: presetatio of food, preeig, leg- ad wig-stretchig, head-scratchig, ad bill-ippig. It always occrred withi 15 m of the estig brrow. Precoplatory behavior. At the oset of precoplatory behavior, both male ad female are ear the brrow. The male ad female pree ad leg- ad wig-stretch for a few mites. The male egages i more of this activity tha the female. The male may the display with primary sog ad its associated postre (fig. 3). While the male sigs, the female sits ear him or i the brrow etrace ad displays with Rasp or ep calls. This activity may stimlate the male to forage ad, po his retr, to preset the female with food. Followig the echage, males ad females may ip each other s bills for a short time. If the female coties to Rasp ad ep, the cycle is repeated; if she ceases, coplatory behavior sally follows. Coplatory behavior. This behavior, termiatig i coplatio, occrred freqetly jst after pair formatio. The highest icidece was drig the first hor after sset. It was observed til mid-april at which time it became rare. By the ed of April, the behavior as sch was abset. The highest rate of coplatio by a pair that I recorded was eight times i 35 mi, althogh most pairs coplated oly oe to three times per eveig. The highest freqecy of coplatio was i March, the lowest, i April. The geeral seqece of a coplatory bot was : male sigig; female moves toward or away from male 1 or 2 m; male ceases sigig; stads p ad looks dow at the female with white patches eposed ad feathers raised; female stads erect ad eposes her white patches bt does ot raise body feathers; male flies to female ad mots. Oce moted, male gives Sog Drig Coplatio with or withot a Male Warble, ad they may termiate with a Tweeter call. Drig the bot, the female may give the Smack call ad Coplatio Warble. While moted, the male sally flaps his wigs bt this appears to be more for balace tha as a display. Whe the male is moted, he may scratch the female s head ad both idividals may bill ip. After termiatio, the male will sally stad for a momet lookig dow o the female, eposig

6 COLOGY AND BHAVIOR OF TH BRROWING OWL 451 his white patches bt withot his body feathers raised, the fly back to where he was previosly sigig. The female may or may ot stad erect with white patches eposed. Coplatory behavior was fairly iform throghot the etire poplatio, althogh variatios o this seqece occrred. gg layig, icbatio, ad broodig. gg layig bega abot the third week of March if Brrowig Owls lay a egg o alterate days as reported by J. D. Ligo (1968) for the lf Owl (Micrathee whiteyi) ad Welty (1962) for certai other owls. This wold apply oly to the earlier arrivals, as some birds were paired til early April. Female Brrowig Owls appear to icbate eclsively. Females aloe develop a brood patch. Bet (1938) reports that both sees icbate, bt more recet works do ot spport his statemet (Colombe 1971; Thomse 1971). The female remais withi the brrow throghot most of the day. I early morig ad eveig she appears at the brrow etrace ad waits for the male to brig food. At this time she ofte gives the Rasp call. The male ifreqetly carries food ito the brrow to the female drig midday. Throghot most of the day the male remais perched qietly ear the brrow. This patter coties til the food demads of the yog become great, at which time the female also gathers food. Territory ad territoriality. A territory cosisted of the Brrowig Owl s brrow at the ceter ad a portio of the arroyo etedig i both directios. The foragig grod was ot iclded withi the bodaries of the territory. Males ad females ehibited itraseal territoriality, which geerally was evidet oly drig pair formatio, althogh oe ecoter was observed as late as 10 May. The primary respose of a male to a itrder more tha 10 m from his brrow was to assme a bet-over postre, hold his wigs over his back ad flat together, ehibit white patches of the throat ad brow, ad give primary sog (fig. 3). If this behavior failed to repel the itrder, the defedig owl approached to withi 1 m of the itrder ad stood erect, withot white patches eposed. If these behaviors were sccessfl ad/or the itrder was withi 10 m of the defeder s brrow, physical cotact might ese. Althogh