Shiny Cowbird Parasitism in Two Avian Communicties in Puerto Rico

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 bucerus)-mes- used in calculating percentage of host nests quite (Prosopisjulijlora) savanna/woodland or parasitized and mean clutch sizes. gumbo-limbo (Bursera simaruba) savanna/ I attempted to determine the cause(s)of nest woodland, with denseepiphytic ball moss(Til- failure for 19 speciesof birds and for parasit- landsia recurvata) throughout. ized species, and compared failure rates between parasitized and non-parasitized neststo METHODS determine if differences existed. Some losses Data presented here were collected between were clearly the result of predation: e.g., rat- 1975 and 198 1. My scheduleof data collection chewed eggsor chicks. Nests where chicks or at Roosevelt Roads was as follows: 1975- eggscleanly disappearedwere assumedto have study area visited at least every other day been plundered (usually by Pearly-eyed throughout the breeding season(March-June), Thrashers, Margarops fuscntus). Disappear- biweekly visits were made during the non- ance of chicks or eggsfrom damaged nestswas breeding season;1976-area visited at least on attributed to black rat (Rattus ruttus) preda- alternate days from July through August; 1977- tion. Exposure was thought to be responsible 1978-daily or alternate day visits through for death of chicksfound in or under nestsafter breeding season,biweekly visits during non- a storm where I found no sign of predation or breeding season; 1979-l 98 1-at least every arthropod infestation. Nests where activity had other day visits through breeding season,ex- ceased before egg-laying or where the adults cept July 1980 when data were collected twice no longer tended their eggs were classed as per week, non-breeding seasonvisits at weekly deserted.Eggs at some desertednests may have or biweekly intervals. been subsequently eaten by predators, which My schedule of visits to the southwestern perhaps led to an underestimate of desertion study area was: 1977 through 1979-data col- and overestimate of predation rates. Nest fail- lected at least once per week during the breed- ures due to nest mites or ants were occasionally ing season,study area visited irregularly during noted. Warble fly (Neomusca [Philornis] pi@ non-breedingseason; 1980 -weekly visits from larva infestation causedthe lossof some chicks. April through June, and August, with daily I defined a nest as “active” when the resident visits during July, irregular visits during non- laid at least one egg or, if parasitized and no breeding season. host eggwas laid, the host incubated the cow- I concentratedon three of the most common bird egg(s).Nests where building occurred but nesting species in each study area: Yellow- no eggs(host or parasite) were laid were not shouldered Blackbird, Yellow Warbler, and included in calculations of nesting effort. A Greater Antillean Grackle (Quiscalus niger). nest from which one or more chicks fledged I found nestsby regularly searchingthe study (either host or parasite) was considered suc- areas. The site of each nest was marked with cessful. a coded tag (unobtrusively placed), and nest I followed statistical methods of Hollander locations were plotted on field maps. Distances and Wolfe (1973) and Zar (1975). Significance and compass directions between active nests level was set at 0.05. When the underlying as- were measured and plotted. At each visit to sumptions (e.g., normality, equal variance) the study areas, I inspected neststo determine were not violated, or only slightly so, I used number of host and, if present, parasite eggs parametric statistical methods to test for sam- and chicks. I inconspicuously marked all eggs ple differences. In other cases,I applied non- with indelible ink, some were measured and parametric statistics. weighed. Weights were taken using Pesola spring scalesof 5-g and 10-g capacities. Mea- RESULTS surements were made to the nearest 0.1 mm. I routinely checked eggs for damage. Hatch- PARASITISM RATES lings were identified from their nestmates by One or more parasitized nests were encoun- food dye (blue, green, yellow, orange) on an tered for 42% (1 l/26) of the non-raptorial land inconspicuous part of the body (e.g., under bird speciesbreeding on the two study areas. wing). When chickswere old enough, I banded At Roosevelt Roads, regular nest inspections them with three colored celluloid leg bands in of 17 bird species (n = 823 nests) revealed unique combinations. cowbird parasitism of eight species(47%; Ta- 168 JAMES W. WILEY ble 1). Prevalenceof parasitismfor thesespecies for Red-legged Thrushes, grackles, and Yel- rangedfrom 2 to 100%. Within this wide range, low-shouldered Blackbirds at Boquer6n For- I found two distinct groups: Gray Kingbirds est, and the singleparasitized Northern Mock- (Tyrannus