nesting of red-tailed hawks and great horned owls in a central new ...

j. Field Ornithol., 64(4):433-439

NESTING

OF

RED-TAILED

OWLS IN A CENTRAL WILLIAM

HAWKS

AND

GREAT

HORNED

NEW YORK URBAN/SUBURBAN

F. MINOR

AND MAUREEN

AREA

MINOR

125 Apple Lane Charlottesville,Vtrgtnia22903 USA MICHAEL

F. INGRALDI 1

Departmentof ForestBtology StateUniversityof New York Collegeof EnvironmentalScienceand Forestry Syracuse,New York •3270 USA

Abstract.--Productivity of Red-tailed Hawks (Buteojamawensis)and Great Horned Owls (Bubovtrginianus)in a 208 km2 urban/suburbancomplexin centralNew York was documented between 1980 and 1989. Mean productivityfor 134 Red-tailed Hawk nesting attemptswithin 24 distinct areas was 1.10 ___0.13 fledglings/nestwith a nest densityof 0.08 q- 0.01 nestingpair per km2. Comparedwith other Red-tailedHawk studiesconducted in non-urban areas,the densityand productivityof nestsitesin this studyare not statistically different (P) 0.05), though the averagedensityof the non-urban studieswas over three times as large. Mean productivityof 34 Great Horned Owl nestingattemptswas 1.63 q0.16 fledglings/nestwith a density of 0.02 ___0.003 nesting pair per km2. Both species successfullynestedcloseto human activity. ANIDAMIENTO

DE BUTEO JAMAICENSIS Y DE BUBO VIRGINIANUS EN UN

AREA URBANA/SUBURBANADE LA PARTE CENTRAL DE NEW YORK Sinopsis.--Entre 1980 y 1989 se document6la productividaddel halc6nButeojamaicensis y del buho Bubovirginianusen un frea urbana/suburbanade 208 km2 de la parte central de New York. La productividadpromedio de 134 intentosreproductivosde los halcones dentro de 24 fireasdiferentes result6 ser de 1.10 ___ 0.13 volantones/nidocon una densidad de nidos de 0.08 q- 0.01 por parejas reproductoraspor km2. Los resultadosobtenidosno son estadisticamentediferentes (P > 0.05) a los de otros estudios en freas no urbanas,

aunque la densidadpromedioen las freas no urbanas result6 tres vecesmayor. La productividadpromediode 34 intentosreproductivos de parejasde buhosresult6ser de 1.63 q- 0.16 volantones/nidosconuna densidadde 0.02 q- 0.003 parejaspor km2. Ambas especies anidan exitosamenteen la proximidad de actividadesde humanos.

Red-tailed Hawks (Buteojamaicensis)and Great Horned Owls (Bubo virginianus) are two of the most widely-distributed raptors in North America and exploit a great variety of habitats, from continuousforest land to fragmentedagricultural landscapes(Brown and Amadon 1968). Both speciesbeing generalistsin diet (Errington 1933, Peterson1979, Terres 1982) and both speciespossessing the ability to travel between patchesof greenspace with relative easemay make thesespeciesideally suitedfor urban/suburban landscapes. The quality of a particular area supportingthesetwo raptor species can be measuredby the reproductivesuccess of a specificnest site. In an earlier paper, Minor and Minor (1981) reportedon 10 nestingsof Red• Currentaddress: ArizonaGameand Fish Department,222• WestGreenwayRoad,Phoenix, Arizona

85023

USA.

433

434]

W. F. Minoretal.

J.Field Ornithol. Autumn

1993

tailed Hawks (Buteojamaicensis) and two nestingsof Great Horned Owls (Bubovirginianus)during 1980 in a suburbanarea to the eastof Syracuse, New York. Minor and Minor continuedobservations from 1981 through 1988 and expandedthe number of nest sitesto 25. Ingraldi continued the censusin 1989. This paper reports 10 yr (1980-1989) of continuous observations,analyzing the nest site selectionand productivityof Redtailed Hawks and Great Horned Owls in an urban/suburbanlandscape. STUDY