physical cotact is rarely employed, as with most species of birds, I observed the followig seqece o 10 May. I drove a male to withi 8 m of aother s brrow. The ower flew slowly directly over the itrder. Givig o idicatio of a comig attack, he dropped o the itrder with otstretched talos, piig him to the grod. The itrder gave a scream ad the defeder released him ad flew back to his brrow. The itrder, slochig, the walked away ito the grass. Female territoriality was observed o oe occasio as I played recordigs of primary sog to a mated male, casig him to sig for a eteded period. His sigig attracted a mlbaded owl that was presmed to be female. She laded at the brrow ad etered. The residet female ra ito the brrow ad mometarily both reappeared at the etrace. They stood erect withi 5 cm of each other, eposig their white facial ad throat patches. At this time the male ra ito the brrow. The residet female bega to give the Defese Warble ad both females pecked at each other s bills. The residet female the sprag p ad grasped the itrder with her talos. Both birds fell off the arroyo rim. Halfway to the grod, the residet female released her grip ad flew to her brrow. The itrder dropped to the grod ad the fled. Territory defese cotied til egg layig commeced, at which time the females remaied i the brrows throghot the day ad the males ceased sigig. This was probably de to recogitio by males of territorial bodaries ad hormoal-psychological chages, althogh the evets of 10 May demostrated the potetial was still preset. FDING BHAVIOR Periods of most itesive foragig behavior were jst after sset ad before srise. Foragig behavior was greater from sset to 24:00, tha from 24:00 to daw. Foragig cosisted of perchig i a bsh or flyig low over a field til a prey item was observed. Adlts did little flycatchig or hoverig while the food demads of the yog were great. Hoverig was performed at 1030 m above the grod. Drig flycatchig, prey were grasped i the talos rather tha the bill. Small prey, e.g. isects, were hopped po ad crshed with the bill. They were immediately eate or carried i the bill to the brrow, althogh prey ifreqetly were trasferred from the feet to the bill i flight. Large items, e.g. rodets, were attacked i a typical raptor maer. These were sally carried to the brrow. Typical storage behavior was ot observed. O occasio prey was killed i midday ad laid at the brrow etrace til eveig

7 452 DNNIS J. MARTIN before beig eate, bt o stock-piles were formed. Males fed females etesively drig pair formatio, icbatio, ad broodig. Males were ever observed eatig food gathered by females. Females gave Rasp calls drig the eveig ad early morig, which appeared to stimlate the males to forage. As the male approached with food, the itesity of the call icreased. po passage of food to the female, the male ofte gave a Rasp call. The female, if hgry, cotied Raspig as she ate. The male appeared to collect all food til the yog were 3 or 4 weeks old. Drig this period, the male bega foragig at sset ad completed two to three food trips per mite. This rate bega to decrease arod 21:00 to 22:O0. The male carried food ito the brrow or passed it to the female at the etrace ad she carried it i. po receipt of the food, the female ofte gave a Rasp call. It cold ot be determied whether the yog gave ay vocalizatios drig this period. Females bega foragig for their ow food ad for the yog after the yog were 3-4 weeks old. By this time the yog walked well. They stood at the brrow etrace givig Rasp calls, which appeared to stimlate both parets to forage. As the adlt approached with food, the itesity of the Rasp icreased ad the yog ra ot to meet the adlt. As a rle, the first chick to meet the adlt received the food item. Other yog owls ot receivig food sally did ot try to take food away from the recipiet. The other yog cotied Raspig ad the adlts wold leave immediately. Drig these food trasfers, the adlts ofte gave Rasp calls. Althogh the females gathered food, it appeared that they made oly abot a third as may trips as the males. By the first of Jly most yog were hoppig ad flyig abot the brrow collectig isects. Toward mid-jly some jveiles accompaied parets o foragig flights