The Condor 99:151-161 0 The CooperOrnithological Society 1997

EXPLANATIONS FOR THE INFREQUENT COWBIRD PARASITISM ON COMMON GRACKLES ’

BRIAN D. PEERS AND ERIC K. BOLLINCER Department of Zoology, Eastern Illinois University, Charleston, IL 61920

Abstract. We determined the factors responsible for the lack of parasitism on Com- mon Grackles (Quiscalus quiscula) by Brown-headed Cowbirds (Molothrus a&r). We found no evidence of parasitism on the 401 grackle nests we monitored in east-central Illinois. By the time cowbirds began laying eggs, 88.5% of all grackle nests were be- yond the point of successful parasitism. Grackles rejected cowbird eggs more frequently during the prelaying stage of the nesting cycle (38.2%) compared to later stages (12.3%). Thirty-three cowbird eggs and nestlings were cross-fostered into grackle nests. Data were collected on 15 cowbird nestlings, of which three fledged. The cross-fostered cowbird eggs that did not hatch appeared to have been incubated ineffectively. These eggs were in clutches that contained four to six eggs, whereas the cowbird eggs that hatched were in clutches of three eggs or less. Grackles also responded more aggressively toward fe- male cowbird models than to Fox Sparrow (Passerella iliaca) models in five of seven response categories. Cowbirds avoid parasitizing grackles even though the opportunity existed. Nest abandonment and infestation of grackle nests with ectoparasitic mites may decrease the suitability of grackles as hosts; however the primary reason for the infre- quent cowbird parasitism on grackles appears to be past rejection behavior. Grackles likely rejected cowbird eggs at a higher frequency in the past and as a consequence grackles were avoided by cowbirds. Rejection apparently decreased in the absence of parasitism due to the high degree of intraclutch egg variability exhibited by grackles which would increase the chances of them mistakenly rejecting their own discordant eggs. Key words: brood parasitism, Brown-headed Cowbird, Common Grackle, egg rejec- tion, host selection, Molothrus ater, Quiscalus quiscula.

INTRODUCTION sites’ egg hatching will decrease (Wiley 1985,

Avian brood parasitism is an uncommon repro- Peer and Bollinger, in press a). After the para- ductive strategy in which females lay their eggs sitic egg hatches, the nestling requires an ad- in the nests of other birds thereby relinquishing equate diet (Friedmann 1929, Middleton 1991) further parental care to the host species. To and parental care (Friedmann 1929, Mills maximize their reproductive effort, brood para- 1988). Finally, host nestlings must have growth sites must locate compatible hosts with life his- rates similar to the parasites’ because if they tory traits similar to their own. Thus, the breed- grow too quickly the parasitic nestling will be ing seasons of parasite and host must overlap at a disadvantage (Friedmann 1963, Fraga (Briskie et al. 1990, Ortega and Cruz 1991) and 1985). hosts should not possess any significant anti- The Brown-headed Cowbird (Molothrus ater) parasite adaptations such as rejection of the is the only obligate brood parasite that is wide- parasitic egg (Rothstein 1975a). spread in North America. It has parasitized at Once a parasites’ egg is in a hosts’ nest it least 220 species of birds, 144 of which have must be incubated effectively to ensure hatch- successfully reared cowbirds (Friedmann and ing (Friedmann 1929, Hofslund 19.57, Rothstein Kiff 1985). The Common Grackle (Quiscalus 1975a). If the hosts’ eggs are considerably quiscula) is an infrequent host of the cowbird, larger than the parasites’ or if the clutch size is with only 20 documented cases of parasitism excessively large, then the chances of the para- (Peer and Bollinger, in press b), and Common Grackles have never been known to fledge cow- birds (Friedmann and Kiff 1985). This is unusual because the Common Grackle is wide- 1 Received 4 January 1996. Accepted 24 October spread and abundant, and ostensibly both grack- 1996. 2 Present address: Department of Zoology, Univer- les and cowbirds have had overlapping ranges sity of Manitoba, Winnipeg, MB R3T 2N2, Canada. and habitat requirements throughout their evo-

