The Auk 109(3):585-593, 1992

PREDATION ON THE AND NESTLINGS OF SCRUB JAYS

RON SCHAUB,1'3 RONALD L. MUMME, 2 AND GLEN E. WOOLFENDENl •Departmentof Biology,University of SouthFlorida, Tampa, Florida 33620, USA; and 2Departmentof Biology,Allegheny College, Meadville, Pennsylvania 16335, USA

AI3STRACT.--Weinvestigated nest predation in a populationof Florida ScrubJays (Apheloco- mac. coerulescens)at Archbold BiologicalStation, Lake Placid,Florida. Frequencyof nest visits by investigatorsdid not influence predation rates.Nest predation increasedas the breeding seasonprogressed and occurredmost often during daylight hours. When seasonaleffects were held constant,nestlings were depredated more often than eggs,and young nestlings were depredatedmore often than old nestlings.Several lines of evidenceindicated that, for Florida ScrubJays, diurnal snakesand were the most importantnest predators,while nocturnalmammals were relatively lessimportant. Late in the breeding season,pairs with helpers experiencedless nest predation than pairs without helpers.This effectwas primarily the result of reduced predation on nestlings.Received 17 June1991, accepted 13 January1992.

NESTpredation is a primary sourceof and Jays exhibit a cooperative nestling mortality in many (Martin in breeding systemin which about one-half of all press),and is increasingly recognized as a sig- breeding pairs are assisted by nonbreeding nificantprocess shaping avian life-historychar- adults (helpers). For pairs with helpers the modal acteristics, selection, and community number of helpers is 1, the mean is 2, and the structure(e.g. Slagsvoid1982, Martin 1988a,b, maximum is 6 (Woolfenden and Fitzpatrick in press). Patterns of nest predation also may 1990).Helpers assistin feeding young, defend- interactwith habitat fragmentationin waysthat ing the territory, guarding the nest and mob- have important implications for conservation bing predators.They do not build nests,incu- biology(e.g. Loiselle and Hoppes1983, Wilcove bate,or brood.Although breeding pairs assisted 1985). However, despite the growing recogni- by two or more helpers are no more successful tion of the importance of nest predation, our than pairs with only a single helper, unassisted understandingof the ecologicalfactors that in- pairsproduce significantly fewer offspringthan fluence the behavior of specific nest predators pairswith helpers(Woolfenden and Fitzpatrick and rates of nest predation in natural popula- 1984, Mumme in press). The major way that tions remains limited (Martin 1987, in press). helpers appear to increasereproductive success In this paper we examine the ecologicaland is by reducing predation on eggs, nestlings social factors that may affect predation on eggs (Woolfenden1978) and fledglings(McGowan and nestlingsof the FloridaScrub (Aphelo- and Woolfenden 1989). Helpers could reduce comac. coerulescens).This jay is restrictedto pen- diurnal predation by serving as sentinels insularFlorida where it inhabitsrecently burned (McGowan and Woolfenden 1989), or by mob- scrub (Woolfenden and Fitzpatrick 1984). bing potential predators(Francis et al. 1989),or Nests are typically positionedabout 1 m above both. However, helpers would be ineffective at the ground in oak shrubs(Woolfenden 1974). reducing the frequency of nocturnal predation Nesting is from March through June. Most (Woolfenden and Fitzpatrick 1984). clutchesare of three or four eggs,rarely two or Basedon data collected over a 10-year period five. Incubation begins after the last or penul- (1969-1979),Woo lfenden and Fitzpatrick(1984) timate egg is laid. Eggs hatch after approxi- concludedthat predation is the primary cause mately 18 days of incubation, and nestlings of nestfailure in the Florida ScrubJay, account- fledge about 18 days after hatching (Woolfen- ing for 67%of egg lossand 85%of nestlingloss. den and Fitzpatrick 1984). Thus, an evaluationof factorsinfluencing nest predationis criticalfor a thoroughunderstand- ing of the breeding biology of this species.In 3present address: Bionetics Corporation, B10-2, this paper, we provide such an evaluation by Kennedy SpaceCenter, Florida 32899,USA. focusingon the following questions:(1) Does 585 586 sc.^u•, MUMME, AND WOOLFEIqDEN [Auk, Vol. 109 the frequencyat which investigatorsvisit nests in an activityindex value of 0.5. If rain obscuredthe influence predation rates?(2) What are the ef- tracks(21 of 100mornings and 11 of 91 evenings),no fectsof habitat and proximity to vehicle trails? censuswas made. Sightings of potential avian nest (3) How is the frequencyof nestpredation in- predatorsalso were recordedand usedto determine fluencedby seasonand neststage? (4) Doesnest their activity rates.The rate of avian predatoractivity was calculatedas number of visual sightingsper hour predation in the Florida Scrub Jay occur pri- of field time within the study tract. The activity rates marily during the day or the night?(5) Which of terrestrialand avian predatorswere combinedinto predatorsare primarily responsible?