ANNUAL SURVIVAL RATES OF BREEDING ADULT ROSEATE TERNS JEFFREYA. SPENDELOWt'2 AND JAMESD. NICHOLSt •U.S.Fish and Wildlife Service, Patuxent Wildlife Research Center, Branch of MigratoryBird Research,Laurel, Maryland 20708 USA, and 2LittleHarbor Laboratory, Inc., 69 AndrewsRoad, Guilford, Connecticut 06437 USA ABSTRACr.--Analysesof the capture-recapturedata on 910 individual RoseateTerns (Sterna dougallii)trapped from 1978-1987as breeding adultson nestson Falkner Island, Connecticut, estimatethe average annual minimum adult survival rate to be 0.74-0.75. There was weak evidenceof year-to-yearvariation in annual survival ratesduring the study period. The Jolly- Sebermodels used to estimatesurvival ratesalso generatedestimates of population size and captureprobabilities. To determine the relative importanceof adult mortality and permanent emigration in contributing to the estimatedannual lossof one-fourth of the breeding pop- ulation will require further study of intercolony movement between all the major colony sites.Assuming that the loss of birds from the Falkner Island colony site is due mostly to mortality rather than permanent emigration, and that the survival rate of this breeding population is typical of the entire North Atlantic breeding population,then the survival rate of this endangeredspecies is low in comparisonto the survival ratesof severalother marine bird speciesin the orders Procellariiformes, Pelecaniformes,and Charadriiformes. Received 23 December1987, accepted18 January1989. ROSE•TET•N (Sternadougallii) breeding pop- STUDY AREA AND METHODS ulations in the western North Alantic have de- clined since the 1930s.It has been suggested Studysite and field techniques.--Descriptionsof the Falkner Island colony site, located in Long Island that the decline has resulted, in part, from in- Sound about 5 km south of Guilford, Connecticut, creasedmortality on the wintering grounds and someof the general methods used to study the (Nisbet 1980,1981a; Buckley and Buckley1981; Roseate and Common (S. hirundo) terns that nest on Gochfeld 1983; RoseateTern Recovery Team the islandwere given by Spendelow(1982). Starting 1988).Rates of changein the size of any pop- at the beginningof the CommonTern breedingsea- ulation are a function of survival rate, repro- son in early May, we searchedthe main RoseateTern ductiverate, and ratesof immigrationand em- nesting areas on the northern and southern ends of igration. Unfortunately, historicalinformation the islandfor new nestsat 1- to 3-dayintervals through is not available on most of theseaspects of Ro- early Augusteach year. Other partsof the islandwhere seate Tern population dynamics,so the direct the Roseatesnest infrequently or in small numbers were searchedfor new nestsat 3- to 5-day intervals. comparisonof current survival rateswith those Once found and properly identified, RoseateTern of earlier periods is not possible.Another ap- nestswere examineddaily (weather permitting) or at proach to the causesof the RoseateTern decline no greater than 3-day intervals until all eggs had is a comparative one in which characteristicsof either been lost or hatched. If possible,we deter- the tern population are comparedwith thoseof mined the initiation date of nests discovered after the other presumably"healthy" marine bird pop- beginningof incubationby back-dating23 daysfrom ulations.We analyzedone aspect of the capture- the known or estimated hatching date of the first recapturedata from a RoseateTern breeding chick. colony on Falkner Island, Connecticut.We es- During 1978-1980, most adult RoseateTerns were timated survival rates from these data and com- trapped on their nests within a few days before or after the expectedhatching dates.Often both adults pared them with estimates of survival rates of were trapped on the same day. Since 1981 we have other marine birds based on similar mark-and- tried to avoid trapping both membersof a pair on the recaptureor mark-and-resightstudies. We also sameday and, where possible,we waited until their used the capture-recapturedata to estimate chickswere severaldays old before trapping a pair breedingpopulation sizes and comparedthese of adultsto reducethe possibilityof investigator-in- with estimates based on nest count data. ducedmortality of eggsand young (seeNisbet 1981b). 367 The Auk 106:367-374. July 1989 368 $PENDELOWAND NICHOLS [Auk, Vol. 106 RoseateTerns that nested in the open, in vegetated less than the amount of intercolony movement that areas,under small boards,or inside tires (Spendelow occurred between the birds on Falkner and Tuxis is- 1982)were trappedin box-shapedor circularPotter- lands. Possibleeffects of the intercolony movement style treadle traps (for design,see figure 2.10 in Ca- on parameterestimates also are discussedbelow. nadian Wildlife Service and U.S. Fish and Wildlife Data analysis.