[I511 152 BRIAN D. PEER AND ERIC K. BOLLINGER

lutionary histories (Mayfield 1965, Fretwell a lake edge, and a residential park. Each site 1973). Furthermore, Common Grackles build was bordered on at least one side by agricul- large, conspicuous nests and usually accept tural fields. cowbird eggs (Rothstein 1975a, Peer 1993). Friedmann et al. (1977:39) made the following BREEDING SEASON statement regarding this enigmatic relationship: We recorded the dates of initiation of grackle “The reason for the lack of parasitism [on clutches and egg-laying dates of cowbird eggs grackles] is not clearly known. The cow- found in 1992 and 1993. Only eggs whose lay- bird may avoid parasitizing species as large ing dates could be determined by direct obser- as the grackle, but the American Robin and vation were used in this study. the Brown Thrasher are nearly as large and have been found to be parasitized many ARTIFICIAL PARASITISM more times than the grackle, even though Artificial cowbird eggs were made of wood, they are rejecter species. Perhaps the grack- painted with waterbased acrylic paints, and les’ colonial nesting may be a factor. It may coated with a clear acrylic sealer. Their dimen- be difficult for cowbirds to escape detec- sions were 23.91 X 16.67 mm and they weighed tion when entering grackle colonies. But 2.5 g. Real cowbird eggs average 2 1.45 X 16.42 many grackles do not nest in colonies, in mm (Bent 1958) and weigh 3.17 g (Ankney and which case other factors may be respon- Johnson 1985). Thus, we feel that our eggs were sible for the low incidence of parasitism.” an effective mimic of real cowbird eggs. In this study we attempted to determine the In 1992, we attempted to simulate natural factors responsible for the lack of cowbird cowbird parasitism by replacing a single grackle parasitism on Common Grackles by (1) deter- egg with an artificial cowbird egg, or genuine mining the synchrony of grackle and cowbird cowbird eggs when available. Although there is breeding seasons, (2) placing artificial cowbird variation in the frequency of host-egg removal eggs into grackle nests to determine whether by cowbirds (Sealy 1992), we followed the there has been a change in the grackles ’ same protocol used by Rothstein (1975a). We egg-rejection frequency since Rothsteins’ also artificially parasitized grackle nests with (1975a) original study, (3) cross-fostering cow- real grackle eggs collected from nonexperimen- bird eggs and nestlings into grackle nests to tal grackle nests during the 1993 season. These determine if cowbirds could fledge from manipulations were performed to determine grackle nests, (4) testing if coloniality is an whether rejection behavior was in response to effective deterrent against brood parasitism by intra- rather than interspecific parasitism. comparing the responses of colonial- and We categorized each nest into one of three noncolonial-nesting grackles to models of fe- stages of the nesting cycle: (1) Prelaying, nest male cowbirds, and (5) observing if noncolo- construction appeared to be complete but no nial grackles are parasitized more frequently eggs had been laid, (2) Laying, eggs were be- than colonial grackles. ing laid, and (3) Incubation, the clutch was com- plete and being incubated. Grackles often be- gin incubation prior to clutch completion (Max- MATERIALS AND METHODS well and Putnam 1972), however we did not STUDY SITE classify nests as in the incubation stage until We monitored grackle nests at 10 sites through- laying was completed. We categorized nests as out Coles County, Illinois during the 1992 and colonial or noncolonial based on the distance 1993 breeding seasons. We collected most of between nearby grackle nests. Colonies con- our data at four sites, including two cemeteries sisted of at least three nests within 10 m of each and two Christmas tree farms. The cemeteries other. Colonial-nesting grackles at our sites contained scattered rows of northern white ce- formed very cohesive groups and responded to dar (Thuja occident&s), 2-3 m in height. The the alarm calls of conspecifics from distances Christmas tree farms had evenly distributed of at least 10 m (pers. observ.). rows of scotch pine (Pinus sylvestris), 2-2.5 m We added one artificial cowbird egg to each in height. The remaining sites consisted of sev- nest between 06:OO and 12:00 CDT because eral roadside thickets, a small nature preserve, cowbirds typically confine egg-laying and host- INFREQUENT PARASITISM ON GRACKLES 153 searching activities to the morning hours (Roth- of the second model. Following the presentation stein et al. 1984, Neudorf and Sealy 1994). of each cowbird model a grackle egg was re- Nests were inspected every l-2 days to record placed with an artificial cowbird egg. No nests host response, and nests were parasitized dur- were tested with the models more than once. ing the entirety of the breeding season. Re- Behavioral responses were recorded by the sponses were considered “rejections” if the ar- same individual (BDP) in all trials and included tificial egg was ejected from the nest, pecked, the following: alarm calls, fly-by investigation, buried under nest lining, or if the nest was de- nest attentive (bird situated between the model serted. We considered a nest deserted if it was and the nest), hovering near the model, indi- abandoned within five days of egg replacement vidual attack, mob by 22 individuals, and (Rothstein 1975a). physically striking the model. Once a model was physically struck the trial ended to preserve CROSS-FOSTERING COWBIRD the model. EGGS AND NESTLINGS STATISTICAL ANALYSES Cowbird eggs and nestlings were collected from other species’ nests in and near our study sites. We used chi-square tests to analyze the egg re- We replaced single grackle eggs with one and jection data. Growth rates of grackles and cow- in some cases two cowbird eggs. All transferred birds were compared from days 2 through 8. cowbird eggs had been laid within l-2 days of Growth was linear during this period, therefore the time the grackle eggs had been laid. In some linear regression was used to determine the cases we cross-fostered cowbird nestlings into slope for each individual bird. To compare mea- nests with grackle nestlings. In all but one of surements of grackles and cowbirds, we used an these cases the nestlings were the same age, the approximate t-test that did not assume equal only exception being a cowbird nestling that variances. In this test, the degrees of freedom was a day younger than its two grackle nest- are determined by approximation and are con- mates. servative (i.e., fewer dfs) relative to t-tests that Nestlings were weighed every l-2 days to the assume the variances are equal (Ryan et al. nearest gram using 50-g and 100-g Pesola 1976). Both standard deviations for linear mea- scales. We measured gape (width of bill at loral surements and standard errors for means were feathering), exposed culmen, and tarsometatar- used as measures of variance. We used P < 0.05 sus length to the nearest 0.01 mm with calipers as our level of accepted significance. according to Baldwin et al. (1931). Grackle and cowbird egg lengths and widths were measured RESULTS to the nearest 0.01 mm. BREEDING SEASON PHENOLOGY The first grackle eggs were laid on 23 March RESPONSE TO COWBIRD MODELS 1992 and 1 April 1993. Clutch initiation for the The aggressive response of grackles to cowbirds two years combined peaked from 12 to 25 April, was tested in 1992 using taxidermically pre- when 215 of 401 (53.6%) clutches were initi- pared models of female Brown-headed Cow- ated (Fig. 1). The first cowbird eggs were found birds. Mounts of Fox Sparrows (Passerella ili- on 16 May 1992 and 17 May 1993 in the nests aca) were used as controls. Although Fox Spar of other hosts. Cowbird egg laying peaked from rows do not breed in Coles County, they are a 24 May to 6 June, encompassing 28 of 45 common spring migrant throughout the study (62.2%) eggs found (Fig. 1). By the time the area during the early portion of the grackle first cowbird egg was laid on our study area in breeding season. Mounts were prepared in the both years, 355 of 401 (88.5%) grackle nests same way and mounted in upright positions. were in the incubation stage or later and thus Models were placed approximately 0.5 m beyond the point of being successfully parasit- from a grackle nest and at the same level as the ized. nest. The Fox Sparrow model was presented first in one half of the trials, and the cowbird FREQUENCY OF COWBIRD PARASITISM model was presented first in the other half of None of the 401 grackle nests that we monitored the trials. Each model was presented for 5 min was naturally parasitized (Table 1). Northern with a 10 min interval before the presentation Cardinals (Cardinalis cardinalis), Song Spar- BRIAN D. PEER AND ERIC K. BOLLINGER

