October2001] ShortCommunications 1069 TheAuk 118(4):1069-1072, 2001 Nest-attendersin the Pied Flycatcher(Ficedula hypoleuca) During Nestling Rearing: A PossibleCase of ProspectiveResource Exploration ULF OTTOSSON,• JOHAN B,•CKMAN, AND HENRIK G. SMITH Departmentof AnimalEcology, Lund University, S-223 62 Lund,Sweden ABSTRACT.--Visitsto nest holes by birds other kenmo 1984,Part and Gustafsson1989), previous in- than their owners is a familiar phenomenonfor stu- formation aboutsite quality might minimize time re- dentsof breedingbiology. In this study,we evaluate quired to find a good nest site. that behavior using a transponderreading system. Nest visitors,different from the breedingpair, are Eighty-fivemales and femaleswere fitted with tran- relatively commonat activenests of severalbird spe- spondersat the end of the incubationperiod or just cies, for example goldeneyes(Bucephala sp., Eadie after hatching.Nest boxeswere fitted with transpon- and Gauthier 1985), Tree Swallows (Tachycinetabi- der readers from just after hatching until all nes- color,Lombardo 1985, 1986, 1987; Stutchbury and tlings fledged. That systemrevealed 123 visits by Robertson 1987) and European Starlings (Sturnus birds to nestboxes other than their own, a visit being vulgaris,H. G. Smith pers. obs.). Normally, those at- defined as at least one visit to a separatenest box on tendantsdo not interact with the breeding birds. In a separateday. Males were more often detected at Tree Swallows, nest attendants visited several nest- other nests than females (53% of males vs. 29% of fe- boxes and were especially attracted to preferred malesvisited) and maleson averagemade more vis- breeding localities (Lombardo 1987). Sexually ma- its than females did (4.8 vs. 2.5 visits). However, both ture attendantsat nestsduring the early breedingpe- males and females devoted time to visiting other riod may be searchingfor possiblebreeding oppor- nestswhile still feeding nestlings.That behavioris tunities (e.g. Leffelaar and Robertson 1985, more commonthan previouslysuspected and is con- Stutchburyand Robertson1987), extra-pair fertiliza- sistentwith birds prospectingfor future nestsites or tions (Westneatet al. 1990), or opportunities to lay investigatingpatch reproductivesuccess. parasitic eggs (Rohwer and Freeman 1989). Later Individuals of many bird speciesreoccupy a fa- during the breeding cycle attendantsmay be indi- miliar area in successiveyears, but within a popu- viduals exploring future nest sites (c.f. Lombardo lation the extent of that breeding philopatry may 1987) or trying to interrupt the breeding attempt to vary between age and sex classes(Greenwood and get their own chance(c.f. Checkand Robertson1991, Harvey 1982).In many casesbirds, onceestablished, Hansson et al. 1997). continueto breed in the same territory throughout The Pied Flycatcher(Ficedula hypoleuca) is a small, their lives. Individuals that dispersebetween breed- migratory, monogamous,or facultatively polygy- ing seasonsmost often do that to obtain a territory nous bird breeding in tree holes and nest boxes of higher quality (e.g. Harvey et al. 1979,Bensch and (Lundbergand Alatalo 1992).A femaleproduces one Hasselquist 1991, Haas 1998). The decision to move clutchper seasonthat she incubatesalone, but par- can be based on the individual's current breeding ents divide nestlingfeeding approximatelyequally successor territory quality (Switzer 1997),but infor- (Alatalo et al. 1988). In this article, we describe the mation about quality of nearby territoriescould also exploratorybehavior of Pied Flycatchersduring nes- be used (Boulinier and Danchin 1997). tling rearing. A migrating bird may face additional difficulties. Methods.--Thestudy was carried out during the Upon arrival, the territory might be occupiedor its breedingseasons of 1995and 1997in smallwoodlots suitability drastically changed.For example,a sec- surrounded by agricultural land surrounding the ondary cavity nester may find its old nest hole de- Revinge area -20 km east of Lund in southernSwe- stroyed or occupied by a con- or heterospecificbird den (55ø41'N,13ø27'E). More than 200 equallysized (Lundbergand Alatalo 1992). If earlybreeding is ad- nest boxes were erected on trees -150 cm above the vantageous(Price et al. 1988), a migrant bird arriv- ground at irregular intervals,normally with a dis- ing at the breeding groundsshould be expectedto tance of >50 m between adjacentnest boxes. minimize time spentin searchof a nestingsite (Ala- Femaleswere trapped during the last days of the talo et al. 1982, Slagsvoid et al. 1988, Part 1995). incubation period or just after hatching, whereas However,because nest sites differ in quality (e.g.As- males were trapped just after hatching. All birds were banded with an aluminum band