(6) How a single index, which was calculatedas the number doesthe presenceof nonbreedinghelpers affect of observations(tracks plus sightings of birds) per the rate of nest predation?In addressingthese observationhour (trackinghours plus field hours). questions,our study extendsearlier analyses Datafrom 552nesting attempts from the years1974- (e.g.Woolfenden and Fitzpatrick1984) and pro- 1979 and 1981-1987 combined were used to calculate vides several revised interpretations of previ- predationrates. The years1969-1973 were excluded because historical information about the breeders was ously publishedwork on this species. sparse,and the year1980 was excluded because of the socialdisruption caused by a majordie-off in fall 1979 METHODS (Woolfenden and Fitzpatrick 1991). The number of fledglingsproduced per groupwithin a breedingsea- We worked on markedjays that resideon the prop- sonwas usedto make between-yearcomparisons. For erty of the ArchboldBiological Station (ABS), a 2,000- ha research station located 12 km south of Lake Placid, this measurementof nestingsuccess the years 1974- 1987,excluding 1980, were not significantlydifferent HighlandsCounty, Florida. Since 1969, Woolfenden (Kruskal-Wallis, H = 17.33, df = 12, P = 0.14). We and colleagueshave monitoreda subsetof the total deletedfrom our analysesthe few neststhat failed to populationof Florida ScrubJays resident at ABS.Up reach the incubationstage, or that experiencedloss to 50 groupsof ScrubJays are censusedevery month or injury of a breeder. and all nestsare found eachyear. Almostall nestsare Daily predationrates were calculated as the number found before or very early in incubation.Through of apparentacts of predationper numberof daysa theseefforts the age, sex,and breedinghistories of nest contained either eggs or nestlings. Lossesthat many individuals are known. occurred at an unknown time between nest visits were In 1987, Schaub and Mumme conducted detailed consideredas having occurredmidway between vis- observationson 76 nestswith contentsowned by 49 its. groupslocated in open oak scrubimmediately south Calculationof daily ratesof predationare basedon of the groupsmonitored by Woolfendenand col- three differentways of recognizingapparent nest pre- leagues.Most nests(n = 60) were found beforein- dation: (1) Individualacts--losses of all or part of a cubationbegan, and all nestswere monitoreduntil clutch or brood between consecutive nest visits that they either fledgedyoung or failed.Schaub visited cannot be attributed to a causeother than predation. somenests twice daily (n = 41) at sunrise(+30 min) and sunset(+ 30 min) and othersapproximately every (2) Ultimatefailures--nests that eventuallylose all eggs or nestlingsas a resultof one or moreacts of apparent third day (n = 35). Actsof predationthat occurred between the sunrise and sunset nest checks are con- predation.(3) Instantaneousfailures--nests that fail as sidered diurnal, and those that occurred between the a result of lossof all eggsor nestlingsbetween con- sunset and sunrise nest checks are considered noc- secufivenest inspections. Predation rates derived from turnal. ratesof individual actsof predation thus provide the most liberal estimate of nest predation, while esti- To determine seasonaland circadianactivity rates mates derived from the rate of instantaneous failure of the snakesand mammalsthat are the probable nest provide the mostconservative estimate. For Florida predators,Schaub censused vertebrate tracks during ScrubJays, we think the mostliberal method provides mostof the 1987nesting season. Tracks on a strip of the most accurate measure. Starvation of nestlings sand1.1 km long and 1 m wide, which extendedalong and subsequentbrood reduction is a rare event for a broad firelane that coursedthrough the study site, FloridaScrub Jays (Woolfenden and Fitzpatrick1984). were censusedtwice daily immediatelyafter the sun- In the few casesobserved during 1987, one member rise nest checksand immediately before the sunset nest checks. Tracks made between the sunrise and of a brood becamevisibly smaller then its siblings sunset censuses are considered diurnal, and those made early in the nestlingperiod, usuallyby day 3. Most between the sunset and sunrise censuses are consid- of these runts survived to day 12. Therefore, we sus- ered nocturnal. Terrestrial-predatoractivity is calcu- pect that brood reductionthat is the direct resultof lated as the number of tracks per number of hours starvation rarely goes undetectedin Florida Scrub since the last census.For example, a censusshowing Jays.The existenceof long-enduringrunts suggests tracksof five potential predators,which were known that most instancesof partial brood loss do in fact to have beenmade during the preceding10 h, results representinstances of partial predation,which sup- July 1992] FloridaScrub Jay Nest Predation 587

T^BEEI. Effectof habitattype on ratesof nestpredation, as measured by bothdaily ratesof individualacts and overall ultimate nest failures, 1974-1979 and 1981-1987 combined.