m Common Grackle (n = 401) sertion (21.9%), pecking the cowbird egg 0 Brown-headed Cowbird (n - 451 (12.5%), and burial (6.2%). Rejections occurred significantly more often during the prelaying stage (13 of 34 nests, n 38.2%) than later in the nesting cycle (19 of 154, 12.3%; xzl = 13.9, P < 0.001). There were no significant differences in egg rejection frequencies between nests presented with cow- bird models compared to those without models during the prelaying (2, = 3.1, P > 0.05) or later stages (xzl = 0.09, P > 0.75). Real cow- Date bird eggs were rejected at similar frequencies FIGURE 1. Percentagesof Common Grackle nests to artificial cowbird eggs (21.0%, 3 of 14 nests, initiated and Brown-headed Cowbird eggs laid in x2, = 0.18, P > 0.50), and all 10 grackle eggs Coles County, Illinois during the 1992-1993 breed- introduced into grackle nests were accepted. ing seasons. CROSS-FOSTERING EXPERIMENTS Thirty-three cowbird eggs and nestlings were rows (Melospiza melodia), and House Finches cross-fostered into grackle nests. Data were col- (Carpoducus mexicanus) were the most com- monly parasitized hosts (Table I), with 85.7% lected on 15 nestlings, of which 10 hatched from transferred eggs and five were nestling of cardinal nests parasitized, 60.0% of Song transfers. In no instance did two cowbird eggs Sparrow nests, and 58.3% of House Finch nests. hatch in a single nest. Three cowbirds fledged. EGG REJECTION One of the cowbirds had no competitors, and Cowbird eggs were rejected at 32 of 188 the other two cowbirds hatched two and four (17.0%) experimentally parasitized nests. Roth- days before any of the grackles. In three cases, stein (1975a) recorded eight rejections from 70 cowbird nestlings died 2-3 days after hatching nests (11.4%). However, as Rothstein per- despite being the sole occupant of the nest. The formed no egg additions during the prelaying other nine cowbird nestlings died in the pres- stage, a direct comparison of our data without ence of grackle nestmates an average of 2.1 prelaying trials yields a similar rejection fre- days after hatching. quency of 12.3% (19 of 154 nests; xzl = 0.02, Grackle hatchlings weighed more than twice P > 0.75). Most rejections (n = 32) in our study as much as cowbird hatchlings (t-test, P < were by egg ejection (59.4%), followed by de- 0.001; Table 2). The differences between