ad caght some of their ow prey. Drig this period, foodgatherig demads po the parets decreased; by mid-agst all birds appeared to be gatherig food idepedetly. BHAVIOR OF TH YONG Yog Brrowig Owls stad abot the brrow etrace each day to s themselves oce they are able to move arod well. Drig feedig periods they also stad abot the brrow etrace. po receipt of food they may dash ito the brrow to eat ad the emerge agai withi secods. Nestligs 3-4 weeks old begi to display wig- ad leg-stretchig, preeig ad hoppig abot while flappig their wigs. They appear to make a effort drig sch. activity to jmp oto some specific object, as if improvig their coordiatio. At this stage of developmet they are capable of flight. I am sre that o two o.ccasios I witessed the first flights of yog Brrowig Owls. At first they were somethig less tha gracefl. The owls jmped p, flapped their wigs wildly, the desceded po some precise poit, e.g., rock or stick, ad sally fell off or missed it. After oly for or five sch short flights, they attempted the width of the arroyo. They took off ito the wid, the chaged directio, ad glided with it to the opposite rim. po ladig, they sally crashed. Flappig flight improved rapidly ad withi two eveigs the yog owls bega to accompay their parets o short foragig flights. Withi a week they were accomplished fliers. After the yog ca hop ad flap their wigs with a degree of coordiatio, they begi practicig prey killig. Two or three yog will alterately jmp o ad crsh a dead isect. The yog later display the same behavior i flight as they hover over the prey ad drop o it. Drig their first foragig flights with the adlts, they are little more tha spectators, bt soo begi to captre their ow prey. By Agst it was commo to observe a etire family leave the brrow site together ad forage i a give area. The yog characteristically hovered at this time, whereas the parets hted from a perch or by flyig low. By mid-agst feedig Rasps give by selfspportig jveiles were igored. O the eveig of 5 Agst 1970, I observed what cold be described as play behavior. Yog Brrowig Owls from the same brrow hovered abot 10 m above the grod ad 10 m apart ad stooped at each other. By their cotied hoverig, stoopig, ad soarig, oe cold coclde that this activity was stimlatig. They cotied this behavior for mi before followig their parets ot to forage. RSPONS TO PRDATORS Brrowig Owls of Cetral New Meico have meros potetial predators. The more commo predatory species the owls display agaist are: Prairie Falcos (F&o meicas), Redtailed Hawks (Bteo jamaicesis), Swaiso s Hawks ( B. swaisoi), Ferrgios Hawks (B. regalis), Marsh Hawks (Circs cyaes), Golde agles ( Aqila chrysaetos), Great Hored Owls ( Bbo virgiias), domestic

8 COLOGY AND BHAVIOR OF TH BRROWING OWL 453 dogs ad cats, ad people. Other possible predators are: sakes, Bar Owls ( Tyto &), badgers, coyotes, ad bobcats (Ly rfs). The followig geeral commets relate to the owls observed o my stdy area i New Meico. Predator displays by Brrowig Owls vary throghot the year, apparetly correlatig with their breedig cycle. I the field, betwee October ad Febrary, Brrowig Owls sally croch low to the grod, r ito a brrow, or fly away qietly whe approached by a predator. I the sprig (March-May) the owls sally give a warig call, the r ito a brrow. Whe the call is give by either the male or the female, both sally r ito a brrow, or the male may remai otside. From Je til the yog are idepedet, if the warig call is give by either paret, all yog r ito the brrow, with the female sally followig the yog. Males sally remai ot less a direct attack is i progress. If the attack is by aother raptor, the male sally retreats ito a brrow; if by terrestrial predator, the male will sally mob the predator. Whe adlt Brrowig Owls with yog observe a terrestrial predator approachig their brrow, they give the Chck warig call, which may be accompaied by a bow (fig. 3). The yog r ito the brrow po hearig this call. The female, if ot already i the brrow, may eter or may fly some distace away. If the predator approaches closer, the female may give Chck ad Chatter calls with bows. The male remais ear the brrow ad gives Chatter calls of three to seve