Habitat type Open scrub Overgrown scrub Pasture Total nest-days 13,193 2,113 731 Individual actsof predation 244 44 19 Predation rate 0.0185 0.0208 0.0260 G = 2.25, n = 16,037, P > 0.25 Neststhat producedyoung 258 40 12 Nestsfailing becauseof predation 112 27 14 Predation rate 0.3027 0.4030 0.5385 G = 7.59, n = 463, P < 0.025

ports our incorporating partial lossesinto estimates amined: recently burned open scrub (burned of nest predationrates. within last20 years),unburned overgrown scrub The time period within the nestingseason, expe- (not burned for more than 20 years),and cattle rience of the breeders,and age of the breedersall pasture with scattered oak shrubs, palmetto affectnesting success in FloridaScrub Jays (Woolfen- den and Fitzpatrick1984). To controlfor the effectof clumpsand tall pines. Although the daily rate season,we dividedthe nestingseason into half-month of nest predation (individual acts) is similar for (15-16 day) and month (30-31 day) intervals.To re- all three , the rate at which nests ulti- ducethe effectsof breederinexperience and senes- mately failed becauseof predation differs sig- cence,in analysesof helper contributionswe deleted nificantly among habitats,with open scrubex- all first breeders and those few breeders older than hibiting the lowest rate (Table 1). 11 years. We analyzed the effectsof proximity to ve- Becausesample sizes and varianceswere unequal, hicle trails on nest predationby dividing nests nonparametricstatistics were used to analyze daily into two categories:those within 15 m of a ve- ratesof predationand predatoractivity. Categorical hicle trail, and thosefarther away. We consider datawere analyzed via contingencytables using log- likelihood ratio tests (G-test) with Williams' correc- it unlikely that a potential predator searching tion (Sokaland Rohlf 1981),or chi-squaretests with for prey from these trails would detect a nest Yates' correction for continuity (Zar 1984). Some of beyond 15 m into the scrub. Analysis shows the following analysesuse individual nests rather proximity to trails had no effect on nest pre- than nestdays as units in order to more closelymeet dation. Nests within 15 m of vehicle trails ex- the assumptionof independence. perienced 104 individual acts of predation in 6,137 nest-days(0.017 acts/day), comparedto RESULTS 183individual actsin 9,372nest-days (0.019 acts/ day; G = 1.23, P > 0.25). Over the entire breed- Effectsof investigatorvisitation.---Rates of nest ing cycle 54 of 172 nests (31.4%) within 15 m predationin 1987were not significantlyinflu- of a trail ultimately failed becauseof predation, encedby the two different nest-visitationtreat- compared to a similar 85 of 271 (31.4%) of the ments.Nests visited by investigatorstwice daily more distant nests (G = 0.00, P > 0.9). experienced32 individual actsof apparentpre- Effectsof season,nest stage, and timeof day.- dationin 1,155nest-days (0.028 acts/day), com- All three methodsof calculatingdaily ratesin- paredto 22individual acts in 979nest-days (0.022 dicatethat nest predation increasedas the sea- acts/day)for nestsvisited approximately every sonprogressed (Fig. 1). Daily ratesof predation, three days (G = 0.37, P > 0.9). Similarly, 19 calculatedby the number of individual acts,the (46.3%)of the 41 nestsvisited twice daily ulti- number of ultimate failures, and the number of mately failed due to predation,compared to 17 instantaneous failures were all significantly (48.6%)of the 35 nestsvisited every three days correlatedwith the advanceof the breedingsea- (G = 0.38, P > 0.9). son (rs = 0.96, 1.00, and 0.96, respectively;n = Effectsof habitatand proximity to vehicletrails.- 7, P < 0.05 for all). In examining the effects of habitat on the rate Predatorstook relatively more nestlingsthan of nestpredation, three majorhabitats were ex- eggs(Fig. 2). The overall daily rate of egg pre- 588 $CHAUB,MUMME, AND WOOLFENDEN [Auk,Vol. 109

0.01 0.20 [] INDIVIDUAL ACTS [] YOUNG NESTLINGS [] ULTIMATE FAILURES [] OLD NESTLINGS "• 0.0, [] INSTANTANEOUS FAILURES rr' 0.15 Z

