854 SHORT COMMUNICATIONS

DEL HOYO, J., A. ELLIOT, AND J. SARGATAL.1996. adox in the Buff-breasted Sandpiper. Am. Nat. Handbook of the birds of the world. Vol. 3. Hoa- 149:1051-1070. tzin to auks. Lynx Editions, Barcelona. LANK, D. B., C. M. SMITH,0. HANOTTE, T A. BURKE, DEWSBURY,D. A. 1982. Ejaculate cost and male AND E COOKE.1995. Genetic polymorphism for choice. Am. Nat. 119:601-610. alternativemating behaviourin iekking male Ruff, DUNNING,J. B., JR. 1993. CRC handbook of avian Philomachusougnax. Nature 378:59-62. body masses.CRC Press, Boca Raton, FL. MARTIN,P A., T.-J.-REIMERS,J. R. LODGE,AND E? J. EMLEN,S. T, l? H. WREGE,AND M. S. WEBSTER.1998. DZIUK. 1974. The effect of ratios and numbersof Cuckoldry as a cost of polyandry in the sex-role- spermatozoa mixed from two males on propor- reversed Wattled Jacana,Jacana jacana. Proc. R. tions of offspring. J. Reprod. Fert. 39:251-258. Sot. Lond. B 256:2359-2364. MILLER, A. P., AND J. V. BRISKIE.1995. Extra-pair pa- GAGE, M. J. G. 1994. Associationsbetween body size, ternity, sperm competition and the evol&ion-of mating pattern, testis size and sperm lengths testis size in birds. Behav. Ecol. Sociobiol. 36: acrossbutterflies. Proc. R. Sot. Lond. B. 258:247- 357-365. 254. OLSSON,M., T MADSEN,AND R. SHINE.1997. Is sperm really so cheap? Costs of reproduction in male GAGE, M. J. G. 1998. Mammalian sperm morphome- try. Proc. R. Sot. Lond. B. 265:97-103. adders, Vipera berus. Proc. R. Sot. Lond. B 264: GOMENDIO,M., ANDE. R. S. ROLDAN.1991. Sperm size 455-459. and sperm competition in mammals. Proc. R. Sot. PARKER,G. A. 1970. Sperm competition and its evo- lutionary consequencesin the insects. Biol. Rev. Lond. B 243:181-185. 45:525-567. HALLIDAY,T, ANDS. J. ARNOLD.1987. Multiple mat- PARKER,G. A. 1982. Why are there so many tiny ing by females: a perspective from quantitative sperm? Sperm compe&tion and the main&an& genetics.Anim. Behav. 35:939-941. of two sexes. J. theor. Biol. 96:281-294. HAR&SON,C. 1975. A field guide to the nests, eggs PIERCE,E. P, AND J. T LIFJELD.1998. High paternity and nestlings of British and European birds. Col- without paternity-assurancebehavior in the Purple lins, London. Sandpiper, a species with high paternal invest- HARRSION,C. 1978. A field guide to the nests, eggs ment. Auk 115:602-612. and nestlings of North American birds. Collins, PITELKA,E A., R. T. HOLMES,AND S. E MCLEAN. 1974. London. Ecology and evolution of social organization in HARVEY,P. H., AND M. D. PAGEL.1991. The compar- arctic sandpipers.Am. Zool. 14:185-204. ative method in evolutionary biology. Oxford PURVIS.A.. ANDA. RAMBAUT.1994. Comoarativeanal- Univ. Press, Oxford. ysis by independent contrasts(CAIk), version 2. HEG, D., B. J. ENS, T. BURKE,L. JENKINS,AND J. I? Oxford Univ., Oxford. KRUIJT. 1993. Why does the typically monoga- SZEKELY,T, ANDJ. D. REYNOLDS. 1995. Evolutionary mous Oystercatcher(Haematopus ostralegus) en- transitionsin parental care in shorebirds.Proc. R. gage in extra-pair copulations? Behaviour 126: Sot. Lond. B 262~57-64. 247-289. - - VAN RHIJN, J. G. 1991. The Ruff: individuality in a LANCTOT, R. B., K. T. SCRIBNER, B. KEMPENAERS, AND gregarious wading bird. T & A.D. Poyser, Lon- P J. WEATHERHEAD.1997. Lekking without a par- don.