dominicensis),Red-legged Thrush- ingbird nest at Boquer6n (Mann-Whitney test, es (Turdus plumbeus), Greater Antillean l-tailed; Table 3). Grackles, and Bronze Mann&ins (Lonchura For some parasitized species or localities, cucullata) had a low rate of nest parasitism the number of cowbird eggsper nest was great- rangingfrom 2 to 17%, and the remaining four er than the number of host eggs. The mean species,Yellow Warbler, Black-whiskered Vir- number of cowbird eggs exceeded the mean eo ( Oreo altiloquus), Yellow-shouldered number of host eggs at nests of the Yellow- Blackbird, and Black-cowled Oriole (Zcterus shoulderedBlackbird (2 1% more cowbird than dominicensis), had a high rate of parasitism, host eggs),both at Roosevelt Roads (3.22 vs. from 75 to 100% of nests examined. An av- 2.63; P < 0.01) and Boquer6n Forest (3.08 vs. erage of 1.1 ? 0.02 [SE] (range l-2) cowbird 2.62; P = 0.025, Mann-Whitney test, l-tailed). eggswere laid per parasitizednest in the species This occurred also for the Puerto Rican Fly- incurring low parasitism rates. The mean catcher(+9%: 3.18 vs. 2.91; P > 0.05), Black- number of cowbird eggsin nests of regularly- whiskered Vireo (+ 15%: 2.56 vs. 2.22; P > parasitized specieswas significantly greater at 0.05), and Black-cowled Oriole (+89%: 3.92 2.7 + 0.24 (range 1-8; P < 0.05, Mann-Whit- vs. 2.07; P < 0.0025), and Yellow Warbler ney test) than in nestsof irregularly-parasitized (+47%: 2.94 vs. 2.00; P = 0.025) at Boquer6n. species. In contrast, host eggs outnumbered cowbird Five of the seven (71%) speciesmonitored eggsat Greater Antillean Grackle (+ 173%: 3.09 at Boquer6n Forest were parasitized at levels vs. 1.13; P = 0.025) and Troupial (+ 50%: 2.40 ranging from 8 to 100% of nestsobserved ( IZ = vs. 1.60; P > 0.05) nests,and Yellow Warbler 154 nests; Table 2). The Northern Mocking- (+27%: 2.07 vs. 1.63; P < 0.025) nestsat Roo- bird (Mimus polyglottos), a speciesfor which sevelt Roads. I found no parasitized nests at Roosevelt Parasitized nestsreceived from one to eight Roads, incurred a low rate of parasitism (8%) cowbird eggs.For some species,the mean total at Boquer6n. The other four parasitized species number of eggsper nest (i.e., host + cowbird (Yellow Warbler; Puerto Rican Flycatcher, eggs)was greater than the average clutch size Myiarchus antillarum; Yellow-shouldered at unparasitized nests:Yellow Warbler at Roo- Blackbird; Troupial, Zcterusicterus) were reg- sevelt Roads-3.6 eggsat parasitized nestsvs. ularly parasitized (mean percentage of nests 2.6 at non-parasitized nests(P = 0.025, Mann- parasitized = 88.5%, range = SO-loo%). I Whitney 2 sample test, l-tailed); Yellow War- found no parasitized Gray Kingbird nests in blerat Boquer6n Forest-4.8 vs. 2.8 (P = 0.05); the southwestern study area. Adelaide’s War- Yellow-shouldered Blackbird at Roosevelt bler (Dendroica adelaidae) did not nest on the Roads-5.7 vs. 3.0 (P = 0.025); Yellow-shoul- study area, but I found parasitized nestsin the dered Blackbird at Boquer6n Forest- 5.6 vs. foothills adjacent to the mangroves. An av- 2.5 (P = 0.05); Greater Antillean Grackle-4.2 erage of 3.0 + 0.05 cowbird eggsper parasit- vs. 3.2 (P < 0.05). Although not statistically ized nest (range l-8) were laid in the nests of significant, the total number of eggsin para- regularly-parasitized species. The single par- sitized nests was greater than that in unparasitized Northern Mockingbird nest contained asitized nestsof Puerto Rican Flycatchers (6.1 one cowbird egg. vs. 4.5) and Black-whiskered Vireos (4.8 vs. The same two-group pattern of parasitism 3.0). The addition of cowbird eggsincreased incidence was evident among the collective the number of eggs in parasitized nests 3 1% breedingpopulations of parasitizedspecies (68 1 (grackle) to 124% (blackbirds at Boqueron) nests;IZ = 11 species)of both study areas.Reg- above non-parasitized clutch sizes (X for five ularly-parasitized specieshatched more cow- species= 65%). bird eggs (R= 1.1 + 0.10 per nest; IZ = 6 For most parasitized species, there was a species)and fledged more cowbird young (R = strongnegative relationship between the num- 0.6 + 0.13) than specieswhere cowbird par- ber of cowbird eggsdeposited in nestsand the asitism was only occasional (X hatched = number of host eggs laid. Only grackles (r 0.02 + 0.08, x fledged = 0.01 + 0.02; n = five [product moment correlation coefficient] = species;both P < 0.001; Fisher-Behrenst-test). 0.57; 0.05 < P < O.lO), Black-whiskered Vir- eos (r = -0.11; 0.10 < P < 0.20), and the FINAL CLUTCH SIZE Yellow-shouldered Blackbird (r = 0.57; 0.10 < In both study areashost clutch sizeswere con- P < 0.20) population at Boquer6n Forest did sistently smaller (average - 12%) in parasit- not show this relationship. Regularly-parasitized nests than non-parasitized nests, except ized speciesexperienced an average reduction