<:•0.03 0.10 n' 0.02

0.05 • O.Ol

DATA...... 0.00 0.00 1 2 3 4 5 6 7 1 2 3 4 5 6 7 PERIOD WITHIN NESTING SEASON PERIOD WITHIN NESTING SEASON Fig. 1. Daily predation rates on nests of Florida Fig. 3. Daily predation rates (individual acts) on ScrubJays calculated three different ways (1974-1979 young (day 8 or younger) and old (day 9 or older) and 1981-1987combined). Half-month periodswith- nestlingsof Florida ScrubJays (1974-1979 and 1981- in nestingseason are: (1) 9-24 March, (2) 25 March- 1987 combined). Asterisks indicate a log-likelihood 8 April, (3) 9-23 April, (4) 24 April-8 May, (5) 9-23 ratio P < 0.05. Half-month periods within nesting May, (6) 24 May-7 June, and (7) 8-23 June. seasonas in Figure 1. dation of 0.0072 (66 individual acts in 9,204 nest (Fig. 3). The overall daily rate of predation on days) was significantly lower than the overall the younger nestlings (0.0404, 115 individual daily rate of nestling predation of 0.0285 (212 actsin 2,847 nestdays) was significantlygreater individual actsin 7,428 nest days;G = 117.06, than for older nestlings (0.0214, 40 individual P < 0.001). Furthermore, the rate of predation acts in 1,869 nest days; G = 13.53, P < 0.001). on nestlingswas higher than that on eggsdur- Predation rates on younger nestlings consis- ing six of the seven half-month periods of the tently exceededthat on older nestlingsduring nesting season(Wilcoxon T = 1, n = 7, P = all six half-month periods for which data were 0.028). Similar resultswere obtained in analyses available (Wilcoxon T = 0, n = 6, P = 0.032) and controlling for age and experience of breeders were significantfor two periods(log-likelihood (Schaub 1990). ratio P < 0.05). Youngnestlings (day 8 or younger)were taken Dawn and dusk nest checks allowed 32 in- by predators more often than older nestlings dividual acts of predation at 41 nests to be cat- egorized as either diurnal (n = 23) or nocturnal (n = 9). Thus, diurnal predation occurred more 0'06r F"]EGGS ] than twice as often as nocturnal predation, and the difference is significant (X 2 = 5.28, df = 1, E' P < 0.025). Becausesome actsof predation cat- (• 0.04 egorized as nocturnal may have occurredin the dim light shortlybefore the early morning nest '• 0.03 , checksor after the evening nestchecks, the true frequency of diurnal predation may have been E0.02 even higher. Activityof potentialnest predators.--Track cen- •__•0.01 susesrevealed that most of the potential mam- mal nest predators were active at night, and 0.00 1 2 3 4 5 6 7 mostof the potential snakenest predatorswere PERIOD WITHIN NESTING SEASON active during the day. In 1987, 1,814 of 1,846 (98.3%) mammal tracks were made between the Fig. 2. Daily predation rates (individual acts)on eggs and nestlings of Florida Scrub Jays(1974-1979 evening and morning track censuses,compared and 1981-1987combined). Asterisks indicate a log- to just 31 of 544 (5.7%) tracks(G = 1,996, likelihood ratio P < 0.05. Half-month periodswithin P < 0.0001). Furthermore, some of the few snake nesting seasonas in Figure 1. trackscounted during the morning censusesmay July 1992] FloridaScrub Jay Nest Predation 589

TAI3Lœ2. Ratesof individualacts of predation(1974-1979 and 1981-1987combined) and ratesof potential nest-predatoractivity (1987).

Predator activity• Diurnal Acts of All Nocturnal Diurnal Diurnal and Half-month periods predationa predators mammals birds snakes birds 9-24 March 2/870 259/565 196/263 32/223 23/79 55/302 0.0023 0.46 0.75 0.15 0.29 0.18 25 March-8 April 30/3,556 280/555 175 •213 31/225 67/117 98/342 0.0084 0.50 0.82 0.14 0.57 0.29 9-23 April 85/4,951 630/584 464 •202 62/224 95/158 157/382 0.0172 1.08 2.30 0.28 0.60 0.41 24 April-8 May 87/3,587 458/559 292 • 194 76/200 84/163 160/363 0.0243 0.82 1.51 0.38 0.52 0.44 9-23 May 48/2,067 339/418 222 •141 35/176 80/101 115/277 0.0232 0.81 1.57 0.20 0.79 0.42 24 May-7 June 44/1,312 474/434 309 •152 36/132 93/150 129/282 0.0335 1.09 2.03 0.27 0.62 0.46 8-23 June 21/439 242/245 136/81 20/78 71/85 91/163 0.0478 0.99 1.68 0.26 0.84 0.56 rswith period in season 0.68 0.61 0.39 0.86 0.96 P > 0.05 > 0.05 > 0.10 < 0.025 = 0.0025 rs with predation rate 0.71 0.57 0.54 0.71 1.00 P = 0.05 > 0.10 > 0.10 = 0.05 <0.005