TABLE 1. Frequency of Brown-headed Cowbird parasitism on hosts in Coles County, IL from 1992-1993.

Number of cowbird eggs per nest Species 0 1 2 3 24

Mourning Dove (Zenaidu mucrouru) 102 - Blue Jay (Cyanocittu cristatu) 1 - - Brown Thrasher (Toxostomurufum) 4 - - Gray Catbird (Dumetella curolinensis) 8 - American Robin (Turdus migrutorius) 14 - - Wood Thrush (Hylocichlu mustelinu) 2 Red-eyed Vireo (Vireo olivaceous) - 1 Yellow Warbler (Dendroicu petechia) 1 - - Common Yellowthroat (Geothlypis trichus) 1 Red-winged Blackbird (Ageluius phoeniceus) 339 5 - Common Grackle (Quisculus quisculu) 401 Baltimore Oriole (I&rus gal&u) 1 - - Northern Cardinal (Cardinalis cardinalis) 2 6 4 2 House Finch (Curpbducusmexicanus) 5 5 2 Chipping Sparrow (Spizellu passerinu) 2 1 - Song Sparrow (Melospiza melodiu) 2 1 2 INFREQUENT PARASITISM ON GRACKLES 155

TABLE 2. Hatching values (mean t SD) for mass, gape, culmen, and tarsometatarsusfor Common Grack- les (n = 42) and Brown-headed Cowbirds (n = 8) during the 1992-1993 breeding seasons.a

Measurement Grackle Cowbird tb P df

Mass k) 5.57 * 0.86 2.53 ? 0.65 11.5

a Tarsometatarsus lengths were recorded only m 1993. For grackles n = 20 and for cowbirds n = 3. b Two-tailed r-test.

grackle and cowbird gape width, culmen length, EGG SIZE and tarsometatarsus length also were signifi- The average size of grackle eggs was 28.76 -t cantly different at hatching (Table 2). Among 1.4 X 21.36 -C 0.8 mm (n = 131), and individuals that fledged, growth rates of grack- cowbird eggs averaged 20.38 + 1.2 X 16.05 -C les also were significantly greater than those of 0.6 mm (n = 19). Of the eight cowbird eggs the cowbirds (t-test, t, = 4.21, P < 0.025). that did not hatch, four appeared to be in- Cowbirds required 2.5 days to reach the hatch- effectively incubated (the remaining cowbird ing values of grackles for weight, gape, and eggs that did not hatch were depredated, tarsometatarsus, and 3.5 days for the culmen rejected, or infertile). These eggs were in (Fig. 2). clutches containing four to six eggs, whereas

(a) Mass

(d) Tarsometatarsus

Age (days) Age Ww) FIGURE 2. Relationship between age and (a) mass (g), (b) gape width (mm), (c) culmen length (mm), and (d) tarsometatarsuslength (mm) for nestling Common Grackles and Brown-headed Cowbirds for the first 10 days after hatching (mean ? SD). 156 BRIAN D. PEER ANDERIC K. BOLLINGER

TABLE 3. Responses of Common Grackles to model female Brown-headed Cowbirds and Fox Sparrows presented near the nest (n = 105 nests). Values indicated are the mean number of seconds (&SE) given per 5 min trial.