AREA

The area is located in central Onondaga County, New York, and includesthe easternhalf of the city of Syracuseand a large portion of the towns of Manlius and Dewitt. The area encompasses roughly 208 km2and containsa human populationof approximately130,000 (Dwayne Koughenour,SyracuseOnondagaCo. PlanningCommission, pers.comm.). The topographyof the area is quite varied, with the Alleghenyplateau to the southand the Oneida Lake plains of the Lake Erie-Ontario lowlands to the north (Davis 1977). The majority of land is in urban or suburbandevelopmentwith scatteredpatchesof greenspace. The southern portion of the area is hilly, with scattered,relatively steepdrumlins,and is dominatedby patchesof upland mixed-hardwoodand old field communities.The dominantfloral speciesincludeeasternwhite pine (Pinus strobus),shagbarkhickory (Caryaovata),sugarmaple (Acersaccharum), and northern red (Quercusrubra) and white oaks (Quercusalba). The northernportionis relativelyflat with scatteredpatchesof wet hardwoods and emergenthardwoodcommunities.The commonfloral speciesin these communitiesare cattail (Typha spp.), Phragmitescommunis,easterncottonwood (Populusdeltoides),northern white-cedar (Thuja occidentalis) and red maple (Acerrubrum). Interstate routes90, 81,690 and 481 are major highway corridorswithin the studyarea and containnestingand foragingareasfor both raptor species. METHODS

Monitoring of nestingareasbeganin early to mid-February to determine the numberof Red-tailedHawk nestsusedby Great Horned Owls. Nest searcheswere made from a vehicleearly in the nestingseasonwhen tree foliagewas absentbecausemany of the nestingareaswere closeto roads.After the foliagebecamefully developedby late spring, areaswere walked to determinenestingactivity.Observationswere madewith a 30 x spottingscope.Incubation by both specieswas assumedif the birds sat low for prolongedperiodson the nests.Periodic nest monitoringwas continuedthroughout the nestling period and continueduntil fledging with many nestsbeing monitoredonly oncea week. Young were considered fledgedwhen they were at least 5 wk of age (Steenhof1987). The number of successfully fledgedbirds at eachnestand the number of nestingattemptswere recordedin Tables 1 and 2. Productivitywas measuredas the number of young fledgedper nestingattempt and per successful nest. Nesting densitywas measuredas the total area of the

Vol.64,No.4

Nesting ofUrban/Suburban Raptors

[435

TABI•. 1. Productivityof Red-tailedHawks nestingin a centralNew York urban/suburban landscape1980-1989.

Nest area # 1 2 3 4 5 6 7 8 9 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Total

# nesting attempts

Mean # young Mean # young # successful fledgedper fledgedper attempts nestingattempt successful nest

7 8

4 6

5 3 8 9 I 2 2 10 7 8 9 9 4 5 3 5 8 5 6 2 1 7

4 0 8 8 0 2 I 3 4 6 7 8 4 5 1 5 5 3 2 2 1 2

1.00 1.33 2.50 2.00 1.67 1.60 1.13 1.00 0.50 2.00 1.00 0.57

1.25 2.50 1.25 0.00 1.88 1.50 0.00 1.50 1.00 2.00 1.50 1.50 1.29 1.50 2.50 2.00 2.00 1.60 1.80 1.67 1.50 2.00 1.00 2.00

91

1.10

1.67

134

0.71 1.88

1.00 0.00

1.88 1.33 0.00 1.50

0.50 0.60 0.86 1.13

studysite dividedby the number of nestingpairs (successful and unsuccessful).

In comparingthis study,conductedin an urban/suburbansetting,with thosein non-urban areas a t-test comparinga single observationwith a samplemean (Sokaland Rohlf 1981:231) was used.Means are expressed as _ the standard

error

unless otherwise RESULTS

AND

noted.