Individual actsof predationper numberof nest-days. Formammals and snakestracks/tracking hour; for birdasightings/field hour, and ratesfor both. have been made during the brief periods of helpers had little effect on rates of individual light beforethe morning censusesor, morelike- actsof nestpredation (Fig. 5). However,during ly, after the evening censuses.Most of the mam- the final month of the breeding season,pairs mal tracks(>80%) were made by raccoons(Pro- withouthelpers experienced significantly high- cyonlotor). er ratesof nestpredation than pairswith help- When all track counts and avian predator ers (Fig. 5A). This apparenteffect of helperson sightings were combined to form an index of late-seasonnest predation was attributable to a the activity of all potentially important nest significantlyreduced rate of predation on nests predators, no significant correlation existswith with nestlings,but not nestswith eggs(Figs. the advanceof the nesting season,but one does 5B and 5C). existwith the rate of nestpredation as measured with individual acts (Table 2). Neither an index 0.06 of nocturnal mammal predator activity nor an rs = 1.00 P: 0.001 index of diurnal avian predator activity is sig- I-- 0.05 nificantly correlatedwith the advanceof the nesting seasonor the rate of individual actsof z 0.04 nest predation. Diurnal snake activity, how- ever, is significantly and positively correlated 0.03 with both the advanceof the nesting season ill and with the rate of nest predation (individual 0.02 acts). Becausemost nest predation is diurnal, ._l the activities of diurnal snakes and birds are 0.01 combined into a single index of potential di- 0.00 urnal predatoractivity. The activity rate of di- 0.1 0.3 0.4 0.5 0.6 0.7 urnal snakes and birds was significantly cor- INDEX OF DIURNAL PREDATOR ACTIVITY related with the advanceof the nesting season Fig. 4. Relationshipbetween an index of diurnal and was perfectly correlatedwith the rate of predatoractivity and daily predationrates (individual individual actsof nest predation (Fig. 4). acts)during seven half-month periods of nestingsea- Effectsof helpers.--In general, the presenceof son shown in Table 2. 590 SCHAVB,MUMME, AND WOOLFENDEN [Auk, Vol. 109

A. NESTS WITH EGGS OR NESTLINGS implicated severalspecies. One snake,the east- 0.12 ern coachwhip (Masticophisfiagellum) is known [] HELPERS PRESENT LLI to take nestlings (Westcott 1970), and it and the [] HELPERS ABSENT ,• 0.10 eastern indigo snake (Drymarchoncorais) are known to take fledglings (Webber 1980,Mumme • 0.08 1987). Convincing evidence of nest predation, • 0.06 including direct observations,also exist for the Red-tailed Hawk (Buteo jamaicensis),Eastern • 0.04 Screech-Owl (Otus asio), (Bubo >';0.02 virginianus),and bobcat (Lynx rufus),as well as for the Northern Harrier (Circus cyaneus), a 0.00 MARCH APRIL MAY JUNE northern migrantwhich may be presentthrough late spring. Despite the enormousnumber of B. NESTS WITH EGGS hours of field observations,the relative impor- 0.12 tanceof theseand other potential ScrubJay nest LU [] HELPERS PRESENT predators (e.g. Swallow-tailed Kites, Elanoides 0.10 [] HELPERS ABSENT forficatus;Fish Crows, ossifragus; Blue Jays,