The Condor 101:854-X59 0 The Cooper Ornithological Society1999

GENETIC MONOGAMY IN LONG-EARED OWLS ’

JEFFREY S. MARKS Montana CooperativeWildlife ResearchUnit, Universityof Montana, Missoula, MT 59812, e-mail: [email protected]

JANISL. DICKINSON AND JOSEPH HAYDOCK HastingsNatural History Reservationand Museum of VertebrateZoology, Universityof California, 38601 East Carmel Valley Road, Cannel Valley, CA 93924

Abstract. We used DNA fingerprinting to study Owls (Asio otus). We detected no extra-pair fertiliza- genetic parentagein socially monogamousLong-eared tions (EPFs) in 59 nestlings from 12 nests. One of these nestswas solitary, but the other 11 had from one to five pairs of owls nesting simultaneouslywithin 30 I Received 5 May 1999. Accepted 26 July 1999. to 250 m. Thus, despite the presumablyhigh potential SHORT COMMUNICATIONS 855

for extra-pair matings, the Long-eared Owls that we hypothesis that EPFs occur in Long-eared Owls. We studied were genetically monogamous. In addition, also use band sharing coefficients from adults to de- based on low band sharing among adults, we found no termine whether nesting aggregationsare composedof evidence that nesting aggregationswere composed of close relatives. close relatives. Genetic monogamy appearsto be the rule for socially monogamousraptors. We suggestthat METHODS the high rate of male parental effort in raptors selects STUDY AREA AND FIELD METHODS againstEPFs becausefemales that engage in extra-pair activities risk losing parental investment by males The study areas were in Lake and Ravalli Counties, whose confidence in paternity is reduced owing to the western Montana, in isolated groves of quaking aspens behavior of their mates. (Populus tremuloides), black hawthorns (Crataegus douglasii), and willows (Salix sp.). The Paul Smith Key words: Asio otus, DNAfingerprinting, genetic study area (Lake County) had six Long-eared Owl monogamy, Long-eared Owl. nests in 1997 and five in 1998. The Victor study area (Ravalli Countv) was 150 km south of Paul Smith and Studies of mating systems based on genetic markers had four nests& 1998. The Airport study area was 14 have shown that cooulationsoutside the oair bond are km east of Paul Smith and had one nest in 1997 and common in many bird species (Westneat et al. 1990). two close-nestingpairs in 1998. Indeed, genetic monogamy seems to be the exception We captured adults at night in mist nets placed in rather than the rule in socially monogamouspasserines natural flight paths to the nest or set in front of flight- (Gowaty 1996). Among socially monogamous non- less young that had recently left the nest. Thus, we passerines,however, extra-pair fertilizations (EPFs) are were reasonablycertain that these owls were the social less common. A recent review showed that EPFs have parentsat the nests where they were captured.We de- occurred in 86% (42/49 species) of passerinesthat termined the sex of capturedadults based on presence have been studied but in only 48% (11/23 species)of or absenceof an incubationpatch. Adults were banded non-passerines(Westneat and Sherman 1997). On both with U.S. Fish and Wildlife Service metal leg bands. empirical and theoretical grounds, the occurrence of Young were banded at three to four weeks of age, just EPFs in birds is influenced by the density and syn- before or shortly after they left the nest. chrony of breeding pairs (Siutchbury and Morton 1995, Johnson and Burlev 1998). In addition. the BLOODSAMPLING AND DNA FINGERPRINTING amount of parental care provided’ by males m& be To determine whether nesting density is associated positively c&related with- their certainty of pateinity with EPFs in Long-eared Owls, we conductedparen- (Mflller and Birkhead 1993). tal-exclusion analyses using multilocus DNA finger- Because Long-eared Owls (Asio otus) sometimes printing. This permitted us to evaluatebackground lev- nest close to one another (Marks et al. 1994), they els of genetic variation in the populationof Long-eared representan important test case for the hypothesisthat Owls in western Montana and to perform paternity- genetic parentageis associatedwith breeding density. exclusion analysis for 59 nestlings from 12 nests dur- Male parental effort is extremely high in all speciesof ing two years of study. Blood samples(100 pL) were owls becausemales provide nearly all of the food to taken from the brachial vein of adults and nestlingsat the female and nestlings from courtship until at least the time of banding. Samples were placed in Queen’s mid-way through the brood-rearing period (Marks et lysis buffer (Seutin et al. 1991) in the field and then al. 1999), although they do not incubate or brood. tiansferred to the lab, where they were frozen at Thus, one might expect owls to be genetically monog- -20°C. We digested 5 to 10 UP of nuclear DNA with amous independentof proximate factors that could in- HaeIII and eviluated the quaiiry and concentrationof fluence mating systems(breeding density and breeding DNA in the restriction digests on a 1% agarose test synchrony). gel stainedwith ethidium bromide. Samplescontaining Investigators have used DNA fingerprinting to as- 5 kg of digested DNA in Ficoll loading buffer were sess relatednessand genetic parentagein three previ- loaded on 27-30 cm agarosegels (1%) aid run along- ous studies of owls. Galeotti et al. (1997) compared side a I-kb ladder at 40 V for three davs. The social genetic similarity within and between communal win- parents were run in lanes adjacent to

