A Nesting Population of Cassin's Sparrows in the Sandhills of Nebraska

J. Field Ornithol., 65(4):472-475

A NESTING POPULATION OF CASSIN'S SPARROWS IN THE SANDHILLS OF NEBRASKA

CARL E. BOCK Departmentof Environmental,Population, and OrganismicBiology Universityof Colorado Bould• Colorado 80309-0334 USA

WILLIAM C. SCHARF Schoolof BiologicalSciences Universityof Nebraska Lincoln, Nebraska 68588-0118 USA

Abstract.--In 1993 a smallpopulation of Cassin'sSparrows (Aimophila cassini•) was found nestingin the sandhillsof Nebraska.There were at leastsix pairs,and three nests,one with five eggsand two with five young,were found. As singingmales may not indicatenesting in thisspecies, numerous earlier sightings have not clarifiedthe breedingstatus of the Cassin's Sparrowin the central and northern Great Plains.These findings suggest that Cassin'sSpar- rowsnest locally and periodicallyin thisregion, especially in substantialgrasslands with high densitiesof grasshoppers.

POBLACION RESIDENTE DE AIMOPHILA CASSINII EN NEBRASKA Sinopsis.--Enel 1993 se encontr6una pequefiapoblaci6n del gorri6n Aimophilacassinii en Nebraska.En la localidadse encontraronun minimo de seisparejas y tres nidos de estas aves.Uno de los nidos contenla cinco huevosy los otros dos cinco pichonescada uno. Avistamientosanteriores no habian demostradoque el ave fuera residentede las Grandes Planicies,ya que el oir a machosde la especiecantando no es sin6nimode reproducci6n. Nuestroinforme sugiereque estegorri6n anidalocalmente y peri6dicamenteen estaregi6n, particularmenteen yerbasalescon densidadesaltas de saltamontes. The Cassin'sSparrow (Aimophila cassini•)is a North American grass- land endemic, whosebreeding range has proven difficult to define. Plain- ly colored and usually secretive, these birds become conspicuousonly when males are singingand performing their characteristicsong-flights (Schnaseand Maxwell 1989). Field workers who have observed singing males,however, often havefound no evidenceof subsequentnesting (e.g., Phillips 1944). This has led to speculationthat Cassin'sSparrows may wander opportunisticallyin summer, maintain a state of reproductive readiness,and then breed only under particular (and largely unknown) circumstances(Hubbard 1977). Cassin'sSparrows nest regularly,if sporadically,from southeasternCol- orado and southwesternKansas, south through Texas, parts of southern New Mexico and southeastern Arizona, and into northern Mexico (Wil- liams and LeSassier1968, Wolf 1977). Breeding habitat consistentlyis grasslandwith scatteredshrubs (Hubbard 1977). Faanes et al. (1979) and Labedz (1986) described and reviewed more northerly records of singingmale Cassin'sSparrows, including a single bird in central Wyoming, another in southwesternSouth Dakota, and numerous sightingsfrom shrub-grasslandsin northeastern Colorado,