0.08 Cyanocittacristata; and raccoons)has been un- clear (Schaub 1990). 0.06 Our study, however, has produced several lines of evidence indicating that mammalian 0.04 predatorsare relatively lessimportant than are snakesand birds. First, twice-daily nest checks and trackcensuses indicate that, although at least two-thirds of the predation on Florida Scrub MARCH APRIL MAY JUNE Jay nestsoccurs during daylight hours, poten- C. NESTS WITH NESTLINGS tial mammalian predatorsare overwhelmingly 0.12 nocturnal in their activity patterns. Second, re- IJJ [] HELPERS PRESENT suits from radio-tracking studies conducted at <• 0.10 [] HELPERS ABSENT * ABS indicate that potential mammalian preda- z 0.08 tots suchas bobcats and raccoonscommonly use O vehicle trails asavenues for travel (Worley 1980, • 0.06 Wassmer et al. 1988). If bobcats and raccoons were important as predators upon jay nests, n• 0.04 proximity of neststo the trails might correlate • 0.02 with increasednest predation (Best1978, Kep- DATA . . pie and Herzog 1978).However, we found that 0.00 the rate of Scrub Jay nest predation is not af- MARCH APRIL MAY JUNE fectedby proximity to vehicletrails. In fact,jays MONTH regularly placetheir nestsat the edgesof clear- Fig. 5. Daily predationrates (individual acts) on ings, including trail edges(Woolfenden 1974). nestswith (A) eggsor nestlings,(B) eggs,and (C) Finally, seasonal activity rates of potential nestlingsfor Florida Scrub Jay pairs with helpers presentand helpers absent (1974-1979 and 1981-1987 mammalian predators were not significantly combined).Nests belonging to noviceand senescent correlatedwith nest predation rates (Table 2). breedersexcluded from analysis.Asterisks indicate a Therefore, we concludethat although bobcats, log-likelihoodratio P < 0.05. raccoons, and other mammals undoubtedly depredatesome jay nests,they appearto be less DISCUSSION important nestpredators than snakesand birds. The snakeswe observedmost frequently in Identifyingthe predators.--The eggs and nest- Scrub Jay habitat at ABS were the eastern in- lings of Florida Scrub Jaysare potential food digo, eastern coachwhip, and southern black for manypossible predators. The continuing22- racer (Coluberconstrictor), although virtually all year studyof ScrubJays at ArchboldBiological the racerswe saw were too small to posea sig- Station (Woolfenden and Fitzpatrick 1990) has nificant threat. Florida pine snakes (Pituophis July1992] FloridaScrub Jay Nest Predation 591 melanoleucus)also were observed, but less fre- Nest mortality usuallyis greateron nestswith quently. All of these speciesare primarily di- eggs than on nests with nestlings (Martin in urnal (Ernst and Barbour 1989), and 94% of the press). However, our data show that Florida snake tracks we recorded were made during ScrubJay nests experience significantly greater daylight. We suspectthat the coachwhip,a large, predation when they have young than when locally common snake that easily traverses they have eggs, and this trend is consistent shrubberyand is strictlydiurnal (Ernstand Bar- throughout the nesting season(Fig. 2). Adult bour 1989), is the most frequent snakepredator jays visit nestsmore frequently when tending of the jays. young than when incubating eggs (Schaub As shown in Table 2, an index of diurnal 1990). Nestling jays beg frequently and loudly, predatoractivity is stronglycorrelated with nest and move about in the nest. Increased activity predation rates. Becauseit combines data ob- may make nestswith young easier to locate by tained from both visual sightings and track certain predators and may contribute to the counts,and includesseveral species of birds and higher predationrates (Hammond and Forward snakesthat vary in conspicuousness,this index 1956,Young 1963,Horn 1968,Knight and Tem- is at best only a crude measureof relative pred- ple 1986). Furthermore, this study has impli- ator activity and shouldbe viewed with caution. cated snakesas the primary nest predator. Be- Nonetheless, the correlation between the index causemany snakeslocate prey by olfaction, in of diurnal predator activity and nest predation addition to vision (Ashton and Ashton 1981), is striking (Fig. 4). Activity of the snakesalone increased olfactory cues emitted by nestlings shows a significant correlation both with the might further facilitate their detection. progressionof the jay nesting seasonand with Our results also indicate that when seasonal jay nest-predation rates. However, activity of effectsare controlled,younger nestlingsexpe- the diurnal nest predators does not cor- riencea greaterrate of predationthan do older relate significantlywith either of thesefeatures nestlings (Fig. 3). This difference may reflect of jay nesting. Becausesnakes are ectotherms, the decreasedsusceptibility of older nestlings their activity would be expectedto increaseover to certainpredators (e.g. Blue Jays, small snakes) the courseof the March-JuneScrub Jay breed- and increased parental defense of nests with ing season. Thus, the positive correlations older nestlings (Montgomerie and Weather- among snake activity, date within the nesting head 1988).Another possibilityis that the more season,and nest predation rates (Table 2, Figs. susceptiblenests are found when nestlingsare 1 and 4) suggestthat, although diurnal snakes young. and birds are frequent nest predators,snakes Rates of nest predation have been found to are relatively more important in our system. decreasewith nestlingage in relatively few spe- Factorsinfluencing nest predation.