BFBM Nl N2 N3 N4 N5 BFBM

Kb

12

10

Band-sharing coefficient

8 FIGURE 2. Distribution of band sharingcoefficients between putative parentsand offspring comparedwith band sharing between presumably unrelated dyads of breedersin the population.Arrow approximatesthe ex- clusion threshold.

from all young that left the nest. Four of the I2 nests were at Paul Smith in 1997 (mean nearest-neighbor 6 distance = 88.8 + 33.8 m, range = 63-140 m, n = 6 nests total, including those with and without finger- printing data), two were at Paul Smith in 1998 (i = 105.2 2 93.8 m. range = 51-245 m, n = 4). one was a solitary nest at Airport in 1997, two were’at Airport in 1998 (nests 46 m apart), and three were at Victor in 1998 (.? = 86.8 2 66.3 m, range = 32-166 m, n = 5 4). All of the nests within each of the three study areas were active simultaneously(within each nestinggroup, all owls were present during the fertile period of each female). Thus, except for the solitary pair at Airport in 1997, each pair for which we had fingerprintingdata had from one (Airport in 1998) to five (Paul Smith in 1997) other nesting pairs within close proximity (32 4 to 245 m) at the time the nest was initiated. Conse- quently, the potential for extra-pair fertilizations within nesting aggregationswas high.

MULTILOCUSDNA FINGERPRINTING AND PARENTAL EXCLUSION The mean band sharing coefficient for dyads of adults that were run no more than two lanes apart (0.25 k 1 0.08) was lower than those for dyads of breeder fe- i 1 males and nestlings (0.60 5 0.08) and breeder males and nestlings(0.62 +- 0.09) (Mann-Whitney u-tests, Z I > 6.3, P < 0.001 for both comparisons;Fig. 2). Par- 3 I ent-offspring band sharing coefficients did not differ significantly between breeder males and females FIGURE 1. DNA fingerprint of a Long-eared Owl (Mann-Whitney u-test, Z = 1.50, P = 0.13). Assum- family. Filter was probed with Jeffreys’ probe 33.15 ing that bands segregate independently and that the (Jeffreys et al. 1985). BM = breeder male, BF = other parent is correctly assigned, the probability of breeder female, N = nestlings. false inclusion of a sire or dam that is unrelatedto the true breeder is I’ = em,where x is the mean band shar- ing coefficient for 22 dyads of breeding adults run no RESULTS more than two lanes auart, and m is the mean number of paternal- or maternal-specificbands in the offspring NEAREST-NEIGHBORDISTANCE (Bruford et al. 1992: Table I). The mean number of Eighteen Long-eared Owl nests occurred in our study paternal-specific bands was slightly higher than the areas in 1997 and 1998, but owing to nesting failures mean number of maternal-specificbands, but the prob- and other complications,we caught the adult males at ability of false inclusion was low for both sexes (5.8 only I2 nests for which we obtained blood samples X 10m6for sires and 4.6 X 10m5for dams; Table I). SHORT COMMUNICATIONS 857

TABLE 1. DNA fingerprinting results for 59 Long-eared Owl nestlings from 12 different nests in western Montana, 1997 and 1998. Means are presented t SD.