170 JAMES W. WILEY

TABLE 2. Summary of breeding bird nest success,productivity, and degree of parasitism by the Shiny Cowbird at Boqueron Forest, southwesternPuerto Rico, 1977-1980.

SpeClW Component PRF GKb Mb YW aQ YsBb Tr Total no. active nests 13 15 13 20 22 66 5 No. successfulnests No.(%) host (Z%, &)43 ($ (4;)41 (1:)55 163($ @p, eggs Mean host clutch size 3.2 2.9 2.9 2.1 2.5 1.5 2.4 No. host eggshatch (% of eggs) (::) (ii) (Z) (& (Z) (ii) (5:) No. host young fledged (% of eggs) d:) (it) (Z) (1;) (ii) (ii) (3;) No. fledged/active nest 1.0 1.9 1.8 0.4 0.8 0.5 0.8 No. fledged/successfulnest 1.4 2.5 2.7 0.8 1.8 1.2 1.0 No. nests parasitized (O/oof nests) (& (i) (i) (& (k (Z) (lOi) Successof parasitized nests 0.6 1.0 0.4 0.4 0.8 Successof nests not parasitized 1.0 0.7 0.7 0.8 0.5 0.1 No. cowbird eggs 35 1 47 182 8 Mean no. cowbird eggs/parasitized nests 3.2 1.0 2.9 3.1 1.6 No. cowbird eggshatched (% of eggs) d:) (10:) (& (Z) (8:) No. cowbirds fledged (O/oof eggs) dt) (!) (1:) (Z) (7:) No. cowbirds fledged/active nest 0.8 0.4 0.6 1.2 No. cowbirds fledged/successful nest 1.2 0 0.8 1.6 1.5

’ Common names: PRF = Puerto Rican Flycatcher, GKh = Gray Kmgbird, Mb = Northern Mockingbird, YW = Yellow Warbler, BQ = Bananaquit, YaBb = Yellow-shouldered Blackbird, Tr = Troupial. of host eggsranging from 13% (nestswith only 2); nest successat non-parasitized nests (nine one cowbird eggadded) below mean clutch size species)averaged 4 1% above that at parasit- at unparasitized neststo 74% at nestswith the ized nests.Examined collectively, nest success maximum observed number of cowbird eggs of parasitized pairs was less than that of non- (eight at three Yellow-shouldered Blackbird parasitized pairs for the 11 species on both nests).Correlation coefficientsfor thesespecies study areas (P < 0.001, Kolmogorov-Smir- were as follows: Puerto Rican Flycatcher- nov test). The single parasitized nests of the -0.97 (P < 0.01; Null hypothesis = 0; t-test Red-legged Thrush, Bronze Mann&in, and for correlation, 2-tailed); Yellow Warbler- Gray Kingbird all failed, whereas the only -0.99 (Roosevelt Roads; P < 0.02) and -0.94 Northern Mockingbird nest parasitized fledged (Boqueron Forest; P -c 0.01); Troupial- - 1.O young (although the cowbird chick did not (P -c 0.001); Yellow-shouldered Blackbird- fledge). Nevertheless, some host speciespop- -0.88 (Roosevelt Roads; P < 0.002); Black- ulations had no better nest successat non-par- cowled Oriole- -0.82 (P < 0.05). asitized nests than at parasitized nests; i.e., Yellow Warblers at Roosevelt Roads, Yellow- HATCHING SUCCESS shoulderedBlackbirds at Roosevelt Roads and Fewer host eggshatched in parasitized nests Boquer6n Forests (P > 0.05, Tables 1 and 2). than in non-parasitized nestsof vireos, Puerto On the average,parasitized pairs fledged67% Rican Flycatchers, and the Boquer6n popu- fewer host chicks than non-parasitized pairs lations of the warbler (Table 3). Numbers of (0.5 host chicks/nestvs. 1.5 chicks/nestat non- host chicks were no different between parasit- parasitizednests for five species),although only ized and non-parasitized blackbird, grackle, Puerto Rican Flycatchers, vireos, and the war- and warbler nests at Roosevelt Roads. Para- bler population at Boqueron showed signifi- sitized blackbirds at Boquer6n Forest hatched cant differences(Table 3). Parasitizedhosts (six more host chicks than did non-parasitized species)fledged an averageof 2.4 times as many pairs. cowbird chicks as host chicks. Only grackles fledged more host chicks (1.36 * 0.28/nest) NEST SUCCESS than cowbird chicks (0.09 ? 0.06; P -c 0.05, Host nest successat the Roosevelt Roads study Mann-Whitney 2 sample test, 2-tailed) at par- area was generally greater at non-parasitized asitized nests.Other speciesfledged more cow- nests than at parasitized nests (Tables 1 and birds than host chicks: Puerto Rican Flycatch-