--Predators cies (e.g. Holcomb 1972). However, severalof may use visual, auditory, or olfactory cues pro- the studies reporting nest-predation rates in- vided by researchersto locatebird nests.There- creasingwith nestling age did not control for fore, in studiesof nestingsuccess, it is important the effectof season(e.g. Young 1963,Best 1978, to determine the effect that investigator visi- Woolfenden and Fitzpatrick 1984). Thus, the tation has on the rate of nest predation (Best increasedmortality on older nestlingsobserved 1978,Gottfried and Thompson1978, Wray et al. in these studiesmay occur becauseolder nest- 1982, Westmoreland and Best 1985, Martin and lings tend to be present later in the breeding Roper 1988). We addressedthis problem in 1987 seasonwhen predation rates are higher. by visiting samplesof nests on two schedules. Nests ultimately failed becauseof predation All nests were checked in the same manner, more often in shrubbypastures and overgrown during one season,and mostly by the sameper- scrubthan in open, recently burned scrub.Fitz- son. Predation ratesdid not differ significantly patrickand Woolfenden(1986) reported similar between nestsvisited twice daily and thosevis- results. However, the daily rate of individual ited every third day. Although nest visits at acts of predation does not differ significantly three-day intervals may influence predation among the three habitats(Table 1). It appears rates (Westmoreland and Best 1985), we con- that individual acts of nest predation are more clude that within the strictures of our experi- likely to lead to complete nest failure in pasture ment, the frequency of investigator visits had and overgrown scrub than in recently burned no influence on the rate of nest predation. scrub. It is unknown whether this is due to 592 SCI•AVS,MUMME, AND WOOLFENDEN [Auk, Vol. 109 among-habitatdifferences in the behavior of nest or a tenest. Our analysis indicates that predators,among-habitat differences in nestde- helpersreduced nest predation only during the fense by jays, or both. final month of the nesting season.We propose Effectsof helpers.--FloridaScrub Jay pairs as- that the resultsof Woolfenden and Fitzpatrick sistedby helpershad a significantlylower rate (1984), suggestingthat helpers enhancerepro- of nest predation than pairs without helpers duction of the first attempts, may have been only during the last month of the breedingsea- confoundedby correlateddifferences in breed- son, and only for nestswith nestlings(Fig. 5). er experience. Pairs without helpers are fre- Our analysis,which controlledfor age and ex- quently inexperiencedbreeders that do not be- perienceof breeders,thus suggests that helpers gin nesting until late in the season,when nest were effectiveat reducing predation ratesonly predationrates are high and successrates are when the predationpressure was at its greatest low (Woolfenden and Fitzpatrick 1984:216). intensity (Fig. 1). Thus, the higher survival observedin seasonal How could the presence of nonbreeding first nestsproduced by pairs with helpersmay helpers reduce predation on nestlings?Al- be attributed to breederexperience rather than though Florida ScrubJays are almostcertainly to the effectsof helpersper se. In contrast,the incapable of defending their nests against at- analysisreported in this paperexamined helper tacksby nocturnal predators(Woolfenden and effectswhile controllingfor ageand experience Fitzpatrick 1984),our studyhas shown that most of breeders. nest predation is attributableto diurnal snakes and birds. Helpers could reducethe frequency ACKNOWLEDGMENTS of successfulattacks by thesediurnal predators by serving as sentinels near nestsand by mob- We are grateful to the staff of Archbold Biological bing potential predatorsonce they have been Stationfor providing accessto the Station'soutstand- ing researchfacilities. We alsothank JackP, Hailman, detected (Francis et al. 1989, McGowan and ThomasE. Martin, Earl D. McCoy, and Henry R. Mu- Woolfenden 1989, Mumme in press). shinskyfor their constructivecriticisms of earlier ver- The finding that helpersdid not significantly sions of the manuscript.Financial support was pro- reducethe rate of predationon eggsis not sur- vided by NSF grant BSR 86-00174 to Mumme, and an prising; nonbreedersnormally do not partici- ABS Grant-in-Aid and Sigma Xi grant to Schaub. pate in reproductiveactivities until after hatch- ing. In fact, breeders usually chase potential LrrER•tURE CITED helpers away from the immediate vicinity of ASHTON, R. E., AND P.S. ASHTON. 1981. Handbook the nestduring laying and incubation(Stallcup of reptilesand amphibiansof Florida;the snakes. and Woolfenden 1978). Furthermore, non- Windward Publishing,Inc., Miami, Florida. breedershave been seen to remove eggsfrom BEST,L.B. 1978. Field Sparrowreproductive success the nests of the pairs with which they are as- and nesting ecology. Auk 95:9-22. sociated(Woolfenden 1974). Although the re- ERNST, C. H., AND R. W. BARBOUR. 1989. Snakes of sultsof Woolfenden and Fitzpatrick (1984) sug- eastern North America. George Mason Univ. gest that the presence of helpers reduces Press, Fairfax, Virginia. predation on eggs, our analysis--which em- FITZPATRICaC,J. W., AND G. E. WOOLFENDF•. 1986. De- ployed a more extensivedata setand controlled mographicroutes to cooperativebreeding in some New World jays.Pages 137-160 in Evolutionof for the potentiallyconfounding effects of sea- behavior (M. H. Nitecki and J. A. Kitcheil, son,breeder experience, and senescence--shows Eds.). Oxford Univ. Press, New York. no such effect. FRANCIS,A. M., J. P. H•aLM•N, AND G. E. WOOLFENDS. Woolfenden and Fitzpatrick (1984:204) re- 1989. Mobbing by Florida ScrubJays: Behaviour, ported that helpers significantly enhance sur- sexualasymmetry, role of helpersand ontogeny. vival of eggsand nestlingsonly during the sea- Anim. Behar. 38:795-816. son'sfirst nestingattempt. For seasonalrenests, GOTTFRIED,B. M., AND C. F. THOMPSON. 1978. Ex- helpers had no significantinfluence on the sur- perimental analysisof nest predation in an old- field habitat. Auk 95:304-312. vival rates. Our analysisdiffers from theirs in HAMMOND, M. C., AND W. R. FORWARD. 1956. Ex- at least one important respect;we investigated periment on causesof nest predation. J. seasonalinfluences by examiningnest preda- Wildl. Manage. 20:243-247. tion in monthly and half-monthly intervals, re- HOECOMS,L.C. 1972. Nest successand age-specific gardlessof whether a particularnest was a first mortality in Traill's Flycatcher.Auk 89:837-841. July1992] FloridaScrub Jay Nest Predation 593

