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Philornis Ectoparasitism of Pearly-Eyed Thrashers. Ii

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Philornis Ectoparasitism of Pearly-Eyed Thrashers. Ii PHILORNIS ECTOPARASITISM OF PEARLY-EYED THRASHERS. II. EFFECTS ON ADULTS AND REPRODUCTION WAYNE J. ARENDT U.S.D.A.Forest Service, Institute of TropicalForestry, Southern Forest Expen'ment Station, P.O. Box AQ, RœoPiedras, Puerto Rico 00928 USA ABSTRACT.--Arain-forest population of the Pearly-eyedThrasher (Margarops fuscatus) suf- fered heavy ectoparasitismfrom the larvae of a tropical fly recently identified as Philornis deceptivus.The impact of parasiticattack was of lessconsequence to adult thrashersthan to nestlings.Nestling mortality was high and was causedmostly by philomid ectoparasitism, other factorsbeing negligible(3%). The extent of larval infestationsfluctuated seasonally and was related to monthly rainfall. Infestationdid not have to be heavy to causedebilitation and death. Larval infestationsites varied with the nestling'sontogeny, especially pterylae development.Overall 4-yr fledging successwas 53.3%,fairly high for a tropical passefine. However, observationsof heavily parasitized nestlings that died during fledging attempts and the deathsof postfledgingjuvenile thrasherssuggest that first-yearmortality may be as high as 80%in nestlingsthat sufferedheavy larval infestations(357 of 448 nestlings).Juve- niles facekeen intraspecificcompetition after leaving the nest.A combinationof high nest- ling and juvenile mortalityand little opportunityfor juvenilesto enter the breedingpopula- tion couldgreatly reduce this host'snumbers. There are signs,however, that the Pearly-eyed Thrasheris adjustingto heavy philomid ectoparasitismby lengtheningthe breedingseason and reproducingwhile fly populationsare seasonallylow (December-March)and, thus,may remain in abundancein the rain forest.Received 31 August1983, accepted 10 June1984. MYIASlS "is the infestation of live human and Thrasher(Margarops fuscatus), a more abundant vertebrate animals with dipterous larvae, cavity-nesting specieswith a similar ecology. which, at leastfor a certain period, feed on the Pearly-eyed Thrasher nestlingsalso are infest- host'sdead or living tissue ..." (Zumpt 1965). ed by anddie fromphilornid larvae. The Pearly- In the Old World, 5 genera of bird-infesting eyed Thrasher is restricted to the West Indies diptera from 3 major families (Neottiophilidae, and hasa patchydistribution, ranging from St. Calliphoridae, and Muscidae) are reported to Vincent in the Lesser Antilles to the Bahamas infest adults and nestlingsor inhabit nestsof (Bond 1979). This rapaciouspasserine, well at least 9 avian orders, 28 families, and some known for its predatory habits (Rolle 1965a,b; 70 speciesof birds (Owen 1954, Zumpt 1965, pers. obs.),is omnivorous,feeding on a variety Pont 1974, Bakkal 1980). In the New World, of fruits, invertebrates, and vertebrates. In the species.ofthe common Europeanbird parasite Luquillo Mountains it is monogamousand a Protocalliphoraspp. (Calliphoridae;Plath 1919a, secondarycavity-nester, often nestingin crev- b) and a unique genus Philornis(Muscidae; ices and tree hollows. Dodge 1955, 1963, 1968;Pont 1972) have been Becauseseveral Philornisspecies are polytyp- reported. Some 30 Philornisspecies have been ic (Dodge and Aitken 1968), the taxonomyof collected and described from birds south of the this group has alwaysbeen extremelydifficult United Statesto the Neotropics(Pont 1972).On (reviewed by Aldrich 1923). In Puerto Rico two Trinidad alone 10 speciesof Philornisassociated Philornisspecies have been reported: P. obscura with 29 avian speciesfrom 17 families are rec- (Van der Wulp) from Ponceand P. pici (Mac- ognized (Dodge and Aitken 1968). quart) from Mayaguez (Wolcott 1948). Recent- In Puerto Rico the endangeredPuerto Rican ly, adult specimensthat emergedfrom puparia Parrot (Amazonavittata), a cavity-nesting bird, that I collected from thrasher nests in the Lu- is parasitizedby larval philornids, which have quillo Mountains were identified as P. decepti- causedthe death of parrot nestlings.As part of vusDodge and Aitken, 1968,by M. S. Couri, of a large-scalerestoration program to save the the National Museum of Rio de Janeiro, Brazil. parrot, study was begun on the Pearly-eyed Philornisdeceptivus previously had been record- 281 The Auk 102:281-292. April 1985 282 W^¾•E J. ARENDT [Auk, Vol. 102 ed only on Trinidad, its type locality (Dodge Over a 4-yr period (1979-1982),I took data on 272 and Aitken 1968, Pont 1972). nests, 681 nestlings, and 105 adults of the Pearly- Although many host recordsinvolving phi- eyed Thrasher.I usedartificial nest boxesdesigned lornid flies have been cited (Pont 1972), few as part of the Puerto Rican Parrot restorationpro- studies have concentrated on the deleterious gram (Snyder1977, Snyder and Taapken1977). Boxes