ReSPOIlSe Cowbird SParrow Pa

Alarm calls 10.5 ? 0.84 4.8 Z 0.74 0.047 Fly-by investigation 23.1 -t 1.73 5.3 ? 0.62 0.001 Nest-attentive 58.1 2 3.78 26.3 2 2.11 0.001 Hover 25.2 5 2.13 14.4 -c 3.29 0.007 Individual attack 1.9 ? 0.23 2.9 ? 0.72 ns Mob by 22 individuals 18.0 + 1.54 11.9 ? 1.64 Physical contact 29.4 2 1.91 7.5 2 1.48 ?005

a Results of Wilcoxon signed-ranks test between models. the eggs that did hatch were contained in press b) and other icterines (Orians 1961, Or- clutches of three eggs or less. tega and Cruz 1991), which presumably makes them less reliable hosts. RESPONSE TO COWBIRD MODELS Grackles responded more aggressively toward EGG REJECTION the cowbird models than to the Fox Sparrow Rothstein (1975b) classified the grackle as an models in five of the seven behavioral catego- accepter species that is rarely parasitized. Rela- ries (n = 105 nests; Table 3). There were tively few grackle nests were available for para- no significant differences in aggressive re- sitism over the course of our study and those sponse to cowbird models between colonial- that were available were not parasitized, indi- and noncolonial-nesting grackles. cating that there is no current selection pressure on grackles to reject parasitic eggs. This is sup- DISCUSSION ported by the fact that there has been no change BREEDING SEASON PHENOLOGY in the rejection frequency of grackles since The early initiation of the grackle breeding sea- Rothstein’s (1975a) study. son may contribute to the lack of parasitism, as Thus, the following question arises: why does only 11.5% (n = 46) of all grackle nests were the rejecter genotype exist at a low level within available for parasitism once cowbirds began grackle populations? Species that have not been laying. This is similar to other species where the subjected to brood parasitism, whether it be absence or low level of parasitism has been intra- or interspecific, generally do not exhibit linked to mismatched breeding seasons with egg rejection behavior as it has no adaptive cowbirds (Briskie et al. 1990, Ortega and Cruz value in the absence of parasitism (Rothstein 1991). Yet, there were more grackle nests avail- 1982b, 1990). The only apparent exceptions are able for parasitism compared to any of the other ground-nesting seabirds that breed in dense hosts at our study sites, with the exception of colonies and risk confusing their eggs with oth- the Red-winged Blackbird (Agelaius phoeni- ers (Tschanz 1959, Birkhead 1978). ceus; see Table 1). Instead of parasitizing grack- There are four potential explanations for the les, cowbirds parasitized individual nests of existence of this behavior in grackles. First, it other hosts repeatedly (such as Song Sparrows; may be in response to intraspecific nest para- Table 1). This, in conjunction with the relatively sitism. Grackles nest semi-colonially and there few cases of parasitism on grackles that have exists ample opportunity for conspecific nest been recorded (Peer and Bollinger, in press b), parasitism. However, recent studies have shown clearly indicates that cowbirds avoid parasitiz- that despite the coloniality of many icterine spe- ing grackles. cies there is little evidence of intraspecific para- Another factor that may contribute to the sitism (Harms et al. 1991, Lyon et al. 1992, avoidance of grackles is that they sometimes Rothstein 1993). We recorded only one incident abandoned nests later in the breeding season. In of intraspecific parasitism in 401 nests (0.2%), May of 1992, grackles abandoned their nests at and only two other cases have been documented three of our study sites. This behavior occurs in Common Grackles (Peer and Bollinger, in frequently in grackles (Peer and Bollinger, in press b). Furthermore, grackles did not reject INFREQUENT PARASITISM ON GRACKLES 1.57 any of the conspecific eggs that were added to cowbirds’ initial hosts during the development their nests. Therefore, it seems unlikely that egg of the parasitic habit. He also stated that grack- rejection has resulted from intraspecific nest les are rejecters of cowbird eggs, but did not parasitism. present any supporting evidence. Larger species Second, egg rejection may result from an ad- in general were likely parasitized more often in aptation that is entirely unrelated to brood para- the past as all but four rejecters of Brown- sitism. Rothstein (1975a) speculated that egg- headed Cowbird eggs, the Cedar Waxwing robbing species such as the Blue Jay (Cyano- (Bombycillu cedrorum), Bullocks’ Oriole (1. cittu cristata) may be ejecters because of the bullockii), Baltimore Oriole (Rothstein 1975a, egg-robbing