HORN,H. S. 1968. The adaptive significanceof co- SolcAn,R. R., ,•lqOF. J. ROHI•F. 1981. Biometry, 2nd lonial nestingin the Brewer'sBlackbird Euphagus ed. W. H. Freeman, New York. cyanocephalus.Ecology 49:682-694. STALLCUP,J.A., ANDG. E. WOOI•FENDEN.1978. Family KEPPIE, D. M., • P. W. HERZOG. 1978. Nest site statusand contributionsto breeding by Florida characteristicsand nest successof Spruce . ScrubJays. Anita. Behav. 26:1144-1156. J. Wildl. Manage. 42:628-632. WASSMER,D. A., D. D. GUENTHER,AND J. N. LAYNE. KNIGHT, R. L., AND S. A. TEMPLE. 1986. Nest defense 1988. Ecologyof the bobcatin south-centralFlor- in the American Goldfinch. Anita. Behav. 34:887- ida. Bull. Fla. State Mus., Biol. Sci. 33(4):159-228. 897. WEBBER,T.A. 1980. Easterncoachwhippredationon LOISELLE,g. A., AND W. G. HOPEES.1983. Nest pre- juvenile ScrubJays. Fla. Field Nat. 8:29-30. dation in insular and mainland lowland rainfor- WES?COTT,P. W. 1970. Ecologyand behavior of the est in Panama. Condor 85:93-95. Florida ScrubJay. Ph.D. dissertation,Univ. Flor- MARTIN,T.E. 1987. Artificial nest experiments:Ef- ida, Gainesville. fects of nest appearanceand type of predator. WESTMORELAND,D., AND L. B. BEST. 1985. The effect Condor 89:925-928. of disturbance on Mourning Dove nesting suc- MARTIN,T. E. 1988a. Nest placement:Implications cess. Auk 102:774-780. for selectedlife-history traits, with special ref- WIthCOVE,D.S. 1985. Nest predation in forest tracts erence to clutch size. Am. Nat. 132:900-910. and the decline of migratory songbirds.Ecology MARTIN, T. E. 1988b. Processesorganizing open- 66:1211-1214. nesting bird assemblages:Competition or nest WOOI•FENDEN,G.E. 1974. Nesting and survival in a predation?Evol. Ecol. 2:37-50. population of Florida ScrubJays. Living Bird 12: MARTIN,T.E. In press. Breeding productivity con- 25-49. siderations:What are the appropriate habitat fea- WOOI•FENDEN, G. E. 1978. Growth and survival of tures for management? In Ecology and conser- Florida ScrubJays. Wilson Bull. 90:1-18. vation of Neotropicalmigrant birds (J. M. Hagan WOOI•FENDEN,G. E.,ANDJ. W. FITZPATRICK.1984. The and D. W. Johnston, Eds.). Smithsonian Institu- Florida ScrubJay: Demography of a cooperative- tion Press,Washington, D.C. breeding bird. Princeton Univ. Press,Princeton, MARTIN, T. E., AND J. J. ROPER. 1988. Nest predation New Jersey. and nest-site selection of a western population WOOLFENDEN, G. E., AND J. W. FITZPATRICK. 1990. of the Hermit Thrush. Condor 90:51-57. Florida Scrub jays: A synopsisafter 18 years of McGOwAN, K. J., ,•lqO G. E. WOOLFENDEN. 1989. A study.Pages 239-266 in Cooperativebreeding in sentinel systemin the Florida Scrub Jay. Anita. birds (P. B. Staceyand W. D. Koenig, Eds.). Cam- Behav. 37:1000-1006. bridge Univ. Press,Cambridge. MONTC,OMImm, R. D., ,•lqO P. J. WEATHERHEAD.1988. WOOI•FENDEN,G. E., ,•,ND J. W. FITZPATRICK. 1991. Risksand rewardsof nestdefense by parentbirds. FloridaScrub Jay ecology and conservation.Pages Q. Rev. Biol. 63:167-187. 542-565 in Bird population studies:Relevance to MUMME,R. L. 1987. Easternindigo snake preys on conservation and management (C. M. Perrins, J.- juvenile Florida ScrubJay. Fla. Field Nat. 15:53- D. Lebreton, and G. J. M. Hirons, Eds.). Oxford 54. Univ. Press, Oxford. MUMME,R. L. In press. Helping behaviour in the WORLEY,D. J. 1980. Nest sites, movement, and ac- Florida ScrubJay: Nonaptation, exaptation, or ad- tivity patternsof the raccoonProcyon lotor in south- aptation?Pages 1317-1324 in Acta XX Congressus central Florida. M.A. thesis, Univ. South Florida, InternationalisOrnithologici. Christchurch, New Tampa. Zealand, 1990. New Zealand Ornithol. Congr. WRAY, T., II., K. A. Stlo, rr, AND R. C. WHITMORE. 1982. Trust Board, Wellington. Reproductive successof grasslandsparrows on a SCH^U•, R. 1990. Predation on the eggs and nest- reclaimed surfacemine in West Virginia. Auk 99: lings of Florida Scrub Jays. M.S. thesis, Univ. 157-164. South Florida, Tampa. YOUNG,H. 1963. Age-specificmortality in the eggs SLAGSVOlvO,T. 1982. Clutch size variation in passer- and nestlings of blackbirds. Auk 80:145-155. ine birds:The nestpredation hypothesis. Oecolo- ZAR,J.H. 1984. Biostatisticalanalysis.Prentice-Hall, gia 54:159-169. Englewood Cliffs, New Jersey.