otes ad bows. If the predator coties to advace, the male begis to fly betwee areas abot 100 m away from the brrow. Whe perched, he gives Chck ad Chatter calls with bows, bt oly Chatter calls while flyig. This display sally cases the predator to follow the owl, thereby protectig the brrow ad the yog. Both parets retr oce the predator is a sfficiet distace from the brrow. The owls attack if their displays are ot sccessfl ad the predator approaches the brrow. Males sally attack, althogh females also may do so. The owls fly i a circle abot 100 m i diameter ad 10 to 20 m high ad give Chatter ad Scream calls. Chatter calls are legtheed to 7 to 15 otes. The owls always dive at the predator from behid ad give a startlig Scream whe withi a meter of the itrder. The owls are very bold whe performig this display ad will strike the predator. I have see male Brrowig Owls strike dogs, cats, ad Great Hored Owls. I also was strck i the head by a Brrowig Owl. Whe a predator is flyig, the male Brrowig Owl watches it qietly til it is close eogh to begi a attack ad the gives a Chck or Chatter call. The male the remais very still ad the female ad yog r ito the brrow. If the female gives the call, she ad the yog sally r ito the brrow, while the male freezes, bt remais otside. A Scream is give by a Brrowig Owl whe startled by a predator, sch as a low flyig Prairie Falco. O hearig this call from the male, the yog r ito the brrow, bt the female freezes if she is more tha abot 2 m from the brrow. If a female gives the call, the male freezes, ad if she is ear eogh ( abot 2 m), she rs ito the brrow. Predator defese behavior employig attack displays does ot appear i jveile Brrowig Owls til they are almost completely selfspportig ad are ecellet fliers. This behavior develops throgh Agst. By the ed of Agst most jveiles display i a maer idetical to the adlts. If the yog are corered before they ca fly, they will bed over, rotate their wigs, as is commo i most owls, ad give the Rasp call. This display is also performed by captive adlts, bt was ever observed i the field. Mobbig. Brrowig Owls display a predator defese approimatig mobbig behavior. As defied by Hartley ( 1950), mobbig is a demostratio of oe bird agaist a potetial eemy of aother ad more powerfl species; mobbig is iitiated by the weaker species, ad is ot a reactio to a attack po the perso, mate, est, eggs, or yog of the mobbig bird. Brrowig Owls sally do ot harass a potetial predator ot withi their territory, less it is a Great Hored Owl. I iitially tried to captre Brrowig Owls by placig mist ets ad a Great Hored Owl at the brrow etraces. I fod that ot oly the residet pair of Brrowig Owls mobbed the owl, bt that adlts from other brrows as far as 300 m away also joied i. Drig these bots o aggressive territorial displays betwee male Brrowig Owls were observed. The display sed agaist the Great Hored Owl was the same as described previosly for a terrestrial predator ear a brrow. Brrowig Owls wold cotie harassmet of the Great Hored Owl for more tha a hor, for three to five bots per week, before cospicos habitatio occrred. Brrowig Owl respose to the Great Hored Owl fol-

9 454 DNNIS J. MARTIN lowed the same patter described previosly for other predators. Two captive jveile Brrowig Owls were show a Great Hored Owl. Oe jveile was eposed at abot 6 weeks of age for 3 mi ad at 8 weeks for 5 mi; o reactio occrred. The other jveile was eposed at 16 weeks of age for 5 mi; o reactio occrred. It was also oted that Bar Owls ested withi 5.2 m of a Brrowig Owl brrow (fig. 1). The Bar Owls were observed flyig withi 10 m of a breedig Brrowig Owl, bt o warig call or displays were observed. The lack of displays i this case may have bee de to habitatio. Bowig. Brrowig Owls are well kow for the behaviors of bowig (fig. 3) ad stadig with their back to a itrder. Bowig occrs drig predator defese ad sally is associated with a Chck or Chatter call. Trig the back to a predator also occrs as the predator approaches to a positio from which it may begi a strike. Bowig occrs before the owl leaves a area, ad after it lads. The distace betwee the owl ad predator is sally greater tha that evokig back-trig. Bowig