habit, rather than it having any- 1977) and Warbling Vireo (Vireo gilvus; Sealy thing to do with cowbird parasitism. The me- 1996) are larger than the cowbird. Cowbirds chanics used in egg predation are similar to subsequently may have been forced to parasit- those required in egg ejection. Rothstein dis- ize smaller hosts once the frequently parasitized counted the egg predation hypothesis and sug- larger species began rejecting cowbird eggs gested that rejection should be more variable (Rothstein 1975a, Mason 1980, Peer and than the 100% recorded for Blue Jays. The Bollinger, in press a). grackle is an egg predator (Peer and Bollinger, It is unlikely that egg rejection has evolved in press b) and its rejection behavior is variable. recently in grackles considering that they are In addition to grackles, Gray Catbirds (Dume- not currently parasitized and have not been tella carolinensis), Baltimore Orioles (Zcterus parasitized in recent history. If grackles were re- galhula), Yellow-headed Blackbirds (Xantho- jecter species, it would have been advantageous cephalus xanthocephalus), and Black-billed for cowbirds to avoid parasitizing them because Cuckoos (Coccyzus erythropthalmus) are all their eggs would be rejected (Sealy and Bazin egg predators and also demonstrate some level 1995). As cowbirds began avoiding grackles, of egg rejection (Rothstein 1975a, Dufty 1994, the rejection frequency likely decreased from Sealy 1994). Rejecters of Shiny Cowbird (M. near 100% to its current level due to the costs bonariensis) eggs also tend to be egg predators of maintaining rejection behavior in the absence (Wiley 1982). of parasitism (see Cruz and Wiley 1989). The However, not all egg predators exhibit rejec- most obvious cost would be ejection of ones’ tion behavior. Corvids (Corvus, Pyrrhocorax own eggs (Marchetti 1992). Grackles exhibit an spp.) in Europe accept foreign eggs (Yom-Tov extremely high level of intraclutch egg variation 1976, Soler and Mgller 1990). Eastern (Stur- (M@ler and Petrie 1991, Peer and Bollinger, in nella magna) and Western (S. neglecta) Mead- press b, S. I. Rothstein, pers. comm.) and these owlarks, also egg predators (Schaeff and Pic- odd eggs would be rejected if grackles displayed man 1988, Picman 1992), are frequently para- the same level of discrimination that is demon- sitized by Brown-headed Cowbirds but appar- strated by typical rejecter species (Rothstein ently do not reject cowbird eggs (Rothstein 1982a). Egg rejection would then be selected 1975a, Friedmann et al. 1977, Friedmann and against in the absence of parasitism. Kiff 1985). As rejection has no apparent adap- Indeed, grackles appear to be more similar to tive value until a species is parasitized, it rejecter species than accepters, because they ex- seems unlikely that the egg predation habit hibit all of the traits of a rejecter. The first of alone results in rejection behavior. Egg preda- these traits is the method of egg rejection. The tion may enhance some hosts ’ ability to evolve most frequent means of rejection displayed by rejection once they are parasitized because of grackles in our study was ejection or pecking the similarity in mechanics between the two the cowbird eggs (72%). On the rare occasions behaviors. This would explain the large num- that accepter species reject parasitic eggs it is ber of egg-predators that also are rejecter spe- usually accomplished by nest desertion or burial cies. of the parasitic egg (Rothstein 1975a). Birds The third possible explanation is that the desert nests for a variety of reasons (Hill and grackle was parasitized by cowbirds more fre- Sealy 1994) and egg burial may simply be a quently in the past and as a consequence continuation of the nest building process (but evolved egg rejection. Fretwell (1973) sug- see Sealy 1995). In contrast, ejection and peck- gested that grackles were probably one of the ing of a parasitic egg are in direct response to 158 BRIAN D. PEER AND ERIC K. BOLLINGER that egg, and as a result are considered to be ness of incubation of the smaller cowbird eggs. clear evidence of true rejection behavior (Roth- In a subsequent study (Peer and Bollinger, in stein 1975a). Grackles also rejected a high per- press a), we demonstrated that it is necessary centage of cowbird eggs during the prelaying for a female cowbird to remove at least one stage (38%) unlike accepter species such as the grackle egg to ensure adequate incubation of her Eastern Phoebe (Suyornis phoebe) and Red- own egg. Cowbird eggs required shorter incu- winged Blackbird which accept cowbird eggs bation periods, hatched earlier