Variable Value Sample size No. of bands scored 21.5 5 9.2 59 nestlings Band sharing among breeders, observed (x) 0.25 ? 0.08 22 dyads No. of paternal-specificbands in nestlings 8.7 _t 4.6 38 nestlings No. of maternal-specificbands in nestlings 7.2 ? 3.5 38 nestlings Observed sibling band sharing 0.60 2 0.10 54 dyads Expected sibling band sharing (s) 0.62 Probability of false inclusion of an unrelatedmale 5.8 x 10-e Probability of false inclusion of an unrelated female 4.6 X lo-” Mutation rate 1.8 x 10-3 Allelic frequency (q)b 0.134 Heterozygosity (h)” 0.928

a Where s = [(4 + 5q - q2)/4(2 - q)] (case i; Jeffreys et al. 1985) b Where * = 2q - q* (Jeffreys et al. 1985). c Where h = 2(1 q)/(2 - q).

We calculated sibling band sharing to be 0.60 ? in close proximity to conspecifics,however, the Long- 0.10, based on a subsetof 54 sibling dyads. Exclusion eared Owls that we studied were genetically and so- analysiswas based on band sharing and the number of cially monogamous.We determined parentage for 56 bands in the offspring that could not be attributed to nestlingsfrom 11 grouped nests in three areasand for either of its putative parents (unattributable bands). three nestlings from one solitary nest, and all were This resulted in the following parental-exclusioncri- sired by their putative fathers. This suggeststhat the teria: (1) we did not exclude a breeder from genetic pattern of genetic monogamy that we observed holds parentage unless an offspring had more than two un- in Long-eared Owls regardlessof nest dispersion.We attributable bands or the band sharing coefficient was recognize that becausewe had only one solitary nest, below 0.343 (observedband sharingfor full sibs minus we have not provided a complete test of the influence 2.57 SD; Table 1). Fewer than 0.5% of first-order rel- of breeding density on the frequency of EPFs. How- atives are expected to have band sharing coefficients ever, our conclusionthat breeding density does not in- below this threshold. (2) In 21 cases (n = 4 nests) fluence the likelihood of EPFs in Long-eared Owls is where we had blood from the male breeder only, we based on the assumptionthat becauseEPFs were ab- used father-offspring band sharing below 0.343 as the sent in close-nesting owls, they would be highly un- basis for exclusion. likely to occur in solitary pairs. Using these exclusioncriteria, we inferred parentage Mean band sharingin Long-eared Owls at two com- for all 59 Long-eared Owl nestlings from 12 pairs in munal winter roostsin Italy was 0.18 within roostsand 1997 and 1998. We observed one unattributableband 0.14 between roosts (Galeotti et al. 1997). Based on in each of three offspring for which we had blood from these data, Galeotti et al. (1997) concluded that com- both parents, but in these cases parent-offspringband munal roosts were not made up of close relatives. sharingexceeded 0.49. We attributedthese three bands Mean band sharing among breeding adultsin Montana to mutation and used this information to calculate the was higher (0.25) than band sharingmeasured by Gal- mutation rate for minisatellite alleles (Table 1). In no eotti et al. Nonetheless, compared with mean band case was parent-offspring band sharing below 0.343. sharing among siblings (0.60), our results suggestthat The mean father-offspring band sharing for nestlings nesting aggregationsof Long-eared Owls in Montana whose mother was not captured was 0.61 -C 0.10 are not composedof close relatives. (range = 0.434.78, n = 21 nestlings), which did not Male raptors provide most of the food during in- differ from the mean for males whose mate also was cubation and brood rearing (Snyder and Wiley 1976). run (0.62 t- 0.08, range = 0.40-0.75, II = 38 nestlings) Indeed, male owls probably provide more parentalcare (Mann-Whitney U-test, Z = 0.26, P = 0.79). Our ex- in terms of food provisioning than do males of any clusion criteria assigned all 59 offspring to their pu- other birds besides hombills (Kemp 1995). To the ex- tative fathers and all 38 offspring for which we had tent that females have some control over EPFs (Go- maternal blood to their putative mothers. Thus, we waty 1997, Stutchbury and Neudorf 1998), one might found no evidence for EPFs. If we assumethat all 59 expect low rates of EPFs in species in which a large of the fertilizations were independent, then the bino- amount of parental care by males is essential.In agree- mial probability is 0.05 that the true frequency of ex- ment with this notion, rates of EPFs are low (0 to 3% tra-pair paternity in this population is 6%. of young) in the handful of socially monogamousrap- tors that have been studied (Table 2). Included among DISCUSSION these speciesare Eleonora’s Falcon (F&o eleonorae) Nesting groups were common in our study areas dur- and Lesser Kestrel (F. naumanni), both of which nest ing both years, but most Long-eared Owls nest soli- in dense colonies but nonethelesstend to be geneti- tarily, with nearest-neighbordistances exceeding 1 km cally monogamous(Swatschek et al. 1993, Negro et (Marks et al. 1994). Even when nesting synchronously al. 1996). 858 SHORT COMMUNICATIONS