--Survivalrates and populationsizes Service 1977). For situations where nestswere hidden were estimatedfrom the capture-recapturedata with under rocks,logs, or large boards,we constructedjust the Jolly-Seber(Jolly 1965,Seber 1965) and related the front 5-10 cm of a treadle trap. After observing reduced-parametermodels (Brownie et al. 1986)for from a nearbyblind how the adultsapproached these openpopulations. We baseddecisions about model nests,we setthe "fronts" in placeto allow easyaccess appropriatenesson goodness-of-fittests and testsbe- to the eggs,but blockedoff otherpossible avenues of tween models(Pollock et al. 1985,Brownie et al. 1986). escape. Computationsof all capture-recaptureestimates and Capturedadult ternswere bandedwith USFWSsize test statisticswere carried out using program JOLLY 2 bandsand released.If bandedpreviously, they were (Brownieet al. 1986).Reasons for preferring model- released after the old band numbers were recorded. basedcapture-recapture estimators (e.g. see North and Worn bandswere replacedas necessary. We usedalu- Morgan1985) to "returnrate" and otherenumeration minum bandson birdstrapped through 1986.In 1987 statisticsoften usedwith bird-banding data were dis- all RoseateTerns were given stainlesssteel (incoloy) cussedin detail by Nichols and Pollock (1983) and bands.It is doubtful that lossof aluminum or incoloy Loery and Nichols (1985). bandsfrom previouslytrapped adults was a problem Estimatesof populationsize alsowere madefrom during the 10-yrstudy period (Nisbet and Hatch 1983). nest count data. Common and Roseateterns usually Time and manpower constraints,as well as nest requirea minimum of 8-10 daysto renestafter a failures that occurred prior to the time a nest was clutch that has been incubated for a week or more considered"trappable" (i.e. after ca. 15 days of in- fails (DiCostanzo 1980, Spendelow 1982). We made cubation),prevented us from trappingadults from all theseestimates by countingthe cumulativetotal num- the RoseateTern nests on the island each year. We ber of nest initiationsby eachnest censusdate and concentratedour effortson thoseareas with the great- subtractingthe numberof nestfailures known to have est number of accessiblenests (i.e. those out in the occurred10 or more daysprior to that date.The re- open, in vegetatedareas, or insidetires). In the first suitingvalues were then plottedagainst time and a 3 yr of the study(Spendelow and Sibley unpubl. data), curve was fitted by eye to determine an approximate a low percentageof the presumablyolder, more ex- asymptoticmaximum. Under the assumptionthat not periencedbirds (those that made the earliestnests) all pairsrenest after a failureand to allow for the were trapped, which producedsome heterogeneity possibilityof a small amount of intercolonymove- in annualcapture probabilities. Possible effects of such mentalong with latenesting by youngerbirds (Hays heterogeneityon parameterestimates are discussed 1978,Spendelow 1982), we roundedthe asymptotic below. maximumup to the nexthighest 5-pair interval and During 1978-1982,perhaps as many as 50 pairsof arrived at a final populationsize estimate.Unlike RoseateTerns nestedwith a small colony of Common CommonTerns (Hays 1984,Wiggins et al. 1984),Ro- Terns about 6 km away from Falkner on Tuxis Island, seateTerns have not been observedraising a second off Madison, Connecticut. At least 3 of 5 adult Ro- brood in a single season,so the estimatesbased on seatestrapped on Tuxisin 1980were trappedon Falk- the nest count data are of the minimum number of her one or more times from 1980 to 1982 (Spendelow pairsnecessary to producethe totalnumber of nests unpubl. data).We presumesome movement also oc- initiated by a given censusdate. curred in the oppositedirection. The Tuxis colony site was deserted in 1983, when rats invaded the is- land, and few terns have nested there subsequently. RESULTS Other colony siteswhere one or more Roseatesare known to have bred before or after being trapped on Comparisonof models.--From1978 to 1987,we Falkner Island include Bird Island, Massachusetts caught910 different adult RoseateTerns at least (about 170 km away, and about 1,700pairs maximum once on Falkner Island. The capture-recapture sizeduring the studyperiod); Great Gull Island,New data for these birds are summarized in Table 1. York (45 km, 800 pairs);Cedar Beach,New York (90 The testof Model D (survivaland captureprob- km, 100pairs); Gardiner's Island, New York (50 km, abilities constant from year to year, Brownie et 75 pairs);Hicks Island, New York (55 km, 70 pairs); al. 1986) vs. Model A (survival and capture Southold,New York (25 km, 25 pairs),and Waterford Island, Connecticut (40 km, 10 pairs). Other