than grackle eggs regardless of when the egg is introduced (Roth- more often, and exhibited the largest hatching stein 1975a, Rothstein 1986, Ortega and Cruz differential in smaller clutches. The frequency 1988). Other traits grackles share with rejecters of unhatched eggs that were considered fertile include eggs that contrast with those of cow- also was greater in larger clutches. birds, a long history of sympatry with the cow- Egg removal appears to be essential if cow- bird, large population size, the removal of fecal birds are to fledge successfully from grackle sacs, a large beak that allows for easy removal nests. The average cowbird incubation period is of the parasitic egg, and large, easily located 11-12 days (Rothstein 1975a), whereas the nests (Rothstein 1975a). grackles’ is 13.5 days (Peer and Bollinger, in The fourth possible explanation for the exist- press a). We found that cowbirds required 2.5- ence of rejection behavior is that it is an ances- 3.5 days to attain the size of a newly hatched tral trait. Rejection is common to all grackle grackle. If cowbirds hatch at the same time or species (Quiscalus) that have been tested, in- later than the grackles it is unlikely that they cluding Great-tailed Grackles (Q. mexicanus; will survive because grackles have faster Carter 1986), Greater Antillean Grackles (Q. ni- growth rates (this study) and grackles typically ger; Wiley 1982, Post et al. 1990), and Carib reduce their broods by starving their smallest Grackles (Q. lugubris; Post et al. 1990). This nestlings (Howe 1976). No grackle hatchlings indicates that egg rejection is an easily evolved under 4.5 g fledged, whereas the largest cow- trait in this genus, or perhaps, it evolved once bird hatchling was only 3.5 g. Each of the three and is lost so slowly that evidence of it can be cowbirds that fledged weighed at least 3.0 g at found in all grackles. If the latter is true, then it hatching. is possible that the Common Grackle was never The mortality of cowbird nestlings that had parasitized that often and simply inherited this no grackle nestmates cannot be linked to com- low level of rejection from an ancestor. How- petition with the larger grackle nestlings. It also ever, we do not favor this interpretation. The is unlikely that the type of food delivered to generalist nature of the Brown-headed Cowbird, cowbird nestlings is a constraint because grack- the large population size of the grackle, the long les, just like other passerines, feed their young history of sympatry with cowbirds, and the suit- mostly insects (reviewed in Peer and Bollinger, ability of the grackle as a host (see below) are in press b). Instead, the problem may be the size all compelling evidence that Common Grack- of the food. Howe (1979) noted that insects de- les should have been parasitized more often in livered to older grackle nestlings were too large the past. In addition, the Common Grackle is for younger nestlings. Adult grackles deliver unique in that it is the only species in North food that is suitable for their own nestlings, yet America that displays a low level of true rejec- it may be too large for cowbird nestlings of the tion behavior throughout its range (Rothstein same age. Only cowbird nestlings that weigh at 1975a). There also is an obvious source of se- least 3.0 g may be physically capable of han- lection against the retention of rejection behav- dling the larger food items. ior in the grackle, namely the high level of in- Nestling survival also may be affected by ec- traclutch egg variation. Further experiments on toparasitic mites. Our hands and arms were cov- the rejection behavior of the other grackle spe- ered by mites (Dermanyssidae) after handling cies will be beneficial in understanding the evo- nestlings. Although our observations are anec- lution of rejection behavior in this genus. dotal, it appeared that the mites contributed to the mortality of nestlings because infested nests EGG SIZE, INCUBATION, AND rarely produced fledglings. The abundance of NESTLING SURVIVAL ectoparasitic mites is likely the result of grackle Our data suggest that the size and number of nesting ecology. Grackles often constructed grackle eggs were correlated with the effective- their nests on top of old nests and members of INFREQUENT PARASITISM ON GRACKLES 159 this family of mites are known to overwinter in except for one, and yet no grackles were para- old nests (Phillis 1972). The incidence of ecto- sitized. Colonial-nesting and aggressive nest de- parasitic infestation also may decrease the suit- fense do not appear to be responsible for the in- ability of