DISCUSSION

During 1980-1989 we observed134 Red-tailedHawk nestingattempts and 34 Great Horned Owl nestingattempts(Tables 1 and 2). Nestingchronology, productivityand nestingdensity.--Exclusiveof renestings,averageinitiation date for Red-tailed Hawks incubatingwas 23 March (range: 13 March-20 April) during the 10-yr period.All nesting Red-tailed

Hawks

had rufous tails and were considered

to be adults. Over

the 10-yr period 134 nestingsof Red-tailed Hawks were observedin 24 nestingterritories (Table 1). Mean productivity for 91 successfulhawk nestingswas 1.67 _ 0.1 young,ranging from 1.0 to 2.5 young/nestsite. The mean productivity for 134 total nestingattempts was 1.10 _ 0.13

436]

W.F. Minoretal.

J.Field Ornithol. Autumn 1993

TABLE 2. Productivityof Great Horned Owls nestingin a central New York urban/ suburbanlandscape1980-1989.

Nest area # 2 3

# nesting attempts 2 1

Mean # young # successful fledgedper attempts nesting attempt 2 1

2.50 1.00

Mean # young fledgedper successfulnest 2.50 1.00

6

1

1

3.00

3.00

7

2

2

2.00

2.00

8 10

1 5

1 4

2.00

2.00

1.20

1.50

11

1

1

1.00

1.00

12

3

3

1.00

1.00

14 15 17 18 19 21 22 23 24 25

2 1 5 2 1 1 1 1 2 2

2 1 3 1 1 1 1 1 2 1

2.00 2.00 1.20 1.00 1.00 2.00 2.00 2.00 2.00 1.05

2.00 2.00 2.00 2.00 1.00 2.00 2.00 2.00 2.00 1.00

Total

34

29

1.63

1.77

young/nest,rangingfrom 0 to 2.5 young/nestsite. The averagenesting density of Red-tailed Hawks in the study area over the 10-yr period (1980-1989) was 0.08 + 0.01 nestingpair/km 2. We observedno human predation on young Red-tailed Hawks. Four Red-tailed Hawk re-nestingswere observedduring the study. All of the re-nestingsoccurredin alternate Red-tailed Hawk nests90180 m from the original nestingattempts. Urbanization is generallythought to have a negativeeffect on raptor populationsdue to the reductionin habitat necessaryto sustainadequate prey populations and the lack of undisturbed nesting areas associated with metropolitan landscapes(Howard and Postovit 1987). The pro-

ductivity (number of young fledged/nestingattempt) and nest density (nestingpair/km 2) of a few Red-tailed Hawk studiesare summarizedin Table 3. These studieswere conductedin rural landscapesof the northeastern, northwestern and the midwestern United States. There was no

significantdifference(P > 0.05) betweenthe mean densityof the nonurban studies(0.274 + 0.09) and mean densityof this urban/suburban population,eventhough the mean densityof the non-urban studieswas over three times as large. The relative low nest densityin this urban/ suburban area can be deceiving.Large parts of the heavily urbanized areasof the city are barren of suitablegreenspace for foragingand nesting. If these areas were excludedfrom the densityestimate,the number of

Vol.64,No.4 TABLE3.

Nesting ofUrban/Suburban Raptors

[437

A comparisonof densityand productivityamongRed-tailed Hawk populations

(mean + SE). Years

Investigator(s) Craighead and Craighead (1956) (W. Wyoming and S. Michigan) Orians

Mean

examined

nest

density•

Mean

nest

productivity2

1947 1948

0.32 0.05

1.70 _+ ? 0.80 _+ ?

1954-1955

0.12 + 0.01

1.45 +_ 0.20

1962-1964

0.89 + 0.01

1.13 _+ 0.15

1966-1967

--

1.70 _+ 0.20

1967-1971

0.18 _+ 0.002

0.94 -4-_ 0.14

1971-1972

0.10 + 0.01

1.19 _+ 0.17

1972-1975

0.31 +_ 0.04

1.46 + 0.16

1976-1977

0.22 + 0.0

1.07 _+ 0.09

1980-1989

0.08 + 0.01

1.10 _+ 0.13

and Kuhlman

(1956) (S. Wisconsin) Gates (1972) (E. Cent. Wisconsin)

Siedenstickerand Reynolds (1971) (S. Cent. Montana) Mclnvaille

and Kieth

(1974) (Cent. Alberta) Johnson (1975) (S. W. Montana) Petersen (1979) (S. Wisconsin) Bohm (1978) (Cent. Minnesota) This study (Cent. New York)