172 JAMES W. WILEY

TABLE 4. Fate of nestswhere Shiny Cowbird laid one or more eggsbefore host lays its eggs,Roosevelt Roads Naval Station (“RR”) and Boquer6n Forest (“BF’) study areas, Puerto Rico, 1975-198 1.

Number eggslaid by host Host’s activity after cowbird laid Host mean Raised Causeof failure specw Locality n 0 I 2 3 4 clutchsize Incubated OffSpring Desertion Predation

YW RR 34 24 4 5 1 1.7 14 5 17 9 YsBb RR 25 20 4 1 1.4 8 5 20 1 BF 11 9 2 1.0 2 10 GAG RR 8 5 2 1 3.3 3 : BcO RR 2 2

* YW = Yellow Warbler, YsBh = Yellow-shoulderedBlackbird, GAG = Greater Antillean Grackle, BcO = Black-cowledOriole.

(30% of nests failed owing to desertion) than dered Blackbirds had higher desertion rates at non-parasitized nests(10% deserted,x2 with than parasitized nests (P < 0.02, Fc2),,,2= Yate’s correction = 29.5, P < 0.0001). Within 169.9), although the difference may have been a species,however, the desertion rates at par- an artifact of the small sample of non-parasit- asitized and non-parasitized nestswere not dif- ized nests (n = 4). ferent, except for the Yellow Warbler popu- In summary, 42% of the resident non-rap- lation at Boqueron (P -C 0.05). torial land bird specieswere parasitized in my Cowbirds occasionally deposited eggs in study areas.These speciesfell into two groups: nestsbefore the host had laid its first egg(Table those which were rarely or occasionally para- 4). Typically, hosts abandoned these nestsbe- sitized (2 to 17% of nestsparasitized) and those fore laying their eggs,i.e., at nestswhere cow- that were regularly parasitized (>75% of the birds laid first, 9 1% of all failures were due to nests). Cowbird parasitism affected hosts by desertion, with predation accounting for only (1) depressingnest successan average of 4 1% 9% of the losses.Host desertion rates where below non-parasitized nests, and (2) reducing parasiteslaid first were higher than thosewhere host productivity. Parasitized hosts produced the host had laid at least one egg.This was the 12% fewer eggsand fledged 67% fewer of their casefor Yellow Warblers (x2 = 4.4; P < 0.05) own chicks than non-parasitized pairs. Never- Yellow-shouldered Blackbirds (x2 = 24.9, P < theless, the total number of chicks (host + O.OOl), and Black-cowled Orioles (x2 = 8.6, cowbird) fledged from parasitized nestswas no P < 0.005). Desertion rates of Greater Antil- different from the number fledged from non- lean Grackles remained about the same re- parasitized nests;i.e., parasite chicks replaced gardlessof whether cowbird or host laid first. some host chicks resulting in fewer host fledg- Most hosts (K = 78%; four species)did not lings. lay eggsin a nest where cowbirds laid the first egg.If the host did lay, however, its clutch size DISCUSSION did not differ from that of other parasitized The first major finding of this study was that nests (P > 0.05, x2). Even when the hosts did only some of the nesting speciesin the Puerto not lay eggs,20% of the pairs stayed with the Rican mangrove community were regularly neststo incubate all-cowbird eggclutches and parasitized by Shiny Cowbirds. Those species 17% raised all-cowbird broods. were nevertheless parasitized at rates ranging Probability of host desertion was inversely from 75 to 100% of the nests examined. Sec- related to reproductive investment; i.e., num- ond, I found that cowbird parasitism generally ber of host eggsdeposited (Fig. 1). Hosts were lowered host productivity. lesslikely to desert nestscontaining completed No specieswere parasitized to an interme- clutchesthan nestswhere the host had not yet diate degree. Although parasitism rates de- laid or where they had laid only one or two scribed for most host populations range from eggs:Yellow Warbler-D Max = 0.423 (Kol- low to moderate proportions of the nests (see mogorov-Smirnov test; Null hypothesis: de- Payne 1977 for examples),high rates have been sertion rate for host with zero eggs= rate with reported for severalpopulations: e.g., Red-eyed one egg = rate with two eggs= . . . = rate with Vireo ( vireo olivuceus)- 72% of nestsparasit- n eggs= 0; P < O.OOl), Yellow-shouldered ized (Southern 1958), Dickcissel (Spiza amer- Blackbird-O.392 (P < 0.0005), Greater An- icana) - 9 5% and Eastern Meadowlark (Stur- tillean Grackle-O.441 (P < 0.0002). Rates of nella magna)- 70% (Elliott 1978), Rufous- desertion for Yellow Warblers and gracklesdid collared Sparrow-66% (Ring 1973). not differ between parasitized and non-para- The high degree of exploitation I found in sitized nests (P > 0.05, 2-way ANOVA, Fig. Puerto Rico is probably related to the Shiny 1). Non-parasitized nests of Yellow-shoul- Cowbirds recent arrival and expansionthrough COWBIRD PARASITISM IN PUERTO RICO 173