grackles as cowbird hosts (see Ortega frequent parasitism, because there was no para- and Cruz 1991). sitism on either colonial or solitary nesters. However, the low survivability of cowbirds Grackles are suitable hosts, although they ap- in host nests does not necessarily preclude para- pear to be at the uppermost size limit for suc- sitism. Fraga (1985) found that 78.1% of Chalk- cessful parasitism. Cowbirds are capable of browed Mockingbird (Mimus saturninus) nests fledging from grackle nests when the following were parasitized by Shiny Cowbirds despite the conditions occur: clutch size has been reduced fact that only 6.2% of cowbirds fledged from to ensure efficient incubation of the smaller the nests of this larger host. Therefore, the lack cowbird egg (see also Peer and Bollinger, in of parasitism on grackles does not appear to be press a), the cowbird hatches at least two days solely a result of the low survivability of cow- earlier than the grackle nestlings, and the cow- birds in grackle nests. bird hatchling weighs at least 3 g. Nest aban- donment and infestation of grackle nests with NEST DEFENSE ectoparasitic mites may decrease their suitabil- Grackles responded more aggressively toward ity as hosts, however the primary reason for the the cowbird models than to the Fox Sparrow infrequent parasitism on grackles appears to be models in five of the seven categories, which past rejection behavior. The grackle likely was suggests that they may recognize cowbirds as a parasitized more often in the past and as a con- unique threat. Yet, it is unclear why there were sequence evolved rejection behavior. Parasitism no differences in the frequency of individual at- of grackles would then be selected against to tacks and mobbing behavior directed toward the avoid wasting eggs on a rejecter species. Re- two models. Grackles may respond aggressively jection by grackles apparently decreased in the toward any species that comes near their nests absence of parasitism due to the high degree of which in effect deters parasitism. Grackles kill intraclutch egg variability exhibited by grack- and eat other passerines (Davidson 1994), so the les which would increase the chances of them risk of injury to the cowbird is great. mistakenly rejecting their own discordant eggs. Nevertheless, aggressive nest defense alone cannot explain the absence of parasitism. Nei- ACKNOWLEDGMENTS ther colonial- nor noncolonial-nesting grackles We thank the land owners who gave us access to their were parasitized, and there were no differences properties. We also would like to thank Julie D. Stan- in the responses between colonial and noncolo- daert for making the artificial cowbird eggs and nial nesters to the cowbird models. Wiley Roger Jansen for assisting in data collection. James V. Briskie, Kyle E. Harms, A. L. A. Middleton, and (1982) found that Greater Antillean Grackles Spencer G. Sealy reviewed earlier drafts of the manu- nesting in isolation and on the periphery of script and offered many useful suggestions. Stephen colonies were parasitized by Shiny Cowbirds I. Rothstein, James W. Wiley, and two anonymous more frequently than those in the middle of the reviewers provided valuable comments on the final colonies. This indicates that nesting in colonies draft. Financial support was provided by an S. Charles Kendeigh grant from the Champaign County decreases the frequency of parasitism but does Audubon Society to BDP, and by David and Karen not eliminate it. Therefore, we would expect Peer and George and Beverly Taylor. parasitism on at least the solitary nesting grack- les. Neudorf and Sealy (1994) also found that LITERATURE CITED Common Grackles only were present at their ANKNEY, C. D., AND S. L. JOHNSON. 1985. Variation nests for approximately half of the 30-minute in weight and composition of Brown-headed time period during which cowbirds were most Cowbird eggs. Condor 87:29&299. BALDWIN, S. P., H. C. OBERHOLSER,AND L. G. WORLEY. likely to parasitize them. Thus grackle nests are 1931. Measurements of birds. Sci. Publ. Cleve- vulnerable to parasitism. land Mus. Nat. Hist. 2:1-121. BENT, A. C. 1958. Life histories of North American CONCLUSIONS blackbirds, orioles, tanagers, and allies. U.S. Natl. Mus. Bull. 211. Despite the early initiation of the grackles’ nest- BIRKHEAD,T. R. 1978. Behavioural adaptations to ing season, there were more grackle nests avail- high density nesting in the Common Guillemot able for parasitism compared to all other hosts Uris aalge. Anim. Behav. 26:321-331. 160 BRIAN D. PEER AND ERIC K. BOLLINGER

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