TABLE 2. Reported frequency of extra-pair fertilizations (EPF) in socially monogamousraptors (Falconiformes and Strigiformes) based on DNA fingerprinting analysis.

Species % EPF Reference Eurasian Kestrel (F&o tinnunculus) 2 Korpimtii et al. 1996 Lesser Kestrel (F&o naumanni) 3 Negro et al. 1996 Merlin (F&o columban’us) 0 Warkentin et al. 1994 Eleonora’s Falcon (F&o eleonorae) 0 Swatscheket al. 1993 Eastern Screech-Owl (O&s asio) 0 Lawless et al. 1997 Burrowing Owl (Athene cunicuZaria) 6 Johnson 1997b Long-eared Owl (Asio otus) 0 This study

The highestrate of EPFs reportedfor a raptor,6.4%, GALEOTTI,I?, A. PILASTRO,G. TAVECCHIA,A. BONEPI, was for a Burrowing Owl population in California AND L. CONGIU.1997. Genetic similarity in Long- (Johnson 1997b). These owls potentially were not so- eared Owl communal winter roosts: a DNA fin- cially monogamous(two nests were attendedby three gerprinting study. Mol. Ecol. 6:429-435. adults), and brood movements among burrows com- GOWATY,P A. 1996. Field studies of parental care in plicated the assessmentof social relationshipsbetween birds: new data focus questions on variation parents and young. Moreover, some Burrowing Owl among females. Adv. Stud. Behav. 25:476-531. populations appear to be highly structuredand-to ex- GOWATY,P A. 1997. Principles of females’ perspec- hibit relativelv high levels of inbreeding (Johnson tives in avian behavioral ecology. J. Avian Biol. 1997a, Millsa;, an;d Bear 1997), indicating ‘that the 28:95-102. mating system in this species may be fundamentally JEFFREYS,A. J., V. WILSON,AND S. L. THEIN. 1985. different from that in most other speciesof owls. Hvpervariable ‘minisatellite’ regions in human On balance, genetic monogamy seemsto be the rule DNA. Nature 314:67-73. - among socially monogamousbirds of prey, although JOHNSON.B. S. 1997a. Characterizationof uooulation relatively few species have been studied. We suggest and ‘family genetics of the Burrowing bwl by that the high rate of male parental effort in raptors DNA fingerprinting with pV47-2. J. Raptor Res. selects against EPFs because females that engage in Rep. 9:58-63. extra-pair activities risk losing parental investment by JOHNSON,B. S. 1997b. Reproductive success,related- males whose confidence in paternity has been reduced ness, and mating patterns of colonial Burrowing owing to the behavior of thkir mates (Whittingham et Owls. J. Rautor Res. Rep. 9:64-67. al. 1992, Koroim&i et al. 1996). Because male owls JOHNSON,K., AN; N. T. BURLEY.1998. Mating tactics provide such a large amount of parental effort, we pre- and mating systems of birds, p. 21-60. In F?G. dict that most species of owls will prove to be genet- Parker and N. T Burley [EDS.], Avian reproductive ically monogamous.Additional studiesof genetic par- tactics: male and female perspectives. Ornithol. entage in birds of prey will be required to evaluatethe Monogr. 