is ever associated with a defiite agoistic or hidig behavior. It appears to arise from a iteral atagoism betwee drives to hide ad to flee. Whe these two drives are balaced, the owl bows. This was demostrated by oe of the yog captive owls. Whe frighteed, the owl wold hide by crochig behid a rock i its cage; if frighteed frther, it wold try to flee by leapig oto the cage wall. A typical bowig seqece whe I eared the caged owl wold proceed like this: the owl first croched behid the rock, the stood p lookig at the wall farthest from me, the it bowed, croched behid the rock agai, the bowed agai; bt po comig p this time, it jmped oto the wall. Back-trig behavior appears to be a itetio movemet precedig flight. Whe a predator approaches too closely, the owl trs its back to the predator, bt sally coties to watch it. If the predator moves closer, the owl flees. This behavior poits the owl i a directio opposite to that of a possible attack by the predator. If a attack eses, the owl is positioed for rapid escape. DISCSSION Habitat reqiremets ad geeral breedig behavior were similar i Colombe s ( 1971), Thomse s ( 1971), ad my stdy poplatios. All breedig sites were i relatively ope areas ad had earby accessory brrows. All the poplatios ihabited distrbed areas ad have adapted to estig i grod sqirrel tels i the absece of prairie dog brrows. Becase of their apparet behavioral plasticity, Brrowig Owls may be oe of the least affected raptors by ma-made evirometal chages. Also, Thomse (1971) feels, as I do, that althogh the owls prefer predg brrows, they are capable of ecavatig their ow. Brrowig Owls i the Califoria poplatio formed pair bods earlier ad retaied them loger (Thomse 1971) tha those i the New Meico poplatio. This is probably a reslt of earlier growig seasos ad a geerally milder climate alog the Califoria coast. Regardless of the timig of the breedig effort, all poplatios were similar i paretal ad foragig behavior ad developmet of the yog. stimated territory size varied cosiderably betwee my stdy poplatio ad Thomse s ( 1971). This apparet territory size differetial is probably more a reslt of habitat differeces tha of fdametal behavioral differeces. Some sigificat poits of dissimilarity are apparet i these stdies. Idividals of my stdy poplatio acted more aggressively drig predator ad alarm displays tha those i Thomse s ( 1971) or Colombe s ( 1971) poplatios. Colombe (1971) did ot report the two-oted primary sog ad its se i pair formatio ( Marti 1973; Thomse 1971). Rather he described a five-oted sog, with the last for otes slrred. I believe a spectrographic aalysis of this call wold reveal that it is two-oted, with the secod ote beig cosiderably loger tha the first (Marti 1973; Thomse 1971). Two other iterpoplatio behavioral differeces are of iterest. Thomse (1971) reported oe coplatio per pair per eveig as ormal, with three coitios as a maimm. Pairs i my stdy poplatio coplated more freqetly. A maimm of eight coitios i 35 mi was recorded. This may be a reslt of the shorter matig period of the New Meico poplatio which reslts i a more itese peak of seal ecitatio. The selectio of estig material varied greatly betwee the stdy poplatios. Thomse (1971) reported divots from a earby golf corse ad grass were sed to lie estig chambers. My stdy poplatio sed aimal feces to lie their ests. Whether this differece is related to habitat or fdametal behavioral differece cold ot be determied with the available data. Reprodctive effort ad sccess of my stdy poplatio was cosiderably higher tha that reported by Thomse ( 1971). Drig two