Density = number of nestingspairs/km2. Productivity= numberof youngfledged/nestingattempt.

hawks nestingper km2 would be increased.Runyan (1987) reporteda nestingdensityof onepair per 3.6 km2 (0.278 nestingpair/km 2) for Redtailed Hawks nestingin the urban landscapeof Richmond,British Columbia. Yet in his densityestimateshe arbitrarily excludedareaswithin his total study area that did not containsuitablenestinghabitat. Productivity was not reported. There was no significantdifference(? > 0.05) betweenthe mean

productivityof the non-urbanstudies(1.27 m 0.11) and the mean productivity(1.10 _+0.13) in this study.It appearsthat with adequatenest sitesRed-tailed Hawks can propagateunder thesehuman-madehabitat conditions just aswell asin non-urbanhabitats.With theapparentendless encroachmentof urban/suburban developmentthis information appears especiallysignificant.

The averageinitiation date for Great-hornedOwl incubationwas 14 Feb. (range: 1-21 Feb.). Mean productivityfor 29 successful nestsof Great Horned Owls was 1.77 + 0.14 young,ranging from 1.00 to 2.50 young/nestsite. For all 34 nestingattemptsthe mean productivitywas 1.63 _+0.16 young/nest,ranging from 0.5 to 3.0 young/nestsite. The averagedensityof nestingGreat Horned Owls (0.016 _+0.003 nesting

438]

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J.Field Ornithol. Autumn

1993

pair/km 2) is not particularly meaningful becauseall but three Great Horned Owl nestingattemptsoccurredin old Red-tailed Hawk nests. Nestingsof Great Horned Owls in otherstructuresmay havebeenmissed. Red-tailedHawk/Great HornedOwl interactions.--Preemption of Redtailed Hawk nestsby Great Horned Owls was common becauseGreat Horned Owls beginto nestabout a month earlier than Red-tailed Hawks

in central New York. Nests of Red-tailed Hawks were usedby Great Horned Owls in 16 of the 25 nestingareas (64%). Great Horned Owls were seenor heard in five additional areas, but their nestswere not found.

SomeRed-tailedHawk nestswere usedalternatelyby Red-tailedHawks and Great Horned Owls. In 1987 and 1988, Red-tailed Hawks success-

fully nestedin the same nest where Great Horned Owls failed in their nestingattempt in February of the sameyear. There

were

instances

of Great

Horned

Owls

and Red-tailed

Hawks

nestingapproximately200 m apart during the sameyear. The proximity of nestingby thesetwo speciesis probablydue to the limited availability

of suitable nestinghabitat within this urban/suburbanlandscape. Throughoutthestudyareathedistributionof suitableundisturbed nesting habitat appearedclumpy. Houston (1975) reportson the proximity of Great Horned Owls and Red-tailedHawks nestingon 'island'clumpsof 'aspen parkland' in Saskatchewan.The short distancesbetween Great Horned Owl and Red-tailedHawk nestsin this studyand that of Houston's(1975) studymay be dueto the similarityin structuralcharacteristics of the two habitats. CONCLUSIONS

Today the populationsof Red-tailedHawks and Great Horned Owls in this urban/suburbanecosystem appear to be relatively stable.We believe from their presenceas successful breedingpopulationsin and around Syracusethat with proper maintenanceof undisturbednesting areas and adequateopen spacefor foraging, they will persist as top predators. ACKNOWLEDGMENTS

We wish to acknowledge the assistance of Dr. Larry VanDruff for introducingus to the followingstudentsfrom the SUNY Collegeof EnvironmentalScienceand Forestrywho aided in collectingnest data: Glenn Johnson,Bruce Hopkins, L. Kroon, Rich Smith, Jeff Zehr, Lisa Truffeli and Dave Seyler.Roy Slackand Dr. Walter Spoffordofferedvaluable adviceand assistance. We alsoappreciatedthe assistance of DorothyCrumb, Dale Hildebrand, Dr. Robert Long, Kay Mcintyre, Dr. F. G. Scheider,Gerald A. Smith and William Johnson,who suppliedus with informationand technicalsupportrelative to the study. Specialthanksgo to GlennJohnsonand Michelle Hirth for reviewingthe manuscript,and to Merry Jo Ingraldi for her patience,understandingand supportover the years. LITERATURE

BOHM,R.T.