49. relationship between parental investment and EPFs KEMP,A. C. 1995. The hombills: Bucerotiformes.Ox- more thoroughly. In this regard, parentage studies of ford Univ. Press, Oxford. raptors in which males exhibit relatively low rates of KORPIMAKI,E., K. LAHTI,C. A. MAY, D. T. PARKIN,G. parental care would be especially valuable. B. POWELL,P. TOLONEN,AND J. H. WETTON.1996. Copulatory behaviour and paternity determined We thank K. Kraaijeveld and E Smit-Kraaijeveld for by DNA fingerprinting in kestrels: effects of cy- helping with the DNA fingerprinting, A. Perkins, C. chic food abindance. ;\nim. Behav. 5 1:945-955. Olson, L. Clough, T. Dial, and B. Petty for helping to LAWLESS,S. G., G. RITCHISON,I? H. KLAIT, AND D. catch owls, P and D. Smith and the ConfederatedSa- WESTNEAT.1997. The mating strategiesof Eastern lish and Kootenai tribes for granting access to their Screech-Owls:a genetic analysis.Condor 99:213- lands, and T. Lubiuhn and three anonymousreferees 217. for commentingon the manuscript.JSM was supported MARKS,J. S., R. J. CANNINGS,AND H. MIKKOLA.1999. by a grant from the University of Montana Research Family Strigidae, p. 76-151. In J. de1 Hoyo, A. and Creativity Committee. Elliott, and J. Sargatal [EDS.], Handbook of the birds of the world. Vol. 5. Lynx Editions, Bar- LITERATURE CITED celona. BIRKHEAD,T R., T BURKE,R. ZAHN, E M. HUNTER, MARKS,J. S., D. L. EVANS,AND D. W. HOLT. 1994. AND A. P KRUPA. 1990. Extra-pair paternity and Long-eared Owl (Asio &us). In A. Poole and E intraspecificbrood parasitismin wild Zebra Finch- Gill [EDS.], The birds of North America, No. 133. es Taeniopygia guttutu, revealed by DNA finger- The Academy of Natural Sciences, Philadelphia, printing. Behav. Ecol. Sociobiol. 27:315-324. and The American Ornithologists’ Union, Wash- BRUFORD,M. W., 0. HANOI, J. E Y. BROOKFIELD, ington, DC. AND T BURKE.1992. Single-locus and multilocus MILLSAP,B. A., AND C. BEAR.1997. Territory fidelity, DNA fingerprinting, p. 226-269. In A. R. Hoelzel mate fidelity, and dispersal in an urban-nesting [ED.], Molecular genetic analysis of populations. population of Florida Burrowing Owls. J. Raptor IRL Press, Oxford. Res. Rep. 9:91-98. SHORT COMMUNICATIONS 859