Greater Antillean Grackle loo+ 0

Yellow Warbler 100

0 l

l t-*=0.79 \\

\ 0 C I I I 3 I I I I 2 3 4 I 2 3 4

Yellow-shouldered Blackbird 100 r2 = 0.42

0 I I 1 I C I I I 1 I 2 3 4 I 2 3 4

NUMBER OF HOST EGGS LAID FIGURE 1. Association of nest desertion with owner’s reproductive investment (i.e., number of eggslaid at the time of desertion) for Yellow-shouldered Blackbirds, Yellow Warblers, and Greater Antillean Grackles at Roosevelt Roads Naval Station studyarea, easternPuerto Rico. For each species,the mean for parasitizedand non-parasitizedindividuals is plotted in the left graph. In the right graph, parasitized individuals are plotted separatelyfrom non-parasitized birds. In the latter plots, circles represent parasitized individuals and dots depict non-parasitized birds. 174 JAMES W. WILEY

an island whoseresident birds presumably lack Punctured host eggsmay have been removed anti-parasite defenses;i.e., those speciessuit- by the cowbird or the host before I inspected able as cowbird hosts and which lack effective the nest, so that they were never counted, lead- anti-parasite defenses are heavily exploited. ing to an underestimate of host clutch size. This pattern of heavy exploitation is not evolu- Thus, the first factor causesa real reduction of tionarily stable because the cowbird may host egg production, whereas egg removal by eliminate certain host speciesor causethem to cowbirds causes a smaller host clutch to be become rare through over-exploitation. A suc- incubated but no reduction in number of eggs cessful parasite should not eliminate its host. produced by the female. Subsequently, lower Logically, when hosts become so scarce that rates of hatching and fledging at parasitized nestsare limiting, it would be adaptive for the nests are partly related to the initial reduced parasite to shift to another acceptable species. clutch size, but other contributing factors exist Concurrently, asnests of some speciesare more (see below). difficult to find becauseof their rarity, the rate Hatching rates at parasitized and non-par- of parasitism may also decline. However, this asitized nestsdiffered considerablyamong host has apparently not yet occurred in the two Yel- species.Fewer host eggshatched at parasitized low-shouldered Blackbird populations I stud- nestsof some speciesthan others, possibly be- ied, despite their continued and accelerating cause of two major factors: (1) differences in decline (Post and Wiley 1976, 1977b; Wiley, egg sizes cause uneven distribution of an in- unpubl. data). Shiny Cowbirds are extreme host cubating adult’s heat, and (2) the sheernumber generalists (Friedmann et al. 1977, pers. ob- of excesseggs from multiple parasitism may serv.). Even as they become rare, primary hosts reduce the chance that the host’s eggswill be (e.g., Yellow-shouldered Blackbird) may still adequately incubated (e.g., Black-cowled Ori- be heavily parasitized. Cowbirds may sustain ole, Yellow-shouldered Blackbird). If the host’s large populations by individuals shifting to egg is smaller than the parasite’s, it may not other, less suitable hosts (e.g., Yellow War- adequately touch the adult’s body for proper blers, Greater Antillean Grackles). Meanwhile, incubation, and, consequently,fewer host eggs the few remaining individuals of the primary than cowbird eggs may hatch. Conversely, host are still vulnerable to the cowbirds and compared to hatching rates in the nestsof other continue to be parasitized whenever encoun- species,a lower proportion of cowbirdshatched tered. Although not well documented, local in nests of grackles. I suggestthat the smaller depression and extinction of host bird popu- relative size of the cowbird eggresulted in poor lations by a parasite may occur. After being contact with the foster parent’s brood patch. parasitized by the Common Cuckoo (Cuculus Mayfield (1960) believed that the Brown- canorus) with increasing frequency over the headed Cowbird eggs in K&land’s Warbler years, a population of Sedge Warblers (Acro- nests have better contact with the incubating cephalus schoenobaenus)became scarce and female than the warbler eggs,which are small- then disappeared(Owen 1933). Similarly, pop- er. ulations of Kirtland’s Warbler (Mayfield 1960, Hosts can efficiently incubate only a certain 1965) declined precipitously after they en- amount of egg mass. The addition of cowbird countered expanding populations of Brown- eggsbolstered the clutches of several species headed Cowbirds. above normal size so that the host may have In Puerto Rico, overall nest successand pro- been unable to cover all eggs,again resulting ductivity were lower at parasitized nests than in lowered hatchability. at non-parasitized nests. Some parasitized Fledging rates of host chicks were typically speciestended toward greatersuccess than non- lower at parasitized neststhan at non-parasit- parasitized nests but these results are likely ized nests,partly (as discussedabove) because artifacts of small sample sizes. of reduced clutches at parasitized nests, but Clutch size was usually smaller at parasit- also becausethe foster adults could raise only ized nests versus non-parasitized nests. Two a limited number of chicks. Perhapsbrood size factors may have been working simultaneously was limited by the adults’ ability to care for a to produce this effect: (1) if a cowbird lays an fixed maximum number of chicks; i.e., chicks eggin a host’s nest without simultaneously re- hatching after the normal brood size was moving one of the owner’s eggs,determinant reached were eliminated through brood re- layers may produce fewer eggsaccordingly, and duction. (2) Shiny Cowbirds commonly puncture and/ Hudson (1920) and Gochfeld (1979) re- or remove host eggsas they deposit their own ported that the activities of Shiny Cowbird eggs (Hoy and Ottow 1964; Post and Wiley young may attract predators and thereby lead 1977b; Wiley, unpubl. data). Some degree of to excessive predation. In contrast, I