Pre-Laying Nest Roosting in the Eastern Phoebe: an Energy-Conserving Behavior?

J. Field Ornithol., 65(1)'52-57

PRE-LAYING NEST ROOSTING IN THE EASTERN PHOEBE: AN ENERGY-CONSERVING BEHAVIOR?

HARMON P. WEEKS,JR. Departmentof Forestryand Natural Resources Purdue University WestLafayette, Indiana 47907 USA

Abstract.--Nestsof EasternPhoebes (Sayornzs phoebe) were examinednocturnally for on- nestroosting during 8 yr on Crane Naval WeaponsSupport Center, Martin Co., Indiana. In early-seasonnests, females invariably roosted in the nestcup from the time the cup was well formed,sometimes several weeks before egg laying. Later in the season,females be- ginning secondattempts exhibited this behaviorin only 56% of nestsobserved. It is hy- pothesizedthat the behavior is an adaptationthat allows energy conservationduring the cool nights experiencedby this early-seasonnester.

PERNOCTACI(•N EN SUS NIDOS, PREVIO A LA PUESTA, POR PARTE DE SAYORNISPHOEBE: LUNA CONDUCTA PARA AHORRAR ENERGIA? Sinopsis.--En un estudioque se 11ev6a cabopot ochoaftos en la compafi[aCrane Naval Weapons Supoft Center, de Indiana, se examinaron nidos de Sayornisphoebe durante la nochepara determinarsi &stosestaban siendo utilizados para pernoctar.A1 principiode la temporadareproductiva, las hembrasdurmieron en los nidos (que son en forma de copa), una vez &stosestaban bien formados.En ocasioneslos nidosrueton utilizadospara dotmir varias semanasantes de la puestadel primer huevo.No obstantepara el final de la temporada, las hembras que comenzabanun segundointento de anidamiento, exhibieron este tipo de conductaen tan s01oel 56% de las ocasiones.Se postulaque estaconducta es una adaptaci6n que permiteahorrar energiadurante las nochesfr•as, ya que estaave comienzaa anidar muy tempranocuando las temperatufasen ocasionesbajan del punto de congelaci6n.

The Eastern Phoebe(Sayornis phoebe) makes considerableuse of hu- man structures,especially bridges and culverts,for nest sites. Nests are thus protectedfrom weather and quite durable, often being used with minor modificationsfor severalyears in succession(Weeks 1979). Phoebes are double-broodedand begin constructionor renovationof first nestsin late March in southernIndiana. Work on early nestsis at a casualpace, whereas between broods, nests are renovated or built very rapidly. In either case,however, seemingly completed nests stand empty for several daysbefore clutch initiation (Klaas 1970, Weeks1978). Phoebesare rarely observeddiurnally at nestsduring this hiatus. Mueller et al. (1982) observeda femaleEastern Phoebe begin nocturnal roostingin the nest cup 3 d prior to the laying of the first egg. Roosting in the nestby cavity nestersis not uncommon,but pre-incubationroosting by a passerine in an open, cup-shaped nest had not been found (e.g., Brackbill 1985, Nolan 1978) before the observation of Mueller et al. (1982). They suggestedthat phoeberoosting habits be examined further beforetheir observationbe brandedunusual; I undertookthis investigation to verify the occurrenceand scopeof this behavior.

52 Vol.65, No 1 NestRoosting zn Eastern Phoebe [53

METHODS The studyarea was Crane Naval WeaponsSupport Center (CNWSC), a 252 km 2 area in south-central Indiana (38ø50'N, 86ø50'W). CNWSC has many bridges,culverts, and natural rock outcropsthat are used as nest sitesby Eastern Phoebes.I conductednocturnal inspectionsof in- dividualnests of known statusand occasionallygroups of nestsknown to be in similar stagesphenologically. I useda flashlightto illuminate nests from a sufficientdistance to avoid flushing birds that might have been present.I knew the completehistory of each nest examined. I surveyedgroups of nestsin 1983 and 1989 beforefirst clutchinitiation; in 1986 and 1988 I examinednests after first broodshad fledgedand prior to secondclutch initiation. In addition,I periodicallymade nocturnal examinationsof over 100 individual nestsfrom 1983 through 1990. Dif- ferencesin occurrenceof roostingin the nest cup prior to early season and late seasonclutches were examinedwith a x 2 2 x 2 contingencytable analysis.