were installed at 0.1-kin intervals between kms 12 effectsthat infesting fly larvae can have on an and 17 alongHighway 191,3-50 m from the roadway avian host over extended periods of time. With within the rain forest. Additional nest boxes were the exceptionof recent studiesby Smith (1968, added until the number of available boxes increased 1980), Bakkal (1980), Hector (1982), and Win- from 26 the first year to 48 the fourth year. From terstein and Raitt (1983), most of the literature January1979 to August 1982nest boxeswere visited on avian ectoparasitisminvolving Old and New throughout the breeding seasonon an average of World dipterans has been limited to the tax- every two days, but often on a daily basis during onomyof the adult fliesand to partial descrip- criticalperiods (laying, hatching, and fledging).Dur- tion of their life history stages(Nielsen 1911; ing the nonbreedingseason (September-November) boxes were checked once a week or once every two Gilbert 1919; Dodge 1968; Ledger 1969; Pont weeks for signsof nesting,depending on the prox- 1972, 1974). Some reports are anecdotal ac- imity of breeding activity. After eggs hatched, I countsof larval impactson host species(Plath checkedthe nestlingsfor fly larvae at every nest vis- 1919a, b; Hindwood 1930). it, notingnumbers, positions, and infestationperiods This study was designed to better under- of the larvae. I trapped larvae in funnels below the stand the host-parasite relationship and to nest cup and allowed them to pupate in small saw- quantify the direct effects that dipteran ecto- dust and woodchipvials coveredwith screens.I also parasitismcan have on an avian host and its collectedpuparia that adheredto nest twigs. During reproduction.Specific objectives were to deter- the 1979and 1980breeding seasonsI removed larvae mine 1) the generalprevalence (percentage of from some nestlingsas soon as they were detected in order to determine thrasher fledging successin host individuals infested;terminology of Mar- the absenceof ectoparasitism. golis et al. 1982)and intensity (mean number To determineif ectoparasitismwas dependent upon of larvae/infested host) of larval infestations, siblinghatch order, I randomlychose 90 parasitized 2) seasonalchanges and how they affect the nestlings(3/nest), separatedthem by hatch order, host-parasiterelationship, 3) at which of the and performed a one-way analysisof variance test host's anatomical sites larvae are most often (c•= 0.05; Shedecorand Cochran 1980) on the mean found, 4) the overall effects on host reproduc- number of infesting larvae/nestling (from the total tive success,and 5) possibleresponses by the number of infesting larvae found during the entire host to counter Philornisectoparasitism. nestling period). Bartlett'stest revealed heteroge- neousvariance among classes,so I useda square-root transformation to stabilize the variance. STUDY •REA AND METHODS I testedthe significanceof the differencebetween This studytook placewithin the 11,200-haLuqui- two proportions(Sokal and Rohlf 1969)to determine llo ]LxperimentalForest located in easternPuerto Rico if adult female thrashersexperienced significantly (18ø19'N,65ø45'W) at an elevation of ca. 800 m within more fly larvaethan adult malesand to determineif subtropicallower montaneforest (Holdridge 1947, site specificityof the infesting larvae on thrashers 1967;Ewel and Whitmore 1973). A detailed descrip- differed significantlybetween adults and nestlings. tion of this forest can be found in Odum and Pigeon To testfor a significantcorrelation between site spec- (1970). ificity of the larvaeon nestlingthrashers (dorsal lo- Annual rainfall varies from 3,000 mm in the foot- cationson young nestlingsvs. ventral siteson older hills to more than 5,000 mm on the highest peaks young), and to test for possibletemporal variation (Odum et al. 1970). Rainfall within the main study (earlyvs. late season)in larvalsite specificity, I used site averaged4,660 mm from 1935 to 1943 (Wads- a nonparametricstest for correlation(phi coefficient; worth 1948).A more recent12-yr summary of NOAA Conover 1971). data (1970-1981) from the Pico del Este weather sta- Adult and nestling thrashers were individually tion located about 5 km from the study site shows a marked with color bands and a metal USFWS band mean rainfall of 4,117 ram, which indicates that rain- for field identification. Outside of the breeding sea- fall patternsremain similar over long periods.Pho- son adult thrasherswere capturedin mist nets locat- toperiod varies little, with 12-14 h of daylight ed in their territories. During the breeding season throughoutthe year. For a 9-yr period, Wadsworth adultswere capturedby the use of pull-string trap- (1948)showed that the temperaturewithin the study doors mounted on the nest boxes. Observations were site averaged21.1øC (range 11.1-32.2øC). madefrom
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