found error in estimating host clutch size is probable. that most nestsof Puerto Rican hosts were no COWBIRD PARASITISM IN PUERTO RICO 175 more likely to be depredated or abandoned where breeding seasonsare shorter, full recy- than non-parasitized nests. Mayfield (1960) cling may preclude the production of one or also reported no difference in predation rates more additional broods which are a regular between non-parasitized nests of K&land’s part of the breeding cycle of many speciesin Warblers and those that were parasitized by the tropics. Thus, there may be little repro- Brown-headed Cowbirds. However, I ob- ductive gain, if any, by deserting a nest late in served higher predation rates at parasitized the laying sequence,whereas early desertion in Yellow-shouldered Blackbird neststhan at non- responseto disturbancesis more likely to re- parasitized nests. The reason blackbirds were sult in the production of additional clutches. exceptional is not clear, but may be related to differences between host and cowbird in their ACKNOWLEDGMENTS stealth at approaching the nest; i.e., cowbirds Throughout my study, I was supportedby the Endangered may be less surreptitious in their approach, Wildlife ResearchProaram of the U.S. Fish and Wildlife allowing detection of the nest by avian pred- Serviceand the U.S. F&est Service. I thank Ray Erickson, H. Randolph Perry, Jr., and Frank H. Wadsworth for their ators. Rate of parasitisim may be comparable encouragement. I received additional logistical support with that for predation in that parasites use from the U.S. Navy and from Sean Fumiss, Caribbean techniquesand cuesto locate host nestswhich Islands National Wildlife Refuge Manager. are like those employed by some nest predators For their assistancein the field, I thank Wayne Arendt, Cheryl Belitsky, Julio Cardona, JoseCol6n, Scott Corbett, (Wiley 1982). CarlosDehumoy, Joseph diTomaso, Nelson Green, Quamie For the most part, desertion rates of para- Greenaway, Mary Hickman, Elizabeth Litovich, Alyson sitized speciesin my study populations were Nethery, Eduardo Santana,Dwight Smith, John Taapken, similar between parasitized and non-parasit- Beth Wiley, Megan Wood, and Michael Zamore. ized nests.That this defense is available in the I thank A. Cruz. M. Gochfeld. and P. Svkes. Jr.. for their many helpful suggestionsto improve an earlier draft behavioral repertoire of the host population of this work. and is regularly used in contexts other than This researchwas submitted in partial fulfillment of the avoiding parasitism suggeststhat desertion is requirementsfor a Ph.D. degreeat the University of Miami, a general response to nest disturbances. The Coral Gables,Florida. For their helpful suggestions,I thank higher frequency of nest desertion at parasit- the members of my graduatecommittee: Theodore Flem- ing, Steven Green, Julian Lee, Earl Rich, and C. Richard ized over non-parasitized nests in one popu- Robins. I am particularly grateful for the guidance and lation (Yellow Warbler at Boqueron Forest) warm friendship given me by Oscar Owre. may indicate the adaptiveness of this defense behavior in avoiding parasitism. Hosts have LITERATURE CITED had little time to evolve anti-parasite defenses, so that the single Yellow Warbler population BIAGGI, V. 1970. Las Aves de Puerto Rico. Editorial with higher desertion rates among parasitized Univ. Puerto Rico, RIO Piedras. BOND, J. 1966. Eleventh supplement to the Check-list nests may represent an early manifestation of of Birds of the West Indies (19561. Academv of Nat- a pre-adaptive behavior application. This be- ural Sciences,Philadelphia, PA. ’ havior may later become fixed if it is effective BUCKLEY, P. A., AND F. G. BUCKLEY. 1970. Notes on in reducing parasitism. Alternatively, gain of the distribution of some Puerto Rican birds and on the courtship behavior of White-tailed Tropicbirds. fitness in using desertion to avoid parasitism Condor 72:483-486. may not be great enough for it to become more BUELL. P. F.. AND P. DANSEREAU. 1966. Analvsis and common as a specific defense against parasit- mapping of the Roosevelt Roads area, p. 46-62. In ism. Studieson the vegetation of Puerto Rico. II. Institute In the parasitized speciesI studied, nest de- for Caribbean Sciences.Univ. of Puerto Rico. Mava- guez, Puerto Rico Sped. Publ. No. 1. _ sertion may have been related to host repro- CLARK, K. L., AND R. J. ROBERTSON. 1981. Cowbird ductive investment; i.e., probability of deser- parasitism and evolution of anti-parasite strategies in tion declined steadily with number of host eggs the Yellow Warbler. Wilson Bull. 93:249-258. laid (see Clark and Robertson 198 1 for similar ELLIOTT, P. F. 1978. Cowbird parasitism in the Kansas tallgrass prairie. Auk 95: 161-167. results with hosts of the Brown-headed Cow- EWEL, J. J., AND J. L. WHITMORE. 1973. The ecological bird). Early desertion of nests would result in life zones of Puerto Rico and the U.S. Virgin Islands. minimal loss of time and energy investment. U.S. Dep. Agric. For. Serv. Res. Pap. ITF- 18. Also, the potential for successful renesting FRIEDMANN, H., L. F. KIFF, AND S. I. ROTHSTEIN. 1977. would be high with early desertion. However, A further contribution to knowledge of host relations of the narasitic cowbirds. Smithson. Contrib. Zool. pairs who deserted complete or nearly-com- No. 235. plete clutcheswould encounter a delay of sev- FRAGA, R. M. 1978. The Rufous-collared Sparrow as a eral days to construct a replacement nest (as host of the Shinv Cowbird. Wilson Bull. 90:27 l-284. would early deserters), build up energy re- GOCHFELD, M. 1979. Brood parasite and host coevolu- tion: interactions between Shiny Cowbirds and two servesfor an additional clutch, and lay the egg speciesof meadowlarks. Am. Nat. 113:855-870. set. Although time limitations are not as con- GRAYCE, R. L. 1957. Range extensionsin Puerto Rico. strainingin Puerto Rico asthey are in the north, Auk 74: 106. 176 JAMES W. WILEY