RESULTS In early spring,before renovation began, nests had no cupsbecause of compactionfrom the previousyear's broods,and birds roostedperched on the nest rim. Once renovationbegan during the early season(20 March-20 April), femalesalways roosted in the nestcup oncea cup with some lining was present. In this season,nocturnal low temperatures averaged3.6 C (range -7.3-16.6 C) and completenests often remained egglessfor extendedperiods. Roosting in the nest cup regularly com- menced10-14 d beforeegg laying and sometimes>20 d (Table 1). The rapidity of nestrenovation or constructionbetween successful first and secondattempts limits time availablefor on-nestroosting. In a normal year (1989), the time betweenthe fledgingof the first broodand the first egg of a secondclutch in the same nest averaged10.9 d (n = 38; range 6-18 d). The roostingpattern during this perioddiffered significantly (x 2 = 14.01, df = 1, P < 0.001) from that of early nests,with only 56% of nestswith equivalentcups being used for on-nest roosting(Table 1). Mean low temperatureduring this period (1 May-1 June) was 12.3 C (range 0.5-20.5 C). In all casesand for all broods,once egg laying beganthe femaleroosted on the nest every night. Diurnal incubationusually beganwith the last eggbut did not reachfull intensityfor a few days;the patterndescribed by Mueller et al. (1982) appearstypical (long periodsof diurnal inat- tentivenessduring first 2 d, little thereafter).

DISCUSSION The EasternPhoebe seems unique amongopen-nesting passerines in the useof nestsfor roostingprior to egglaying (Brackbill 1985, Nolan 1978). The phoebeis, however,a structurenester and thus may be thought of as intermediatebetween ground/vegetation nesters and hole nesters. 54] H.P. Weeks,Jr. j. FieldWinterOrnithol. 1994 Vol.65, No I NestRoostzng •n Eastertl Phoebe [55

My resultsprovide someinsight as to the origin of this behavior and its adaptive function. At least three possibilitiesexist: (1) nest-sitedefense againstconspecifics, (2) assessingsecurity of sitefrom predation(Mueller et al. 1982), and (3) energysavings (Mueller et al. 1982). Competition for nest sites undoubtedlyexists in the Eastern Phoebe, as suitable sitesare not uniformly distributed. On CNWSC, occupancy of suitablestructures is almost 100%, and platformserected in previously unusable pipe culvertswere occupiedimmediately (29 of 30) with no abandonmentof adjacenttraditional sites(Weeks 1984). Territorial be- havior restrictsstructure occupancyto a single pair. Even though com- petitionis keen,it is difficultto surmisewhy on-nestroosting would better establishoccupancy than roostingon the nestrim or at someother location on the structure. I doubt that the primary function of pre-laying roostingis assessing securityfrom nocturnal predators.Birds (principally females)roost at the nest continuouslyfrom the time territorial occupancyis established, on-rim early and then shifting to an on-nest position. It is difficult to believethat one positionwould be more effectivein monitoring than the other. Furthermore, in the almost total darkness within a culvert or beneath a bridge, monitoring predator activity would be virtually im- possible.Additionally, I have observedthat >98% of all neststhat are initiated receiveeggs unless destroyed by predators,suggesting that aban- donmentsdue to proximity of predatorsrarely occur. The final possibility,that on-nestroosting imparts an energysavings due to the insulative value of the nest, seemsmore plausible. Eastern Phoebesare very early nestersand nocturnaltemperatures in late March occasionallyfall below 0 C. As phoebesbegin breeding before insect abundanceincreases, it is possiblethat femalesare food (energy) limited during this period. Breeding is linked to prevailing weather conditions, principally temperature,and beginssubstantially earlier in warm springs (M. T. Murphy, pets. comm.;Klaas 1970). M. T. Murphy (pets. comm.) alsofound larger mean eggsize in a warm springthan in normal springs and a sequentialincrease in intraclutch eggsize in normal springs,which he concludedresulted from increasesin food availability over the laying period.Weeks (1978) foundthat early seasonclutch sizes were larger for birds reusing old as opposedto constructingnew nests,suggesting par- titioning of limited energy supplies.Additionally, there is evidencethat the hiatusbetween nest completion and laying (Klaas 1970, Weeks 1978) is related to the energy drain associatedwith nest constructionand cool environmentaltemperatures. It might be surmisedthat other early nesting passetinesshould have developedthis trait as well. The Eastern Phoebe is the earliest breeding, open-nestingpassefine in this region, with the possibleexception of the American Crow (Corvusbrachyryncos). Eastern Phoebenest-building and clutch initiation usually predatesthat of the early nesting Blue Jay (Cyanocittacristata), American Robin (Turdus migralorius)and Northern Cardinal (Cardinaliscardinalis) by 2-3 wk. Additionally, its habit of frequently reusing old nestsand building in 56] H.P. Weeks,Jr. J.Field Ornithol. Winter 1994 shelteredlocations maximizes the potential of developmentand fixation of this trait in the EasternPhoebe. Roosting in the nestcup oncethe first egg is presentis requisite to prevent freezing of eggsduring this period of vacillating temperatures. Nestsgenerally are concededto functionto insulateeggs and young as well as simply servingas a receptacle(Collias and Collias 1984). Nolan (1978) for Prairie Warblers (Dendroicadiscolor) and Ramsey(1987) for Carolina Wrens (Thryothorusludovicianus) have documentedan inverse relationshipbetween seasonal temperatures and nest size. Walsberg and King (1978) calculatedthat the heat expenditureof an incubatingfemale White-crowned Sparrow (Zonotrichialeucophrys) was 15% lower than for a bird perchedoutside the nest;a savingsof this magnitudecould be critical for small birds. Mueller et al. (1982) mentionedsuch an energy savingsas being an explanation of on-nest roosting, but dismissedit becausetemperatures were unseasonablywarm during that incident.Viewed in the contextof a universal early-seasonbehavior rather than an isolated occurrence, however, it seems that selective forces have fixed this behavior in the population to such a degree that transient deviationsfrom the normal cool night temperaturesof early spring do not modify behavior. The reduceddegree to which femalesroost on the nestbetween first and second clutchesmay simply be an artifact of extendedcare of recently fledged young.The relatively warm nightsof late May, however,coupled with an abundantarthropod biomass (Robins 1970), shouldgreatly reduce the energyconstraints under which female EasternPhoebes operate. As nests are occasionallydestroyed by nocturnal predators,the behavior would becomerealadaptive once the energeticadvantages are assuaged.

