THE CONDOR JOURNAL OF THE COOPER ORNITHOLOGICAL SOCIETY Volume 87 Number 2 May 1985 The Condor87:165-176 0 The CooperOrnithological Society 1985 SHINY COWBIRD PARASITISM IN TWO AVIAN COMMUNITIES IN PUERTO RICO JAMES W. WILEY ABSTRACT. -The Shiny Cowbird (Molothrus bonariensis),a brood parasite, has recently expanded its range from South America to Puerto Rico via the Lesser Antilles. This speciesis a host generalistand, on reaching Puerto Rico, encountered avian specieswith no history of social parasitism. In mangrove habitat study areas, 42% of the resident non-raptorial land bird specieswere parasitized. Some specieswere heavily parasitized, e.g., Yellow Warbler (Dendroicapetechiu)- 76% of nestsparasitized, Black-whiskered Vireo ( Vireoaltiloquus) - 82%,Puerto Rican Flycatcher (Myiarchusuntillurum)--85%, Yellow-shouldered Blackbird (Ageluius xunthomus)-95%, Troupial (Icterus icterus)- lOO%, Black-cowled Oriole (I. dominicensis)-100%. Others suffered low rates of parasitism (2 to 17% of nests examined); e.g., Gray Kingbird (Tyrunnusdominicensis), Red-legged Thrush (Turdusplumbeus), Bronze Mann&n (Lonchurucucullata), Northern Mocking- bird (Mimus polyglottos),Greater Antillean Grackle (Quisculusniger). Cowbird parasitism affected hosts by (1) depressingnest successan average of 41% below non-parasitized nests, and (2) reducing host productivity. Parasitized host nests hatched 12% fewer eggs and fledged 67% fewer of their own chicks than non- parasitized pairs. In contrast to the host specialization shown by Rufous-collared Sparrow, Zonotrichiacapen- most other parasites,some speciesof cowbirds sis,in Argentina, Ring 1973, Fraga 1978; on spread the burden of parasitism among many Yellow Warbler, Dendroicapetechiu, in Bar- species.This habit reduces selective pressures bados, Bond 1966; and on Yellow-shouldered that favor the host’s development of counter- Blackbird, Agelaiusxunthomus, in Puerto Rico, adaptive responsesagainst the parasite, and Post and Wiley 1976, 1977b). makes more foster parents available to the Most cowbird research has been conducted cowbird. A nonspecific parasite need not be in areas where the parasites have long coex- regarded as primitive or unable to specialize, isted with host populations. During this long becauseremaining a generalist may be an im- coexistence,cowbird and host populationshave portant adaptation in itself, allowing oppor- coevolved patterns of exploitation and avoid- tunistic exploitation of many speciesencoun- ance, some of which are specialized enough to tered by the parasite (Gochfeld 1979). The obscure their origin or possible evolutionary abundance and expansion of both the Brown- route. The Shiny Cowbird reached Puerto Rico headed Cowbird (Molothrus ater; Mayfield by “island hopping” (perhaps aided by intro- 1965, Rothstein et al. 1980) and the Shiny ductions on some islands) through the Lesser Cowbird (M. bonuriensis;Post and Wiley Antilles from South America and Trinidad 1977a) show that generalisthabits can be suc- (Post and Wiley 1977a). Although the cowbird cessful. was first recorded at the northeasternmostpart Although a generalisthabit may enable host of the island in 1955 (Grayce 1957), there is populations to maintain parasitic loads more strongcircumstantial evidence that the species easily (reviewed in Payne 1977), some cowbird arrived before that time (Post and Wiley populations have had extreme depressive ef- 1977a). In 1963, Biaggi (1970) saw more than fects on the reproductive successof certain host 200 cowbirds at Yabucoa, southeasternPuerto species(e.g., Brown-headed Cowbird on Kirt- Rico. M. Gochfeld (Buckley and Buckley 1970) land’s Warbler, Dendroicakirtlundii, Mayfield found 250 near the Guajataca cliffs in north- 1972, Walkinshaw 1983; Shiny Cowbird on western Puerto Rico in 196 5. It is not known 11651 166 JAMES W. WILEY when cowbirds reached southwestern Puerto Buell and Dansereau (1966) have describedthe Rico, but it is likely that they spreadwestward vegetative structure of Roosevelt Roads. through the island and arrived there somewhat Because I wanted to restrict my work to a later. vegetatively simple ecosystem,I chose to con- The study of the Shiny Cowbird and its re- duct the researchwithin the mangrove forests. cently-parasitizedhost populations in the West This also made comparisons between the Indies offers a unique opportunity to examine southwesternand eastern study areasmore eq- the biology of brood parasitism early in the uitable becausethe forestsare composed of the history of interaction between a generalistpar- samedominant species,although structuredif- asite and a ndive local avifauna. In a seven- fers somewhat. year study, I investigated the relationships At Roosevelt Roads, the forest is composed between Shiny Cowbirds