HOLLANDER,M., AND D. A. WOLFE. 1973. Nonpara- POST,W., AND J. W. WILEY. 1977b. Reproductive in- metric statisticalmethods. John Wiley and Sons,Inc., teractions of the Shiny Cowbird and the Yellow- New York. shoulderedBlackbird. Condor 79: 176-l 84. HOY, G., AND J. OTTOW. 1964. Biological and oological PUERTORICO DEPARTMENTOF NATURAL RESOURCES. studiesof the molothrine cowbirds (Icteridae) of Ar- 1976. The master plan for the Commonwealth for- gentina. Auk 8 1:186-203. ests of Puerto Rico. San Juan, Puerto Rico. HUDSON,W. H. 1920. The birds of La Plata. J. M. Dent ROTHSTEIN,S. I., J. VERNER,AND E. STEVENS.1980. Range and Sons, Ltd., London. expansion and diurnal changesin dispersion of the KING. J. R. 1973. Renroductive relationshinsof the Ru- Brown-headed Cowbird in the Sierra Nevada. Auk f&s-collared Sparrow and the Shiny Cbwbird. Auk 971253-267. 90: 19-34. SOUTHERN,W. E. 1958. Nesting of the Red-eyed Vireo MAY~ELD,H. 1960. The Kirtland’s Warbler. Cranbrook in the Douglas Lake region, Michigan, Part 2. Jack- Inst. Sci. Bull. 40. Pine Warbler 36: 185-207. MAYL~ELD,H. 1965. The Brown-headed Cowbird, with WALKINSHAW,L. H. 1983. Kirtland’s Warbler: the nat- old and new hosts. Living Bird 4: 13-28. ural history of an endangeredspecies. Cranbrook In- MAYFIELD,H. 1972. Third decennialcensus ofKirtland ’s stitute of Science, Bloomfield Hills, MI. Warbler. Auk 89:263-268. WILEY, J. W. 1982. Ecology of avian brood parasitism OWEN,J. H. 1933. The cuckoo in the Felsted district. at an early interfacing of host and parasite popula- Rep. Felsted School Sci. Sot. 33:25-39. tions. Ph.D. diss., Univ. of Miami, Coral Gables, FL. PAYNE.R. E. 1977. The ecology of brood parasitism in ZAR, J. H. 1975. Biostatistical analysis. Prentice-Hall, biids. Annu. Rev. Ecol. Sy%. 811-28. _ NJ. POST. W.. AND J. W. WILEY. 1976. The Yellow-shoul- derei Blackbird-present and future. Am. Birds 30: Puerto Rico Field Station, Patuxent Wild&if Research 13-20. Center,U.S. Fish and WildlifeService, P.0 Box 21, Palm- POST,W., AND J. W. WILEY. 1977a. The Shiny Cowbird er, PuertoRico 00721. Received 3 November 1983. Final in the West Indies. Condor 79: 119-l 2 1. acceptance31 December 1984.