ACKNOWLEDGMENTS

I would like to thank the personnelof CNWSC, especially Lynn Andrews, for their cooperationon this project.I am grateful to R. K. Swihart, P. J. DuBowy, E. E. Klaas, I. G. Warkentin and two anonymousreferees for their thoughtful commentson earlier drafts of this paper. This is Journal Paper No. 13153 from the Purdue Agricultural Experiment Station.

LITERATURE CITED

BRACKBILL,H. 1985. Initiation of nest-roostingby passerineswith open nests.J. Field Ornithol. 56:71. COLLIAS,N. E., AND E. C. COLLIAS. 1984. Nest building and bird behavior. Princeton Univ. Press,Princeton, New Jersey.336 pp. KL^^S, E. E. 1970. A population study of the eastern phoebeand its socialrelationship with the brown-headed cowbird. Ph.D. thesis. Univ. Kansas, Lawrence, Kansas. MUELLER, H. C., N. S. MUELLER, AND K. D. MEYER. 1982. Unusual nest attentivehess of an Eastern Phoebe.J. Field Ornithol. 53:421-422. NOL^N,V., Je,. 1978. The ecologyand behaviorof the Prairie Warbler, Dendroicadiscolor. AOU, Ornithol. Monogr. 26. 595 pp. RAMSEY,A. O. 1987. Thermal adaptationsof the Carolina Wren. N. Am. Bird Bander 12:64. ROBINS,J. D. 1970. The relationshipof food supply to the timing of breedingin aerial foragers. Kansas Ornithol. Soc. Bull. 21:9-15. Vol.65, No. 1 NestRoostzng znEastern Phoebe

WALSBERG,G. E., ANDj. R. KING. 1978ß The heat budgetof incubatingmountain white- crownedsparrows (Zonotrichza le•cophrys orzantha) in Oregon. Physiol. Zool. 51:92- 103. WEEKS,n. P., JR. 1978. Clutch sizevariation in the Eastern Phoebein southernIndiana. Auk 95:656-666. 1979. Nesting ecologyof the Eastern Phoebein southernIndiana. Wilson Bull. 91:•41-454. ß 1984. Importanceand managementof riparian bridgesand culvertsfor nesting passetines.Pp. 163-175, in W. C. McComb, ed. Trans. Workshop Mgmt. Non-game Speciesand Ecol. Comm., Lexington, Kentucky. Received13 Nov. 1992; accepted1 Mar. 1993.