and their hosts in of four mangrove species:black mangrove (Av- Puerto Rico with three objectives in mind: (1) icennia germinans), white mangrove (Lagun- to determine the effects of Shiny Cowbird par- &aria racemosa), button mangrove (Cono- asitism on an avian community at the frontier carpuserectus), and red mangrove (Rhizophora of the parasite’s expanding range, (2) to ex- mangle). Rhizophora is nearest the sea and amine factors critical to selection of hosts by Laguncularia and Conocarpusfarthest inland. the Shiny Cowbird, and (3) to determine if Inner mangrove areaswith sandy soilsare often nesting birds at this frontier, presumably in- characterized by a saltwort (Batis maritima)- experienced with social parasitism, had anti- Avicennia scrub and glasswort(Salicornia) or parasite habits and, if so, the origins of these sea-purslane (Sesuvium portulacastrum) mat. patterns. Here I will present data that show the The prevalent vegetation type in the elevated extent and effect of brood parasitism on avian areas surrounding the mangroves is thicket communities which have recently encountered dominated by the introduced leadtree Leu- the Shiny Cowbird. caena leucocephala.Open hillsides and aban- Becausethe cowbirds and the resident avian doned pastureland characteristically are cov- community have recently met, I predicted that ered with graminoids, especially guinea grass the incidence of parasitism would be high ow- (Panicum maximum). ing to the na’ivete of nesting birds to this form As is true throughout coastal Puerto Rico, of exploitation. Since brood parasitism has the Roosevelt Roads mangrove forests have been shown to depress host productivity in been extensively disturbed (Buell and Danser- systemswhere host and parasite have a longer eau 1966), althoughthe resultingsalt flats, scrub, history of coexistence(Payne 1977), I expected and savannas are now returning to forests. host nest successand productivity at parasit- Some mangrovesserved as depositoriesfor soil ized nestsin Puerto Rico would be lower than removed from graded uplands. Dredge mate- in non-parasitized nests. rial from the harbor and accesschannels was deposited on mangroves or on areas seaward STUDY AREAS of existing land. These have revegetated pri- I studied brood parasitism in mangrove hab- marily with Avicennia. One such fill (“LT”) itats at Roosevelt Roads Naval Station at the served as one of my two study sites at Roo- easternmost point of Puerto Rico, and at Bo- sevelt Roads. The other main site (“OC”) was quer6n Forest in southwestern Puerto Rico. a lessdisturbed, older mixed-mangrove forest. The Roosevelt Roads study area is about 60 The southwesterncoastal study site lies be- km east of San Juan. Ewe1 and Whitmore tween Bahia Sucia and Bahia Montalva, but (1973) included the 3,260-ha naval station primarily within the Commonwealth Boque- within the Subtropical Dry Forest Zone. An- r6n Forest. Ewe1 and Whitmore (1973) also nual rainfall averages 162 cm, with most rain characterized this area as within the Subtrop- falling in two periods:May, and Augustthrough ical Dry Forest Zone although, at a mean of November (data from Fajardo Station, 6 km 68 cm, annual precipitation is less than half west of Roosevelt Roads). Mean annual tem- that at Roosevelt Roads. Like the easternstudy perature is 25.9”C (1975-1981). area, most rainfall occursin two periods: May, Mangrove forest dominates the extensive and August through November. Mean annual tidal lands and occupies about 25% (814 ha) temperature (1975-l 980) is 26.8”C. of the station’s land area. The remainder con- The Boqueron Forest study area consistsof tains level, alluvial valleys and low hills (less a red mangrove fringe (approximately 25% of than 150 m above sea level) which are vege- the area; Puerto Rico Department of Natural tated with grasses,various scrubs, and inter- Resources1976), black mangrove (about 55%), mediate successionalstages. Military instal- and salt flats (10%). I inspectedthe other main lations and housing/recreation complexes association (salt spray exposed sandy beach occupy a considerable portion of the station. thicket) irregularly. The mangrove forestscon- COWBIRD PARASITISM IN PUERTO RICO 167 sist of old, tall, red and black mangrove stands Some nestswere manipulated, either for ex- as well as re-vegetating salinas. In contrast to periments or to reduce cowbird loads (e.g., Roosevelt Roads, there are many offshore cowbird egg removal). I did not include these mangrove cays, which I visited regularly to manipulated nests in analyses that would be check nests. Inland from the mangrove, the biased by such experiments (e.g., nest success, dry hillside and alluvial fall is characterized hatchability, fledgingsuccess), but several were by an oxhorn bucida (Bucidu