MILLER, A. F?,AND T R. BIRKHEAD.1993. Certainty Parker and N. T. Burley [EDS.], Avian reproductive of paternity covaries with paternal care in birds. tactics: male and female perspectives. Omithol. Behav. Ecol. Sociobiol. 33:261-268. Monogr. 49. NEGRO,J. J., M. VILLARROEL,J. L. TELLA,U. KUHN- SWATSCHEK,I., D. RISTOW,W. SCHARLAU,C. WINK, LEIN, E HIRALDO,J. A. DONAZAR,AND D. M. BIRD. ANDM. WINK. 1993. Population genetics and pa- 1996. DNA fingerprinting reveals a low incidence ternity analysis of Eleonora’s Falcon (F&o eleo- of extra-pair fertilizations in the Lesser Kestrel. norm). J. Ornithol. 134:137-143. Anim. Behav. 51:935-943. WARKENTIN,I. G., A. D. CURZON,R. E. CARTER,J. H. WE~ON, F?C. JAMES,L. W. OLIPHANT,AM) D. T SEUTIN,G., B. N. WHITE,AND I? I‘ BOAG.1991. Pres- PARKIN.1994. No evidence for extra-pair fertiliza- ervation of avian blood and tissue samples for tions in the Merlin revealed by DNA fingerprint- DNA analyses.Can. J. 2001. 69:82-90. - _ ing. Mol. Ecol. 3:229-234. _ SNYDER, N. E R., AND J. W. WILEY. 1976. Sexual size WESTNEAT,D. E, AND I? W. SHERMAN.1997. Density dimorphism in hawks and owls of North America. and extra-pair fertilizations in birds: a comparative Omithol. Monogr. 20. analvsis.Behav. Ecol. Sociobiol. 41:205-215. STUTCHBURY,B. J., AND E. S. MORTON.1995. The ef- WESTNEAT,D. E, l? W. SHERMAN, AND M. L. MORTON. fect of breeding synchrony on extra-pair mating 1990. The ecology and evolution of extra-pair systemsin songbirds.Behaviour 132:675-690. copulation in birds. Current Omithol. 7:331-369. STLJTCHBURY,B. J., ANDD. L. NEUDORF.1998. Female WHITTINGHAM,L. A., F?D. TAYLOR,AND R. J. ROB- control, breeding synchrony,and the evolution of ERTSON. 1992. Confidence of paternity and male extra-pair mating systems, p. 103-121. In P G. parental care. Am. Nat. 139:1115-1125.

The Condor 101:859-862 0 The Cooper Ornithological Society 1999

BROWN THRASHER NEST REUSE: A TIME SAVING RESOURCE, PROTECTION FROM SEARCH-STRATEGY PREDATORS, OR CUES FOR NEST-SITE SELECTION? ’

JOHNE CAVIT?, AARON I‘ PURSE AND TODDA. MILLER’ Division of Biology, Kansas State University, Manhattan, KS 66506

Abstract. We examined the potential functions of Key words: Brown Thrasher, nest-reuse, nest-site old nestsin a populationof Brown Thrashers(Toxosto- selection, old nests, Toxostomarufum. ma rufam) nesting on the Konza Prairie ResearchNat- ural Area in northeastern Kansas. We determined Considerablevariation exists in the longevity of open whether thrashersreuse nests constructedin previous cup nests built by passerines.Some nests deteriorate years, and tested predictionsof the hypothesisthat old during, and shortly after, a nesting attempt (Skutch nests function to reduce the risk of nest predation by 1976, Briskie and Sealy 1988), whereas others may saturatingthe cues used by search-strategypredators. last for several years (Watts 1987). The accumulation We also manipulatedold-nest densities to test the hy- of old nests on the territories of breeding birds has led pothesis that old nests are used as indirect cues for to the suppositionthat they may provide an adaptive nest-site selection. Thrashers were found to reuse function. Three hypothesesproposed for the function nests, albeit at low rates (4% of nests monitored). We of old nests include: (1) old nests may be reused and found no significant relationshipsbetween the density thus, provide a savings in time and energy to parents, of old nests and the successof active nests, and ex- (2) the accumulationof old nests may provide protec- perimentally removing nestsdid not influence nest-site tion from search-strategypredators (Watts 1987), and selection.These resultssuggest that old nestsmay only (3) old nests may function as an indirect cue for nest- benefit thrashers in this population as a resource to site selection (Erckmann et al. 1990). These hypothe- reduce the time spent in nest construction. ses are not mutually exclusive and may act in concert, dependingon the speciesand the local environment in which it breeds.Despite the potential adaptivefunction ‘Received 12 January 1999. Accepted 22 June of old nests, these hypotheseshave been largely un- 1999. tested.The reuseof nestsconstructed in previousyears 2Current address: Department of Zoology, Weber has been well documentedfor cavity breeders(Nilsson State University, 2505 University Circle, Ogden, UT 1984, Brawn and Balda 1988), and species that place 84408-2505. their nests on ledges (Skutch 1976). However, few 1 Current address:Schendel Services, 528 N. Wash- open nesting passerineshave been found to reuse old ington, Wichita, KS 67214. nests (Clark and Mason 1985) and only recently has