The Condor 87:176 0 The Cooper Ornithological Society 1985

RECENT PUBLICATIONS

The grouseof the world.-Paul A. Johnsgard.1983. Uni- Ecological study of bird hazards at Indian aerodromes. versiiy of Nebraska Press, Lincoln. 413. p. $42.50. The Phase II, First &nual report (1982-83).-Salim Ali and sixteen speciesof grouse and ptarmigans are distributed Robert B. Grubh. 1984. Bombay Natural History Society. over desert scrub,grasslands, forests, and tundra of North 96 p. Paper cover. No price given. Source:Bombay Nat. America and Eurasia.As important game birds, they have Hist. Sot., Hombill House, Shahid Bhagat Singh Road, received enormous attention yet no comprehensivetreat- Bombay 400 023, India. Bird strikes to aircraft are a se- ment sinceElliot ’s (1864-1865) monograph.That lack has rious hazard in India becauseenvironmental conditions now been admirably met with this book, the latest of at airfields attract large numbers of vultures and kites, as Johnsgard’s treatises on avian families. Its plan is con- well as other birds. Researchersof the Bombay Natural ventional: a comparative overview of the group, followed History Societyhave investigatedthis problem for several by speciesaccounts. The overview, however, is consid- years, sponsoredby the Aeronautics R&D Board in the erably fuller here than it was in his books on waterfowl, Government of India’s Ministry of Defense. This report cranes,and shorebirds.Occupying nearly one-third of the opens with an analysisof Indian bird strike data and then text, it surveysgrouse with regard to their evolution and examines the problem at three airports (additional to two taxonomy, physical and physiological traits, hybridiza- which were studied earlier). Following chaptersdeal with tion, breeding biology, population dynamics, social be- potentially hazardousspecies ofbirds and with factorsthat havior and vocalizations, aviculture, hunting and conser- attract them in and around airports. Follow-up work at vation. The chapters on individual speciesare organized the two first-studied airports and the causesof bird strikes as to range and subspecies,measurements, identification, enroute are discussed.In closing,the report makes several field marks, age and sex criteria, distribution, population recommendations for preventive measuresto reduce the density, habitat requirements,food and foraging, mobility numbers of problem birds at and around airports. Since and movements, reproductive behavior, and evolutionary these suggestionsarise from an ecological study of the relationships. The volume is generouslyillustrated with situation, they are sensibleand likely to be more effective color and monochrome photographs,range maps, and the in the long run than the mass killing of vultures, which author’s pen-and-ink drawingsof displays.Obviously, this has been advocated as an immediate solution. These spe- book will be widely useful as an inclusive source of in- cific proposalshave little or no application elsewhere,yet formation. In addition, it should remind North American the approach leading toward them should be universal. researchersof how much is known about Old World grouse Teachers of ornithology might well use this material as and ptarmigans, and facilitate more comparative evolu- another example of the interactions between birds and tionary thinking about the group. Appendices, references, human affairs-and how to alleviate them. Illustrations, index. appendices.