10 COLOGY AND BHAVIOR OF TH BRROWING OWL 455 smmers observatios I recorded oly oe paired ad/or obreedig Brrowig Owl. Thomse s (1971) stdy poplatio cotaied 40% obreeders drig oe smmer. The mea reprodctive sccess of the New Meico poplatio was 4.9 yog per pair, whereas the Califoria poplatio s mea reprodctive sccess for 2 years was 2.7 ad 1.9 yog per pair. Oe caot hope to show case for this variatio withot a log-term stdy of both poplatios, bt the migratory behavior of the New Meico poplatio may be sigificat. This behavior may reslt i a higher death or dispersal rate, resltig i lower year-rod poplatio desities i the viciity, especially drig the breedig seaso. Ths, limitig resorces may be more abdat to the New Meico poplatio, allowig them to maimize atality i their reprodctive strategy. SMMARY A breedig poplatio of Brrowig Owls (Speotyto ciclaria) i Cetral New Meico was stdied drig 1970 ad The coloy ested withi a arroyo ad railroad ct-throgh. Breedig sccess of this poplatio was high. Fiftee breedig pairs fledged 74 yog i Yog Brrowig Owls, althogh idepedet, remaied o the breedig grod til mid-agst. Brrowig Owls ehibit seal dimorphism. Female owls are smaller ad sally darker tha males. It appears some Brrowig Owls i Cetral New Meico wader etesively drig the witer while others migrate. Time of departre is from Agst throgh September. Time of earliest arrival is mid-march. Witer residece of this poplatio is kow. Retrig males occpy the same brrows they occpied the previos seaso. Females did ot ehibit a strog bod to ay particlar brrow. Pair formatio i some Brrowig Owls appears to occr before arrival o the breedig grods. Pair formatio o the area may take place withi a sigle ight. Brrowig Owls do ot appear to pair for life. Males appear to do all the brrow modificatio, althogh females may help with the chamber liig. No Brrowig Owls dg their ow brrows. They sed Rock Sqirrel (Spermophils ariegats) tels eclsively. Nest chambers, tels, ad brrow e traces were lied with dry feces. This probably aids i camoflagig the owl s scet. Cortship behavior occrs from the time of pair formatio or retr to the est site til mid-april. Brrowig Owls ehibit itraseal territoriality, which is most apparet drig pair formatio. Territory size appeared larger tha that of more typical poplatios de to the sal spatial relatioships of the owls to oe aother. Females apparetly icbate ad brood eclsively. Females are fed by their mates from the time of pair formatio til some time after broodig of the yog ceases. Brrowig Owls ehibit predator mobbig, althogh it appeared to vary with the hormoal-psychological state of the owl. ACKNOWLDGMNTS This ivestigatio is based po a thesis sbmitted i partial flfillmet of the reqiremets for the MS. degree at the iversity of New Meico. J. David Ligo provided helpfl sggestios throghot the corse of this stdy. He also was a ivalable aid i the preparatio of this mascript; for his gidace ad bodless patiece I am gratefl. I also wish to thak hlarvi L. Riedesel, James S. Fidley, James R. Gosz, ad Keith L. Dio for readig a earlier versio of the mascript. Advice o statistical matters was offered bv lames R. Gosz. Live tracig ad badig was aided by Barbara McKight. LL y The help of my wife, Laa, i typig the may drafts of the paper ad toleratio of my smmer ight activities is also grateflly ackowledged. LITRATR CITD BNT, A. C Life histories of North America birds of prey. Part 2..S. Natl. Ms., Bll BST. R Habitat. aal cvcle. ad food of Brrowig Owls i sothwester New Meico. pbl. M.S. Thesis, N. Meico State iv., Las Crces. BRNCKL, J. F The migratio of the wester Brrowig Owl. Bird-Badig 7: COVS, Birds of the Northwest..S. Geol. Srv. Terr. Misc. Pbl. No. 3. COLOMB, H. N Physiological ad physical aspects of temperatre reglatio i the Brrowig Owl, Speotyto ciclaria. Comp. Biochem. Physiol. 35 : COLOMB, H. N Behavior ad poplatio ecology of the Brrowig Owl i the Imperial Valley of Califoria. Codor 73: ARHART, C. M., AND N. K. JOHNSON Size dimorphism ad food habits of North America owls. Codor 72: HARTLY, P. H. T A eperimetal aalysis of iterspecific recogitio. Symp. Sot. ptl. Biol. 4: LIGON, J. D The biology of the lf Owl, Micrathee &eyi. iv. Mich. Ms. Zool. Misc. Pbl. 136.

11 456 DNNIS J. MARTIN LIGON, J. S New Meico birds ad where to SOKAL, R. R., AND F. J. ROHLF Biometry. fid them. iv. New Meico Press, p W. H. Freema ad Co., Sa Fracisco. MARTIN, D. J. 1971a. A trappig techiqe for THoMsN, L Behavior ad ecology of Br- Brrowig Owls. Bird-Badig 42:26. rowig Owls i the Oaklad Micipal Airport. MARTIN, D. J. 1971b. iqe Brrowig Owl pel- Codor 73: WLTY, J. C The life of birds. W. B. Salets. Bird-Badig 42: ders Co., Philadelphia ad Lodo. MARTIN, D. J A spectrographic aalysis of Brrowig Owl vocalizatios. Ak 90: Accepted for pblicatio 11 Jaary 1973.