and fitted with a transponderglued to two color-bands.In 1995, 33 Present address: 18A, rue de Mamer, L-8280 Keh- adults (16 males and 17 females) and in 1997, 52 len, Luxemburg.E-mail: [email protected] adults (24 males and 28 females) from 20 and 28 nest 1070 ShortCommunications [Auk,Vol. 118 TABLE1. Number (percentage)of visitsto nestbox- ents in differentnest boxeswere made during the es other than the one a bird was breedingin by nestlingstage up to 10 days after their nestfailed. A male and female Pied Flycatchersbefore and after higher proportion of males (52.5%) than females the last recordingof the bird in its own nest.Suc- (28.9%) were detected at nests other than their own cessfulnests produced fledgling, whereas unsuc- cessful nests were either depredated or aban- (G-test, G = 5.99, P = 0.014). Each male detected at doned. another'snest also made a highernumber of detected individual visits (4.8/male to on average2.1 nest Before After boxes)than did females(2.5/female to on average 1.6 nestboxes; separate variance t-test, t = 2.13, df = Success- Unsuccess- Success- Unsuccess- ful ful ful ful 25.4, P = 0.043). After predation or abandonment, visitation rate at foreign nestsseemed to increasefor Females 8 (57.1) 6 (35.3) 9 (60.0) 9 (11.8) males, but not for females (Table 1). Males 6 (42.9) 11 (64.7) 6 (40.0) 67 (88.2) Discussion.--Thisstudy clearly showsthat adult Pied Flycatchersvisit nest boxesother than their own during the nestlingand postbreedingperiods. Even boxes,respectively, were fitted with transponders. while feedingnestlings, at leastsome breeding birds Beginningjust after hatchinguntil the last nestling devotetime to visitingnest boxes up to 700m away. had left the nest box, we attacheda transponder The phenomenon of nest attenders is well known readingsystem (Trovan • TrovanLtd., United King- (e.g. Eadie and Gauthier 1985, Lombardo 1985, dom) to a number of nest boxes (for details see Ot- Stutchbury and Robertson1987), but it is often as- tossonet al. 1998).The transpondersystem allowed sumedthat visitorsare youngor nonbreedingindi- us to record all visits by transponder-markedfly- viduals seekingbreeding opportunities (e.g. Lom- catchers to a nest box. Number of available readers bardo1987). This studydemonstrates that breeding was 12 in 1995and 20 in 1997.We fitted asmany nest individuals of both sexesregularly visit nestsof oth- boxes as possiblewith transponderreaders. Of er pairs. knownbreeding attempts in nestboxes in the study Nest visitation may lead to current or future fit- area,about half werefitted with transponderreaders nessbenefits. For females,intraspecific brood para- in 1995 and almost all in 1997. Because of technical sitism is a possible reason to visit other nest sites problemsand predation,number of days a nestbox (Yom-Tov et al. 2000). However, becauseall visits in was fitted with a workingtransponder reader varied this study occurredwhen the visitednests contained from a few h up to 16 days. nestlings,that is highly unlikely.For males, pursuit Oftena visitby a transponder-markedflycatcher to of extrapair copulationsis a possiblefitness benefit a nest-boxincluded multiple readingswith shortin- (Alatalo et al. 1984,Lifjeld et al. 1991).However, be- tervals. In this paper, we defined a "visit" as read- causefemales at visited nestswere postfertile,that ings at separatenest boxes or on separatedays. seemsunlikely. Birds could be prospectingfor alter- nativebreeding sites in casethe current reproductive Results.--Eighty-fiveadults were recorded making 132 different visits to nest boxes other than their attemptfailed. However,we did not detectany bird own. Note that all visits were made to nest boxes producinga replacementbrood after failureduring the nestling period. Furthermore,that fitnessbenefit containingnestlings. Only in onecase was it possible cannotexplain why birds visited nestsduring the that the male wasvisiting his secondaryfemale's nest periodthey reared dependent fledglings, because the box. Singleindividuals were detectedat up to four probabilitya replacementclutch will be produced different nest boxes other than their own, and indi- then is extremelylow (Lundbergand Alatalo 1992). vidual nestboxes were visitedby up to sevendiffer- However,our resultsare compatiblewith birdspros- ent individuals other than the breedingpair. Visits pecting for future nest sites (Eadie and Gauthier during nestling rearing occurred during feeding 1985) or investigatingpatch reproductivesuccess pausesthat normally were •1 h long (range 9-101 (Doligez et al. 1999). min). Distance between the box in which a bird was In the Pied Flycatcher,rapid establishmentat a breedingand the oneit visitedranged from 60 m to nest site after arrival in spring confersa fitnessad- almost3