CITED

1978. A studyof nestingRed-tailed Hawks in central Minnesota. The Loon:

129-137.

BROWN,L., AND D. AMADON. 1968. Eagles, hawks, and falconsof the world. McGrawHill Book Co., New York, New York. 945 pp.

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Nesting ofUrban/Suburban Raptors

[439

CRAIGHEAD, J. J., ANDF. C. CRAIGHEAD, JR. 1956. Hawks, owls, and wildlife. Stackpole Co., Harrisburg, Pennsylvania.443 pp. DAVIS, J. A. 1977. An ecosystemclassificationof New York State for natural resources management.N.Y. Fish and Game J. 24:129-143. ERRINGTON,P. L. 1933. Food habits of southern Wisconsin raptors part II. Hawks. Condor

35:19-29.

GATES,J. M. Wilson

1972. Red-tailed hawk populationsand ecologyin east-centralWisconsin.

Bull.

84:421-433.

HOUSTON,C.S. Auk

1975. Closeproximity of Red-tailed Hawk and Great Horned Owl nests.

92:612-614.

HOWARD,R., AND B.C. POSTOVIT. 1987. Impacts and mitigation techniques.Pp. 183213, in B. A. Giron Pendleton, B. A. Millsap, K. W. Cline, and D. M. Bird, eds. Raptor management techniquesmanual. Natl. Wildl. Fed., Washington, D.C. JOHNSON,S.J. 1975. Productivity of the Red-tailed Hawk in southwesternMontana. Auk

92:732-736.

MCINVAILLE, W. B., ANDL. B. KEITH. 1974. Predator-prey relationsand breedingbiology of the Great 88:1-20.

Horned

Owl

and Red-tailed

Hawk

in central

Alberta.

Can.

Field-Nat.

MINOR, W. F., AND M. MINOR 1981. Nesting of Red-tailed Hawks and Great Horned Owls in central New York suburbanareas. Kingbird 31:68-76. ORIANS,G., AND F. KUHLMAN. 1956. Red-tailed Hawk and Horned Owl populationsin Wisconsin.

Condor

58:371-385.

PETERSON, L. 1979. Ecologyof Great Horned Owls and Red-tailed Hawks in southeastern Wisconsin.Tech. Bull. No. 111 Dept. of Natl. Res., Madison, Wisconsin.79 pp. RUNYAN, C. S. 1987. Location and density of nests of the Red-tailed Hawk, Buteojamaicenszs, in Richmond, British Columbia. Can. Field-Nat. 101:415-418. SEIDENSTICKER, J. C., AND H. V. REYNOLDS. 1971. The nesting,reproductiveperformances, and chlorinated hydrocarbonresiduesin the Red-tailed Hawk and Great Horned

Owl

in south-central

Montana.

Wilson

Bull.

83:408-418.

SOKAL,R. R., AND F. J. ROHLF. 1981. Biometry. W. H. Freeman and Co., New York, New York. 859 pp. STEENHOF,K. 1987. Assessing raptor reproductivesuccess and productivity.Pp. 157-170, in B. A. Giron Pendleton,B. A. Millsap, K. W. Cline, and D. M. Bird, eds. Raptor managementtechniquesmanual. Natl. Wildl. Fed., Washington, D.C. TERRES,J. K. 1982. The AudubonSocietyencyclopedia of North Americanbirds.Alfred A. Knopf, New York, New York. 1109 pp.

Received28 Feb. 1991; accepted30 Sep. 1992.

ERRATUM

The natural leg injury rates for Black-neckedStilts (Himantopusmexicanus) containedin Bandingis infrequentlyassociated with footlossin SpottedSandpipers by J. M. Reed and L. W. Oring (1993:146,J. Field Ornithol. 64:145-148), as a personalcommunicationfrom C. Gratto-Trevor and H. L. Dickson,is erroneous. The reportedratesare for Stilt Sandpipers(Caliclrishimantopus). J. Michael Reed and Lewis W. Oring