472 Vol.65, No. 4 Cassin'sSparrows in Nebraska [473 western Nebraska,and southwesternNebraska. There apparently is only one record of an actual breeding attempt in this entire region, however, a nestwith eggsfrom PerkinsCounty in southwesternNebraska (Williams 1974). In June 1993,we discovereda smallnesting population of Cassin'sSpar- rowsin the sandhillsprairie of westernNebraska, about 55 km north of the 1974 PerkinsCounty record. The localitywas along an abandoned road, about 5 km north of Lake McConaughy, in Keith County. There were a minimum of six malesperforming song-flights,and we discovered three nests. Perennial mid-height native grasseswere the predominant ground cover where the Cassin'sSparrows were nesting.Common specieswere nee- die-and-threadgrass ( Stipa comata),western wheatgrass (Affropyron smith- ii), little bluestem (Andropogonscoparius), and variousgramas (Bouteloua spp.). Scattered Yuccaglauca were conspicuousin the grazed pastures along both sidesof the road. The Cassin'sSparrows appeared to spenda disproportionateamount of time in ungrazedtaller and denservegetation between the fences and the abandoned roadway,where the grasslands included a higher proportion of weedy plants, especiallyprickly poppy (Argemonepolyanthemos). Grasshoppers, which are the major summer food of Cassin'sSparrows (Bock et al. 1992), also appeared to be very common in this roadside habitat, although we did not quantify their abundance.Other bird speciesnesting along the roadsidewere Lark Spar- row ( Chondestesgrammacus), Grasshopper Sparrow (Ammodramussavan- narum), and Western Meadowlark ( Sturnella neglecta). We spentabout 20 h over 9 d at the site,between 17 June and 12 July, and made the following observations. 17 June. At leastfour maleswere givingsong-flights, each in the pres- ence of a non-singingbird we assumedto be a female; two birds of unknown sex were seen carrying nest material. 29 June. At least three pairs were feeding together along roadsides; singingcontinued, but lessactively than before. 30 June. Nest 4/1 discovered,with five eggsbeing incubated. The nest wasbuilt on the ground, about 2 m from the road, with its sidessupported by dead stalksof a prickly poppy. 1 July.At least six maleswere performing song-flightsin the area, four along the road and two about 100 m out into adjacentgrazed pastures. 7 July.Nest #1 wasstill being incubated. 8 July. One newly-hatchedyoung and four eggswere in nest #1. We alsodiscovered two new nests;each held five downyyoung, whoseflight featherswere still completelysheathed. Nest #2 wasbuilt about 10 cm off the ground, deep in a yuccaplant, about 50 m out into a grazed pasture. Nest #3 was built on the ground next to the roadside fence, supported and partiallycovered by stalksof a smallsandhill sage (Artemisiafilifolia). 9 July.Nest #2 wasempty, whereas nests #1 and #3 were still occupied. 11 July. Nest #1 was empty; nest #3 still had five young, nearly fully feathered,with primariesabout 40% grown. 474] c. E. Bockand W. C. Scharf J.Field Ornithol. Autumn 1994

12 July. Nest 4/3 was empty. All nestlingswere too young to have fledged by the dates we found their neststo be empty. Unknown predators may have causedthe nests to fail, but it seemsequally likely that they were abandonedby the adults becauseof human interference. On 9 July, a road construction crew occupied the abandonedroad as a site for mixing blacktop and for parking heavy equipment. Human activitywas intense for the subsequent2 wk, and we sawno more birds after the final nest waslost on 12 July. Whatever causedthe neststo fail, their potential productivity (clutchesand broods of five) wasas high as has been found elsewhere(e.g., Johnsgard 1979, Maurer et al. 1989), suggestingthat sandhillsprairie can be suitablenest- ing habitat. The mysterysurrounding the geographicalecology of the Cassin'sSpar- row may be somewhatoverrated. Certainly these birds are patchily dis- tributed, and much remains to be learned about factors affecting their breeding range and reproductivesuccess. The presenceof malesperforming song-flights(skylarking), however, probably does indicate nesting in most cases.Schnase et al. (1991) found that unmated males rarely sing in flight, so observationsof skylarkingat least should be evidence of paired birds. Furthermore, Cassin'sSparrows are so wary and secretive that brief observationsof these birds, without using blinds, are unlikely to reveal the presenceof either nestsor nestingbehavior (Johnson1956). Breeding Bird Surveydata indicate that the Cassin'sSparrow is declin- ing in abundancethroughout its range (Knopf 1994). The causesof this decline are unknown, but Cassin'sSparrows were negativelyaffected by livestockgrazing in southeasternArizona (Bock and Bock 1988), apparently becausethey require the vegetativecover and grasshoppersassoci- ated with ungrazedhabitat. It is noteworthythat the birds we found nesting in Nebraska were concentrated along an abandoned ungrazed roadside, where both vegetation cover and grasshopperdensities appeared to be much higher than in adjacentgrazed pastures.

ACKNOWLEDGMENTS

We thank Paul Johnsgard,Brian Maurer, and John Schnasefor commentson an earlier version of the manuscript.

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