Hosts feeding only Brown-headed Cowbird fledglings: where are the host fledglings?

ABSTRACT Brown-headed Cowbirds (Molothrus ater) impose costs on their hosts, usually resulting in fewer or no host young being raised. In 95% of the studies where attempts were made to estimate the cost of parasitism, cost was measured as the difference in host nestling survival in parasitized and unparasitized nests. For hosts that fledge at least some of their own young along with cowbirds, the magnitude of further brood reduction beyond the nestling stage is not known. To examine the possibility that further brood reduction occurs during the fledgling stage, we compiled 102 observations of 45 species feeding fledgling cowbirds. In 97 cases, observers did not record hosts feeding their own young, suggesting that many young die shortly after fledging from parasitized nests. These observations suggest that measuring the cost of parasitism as the difference between the number of host young fledged from parasitized and unparasitized nests probably overestimates the reproductive output of many hosts. If costs increase during the fledgling stage, then the costs of brood parasitism have been underestimated in many studies. El tordo (Molothrus ater) impone un costo a las especies que parasita, usualmente produciendo como resultado que el hospedero no pueda criar sus pichones. En el 95% de los estudios en donde se trató de determinar el costo del parasitismo, dicho costo fue medido como una diferencia en la supervivencia de los pichones del hospedero en nidos parasitados y en noparasitados. En nidos del hospedero en donde al menos voló un pichón de este, junto a pichones de la especie parasítica, la magnitud de la reducción en la productividad, a nivel de los volantones, se desconoce. Para examinar la posibilidad de mayor reducción durante la etapa de volantones, compilamos 102 observaciones de 45 especies alimentando volantones de tordo. En 97 de los casos, no observamos a la especie huesped alimentando a sus propios pichones, lo que sugiere que muchos pichones de nidos parasitados mueren durante la etapa de volantones. Estas observaciones sugieren que medir el costo del parasitismo como la diferencia en el número de pichones que dejan el nido (entre nidos parasitados y no parasitados), probablemente no determina adecuadamente la producción reproductiva de la especie parasitada. Si el costo aumenta durante la etapa de volantones, entonces el costo del parasitismo reproductivo ha sido subestimado en muchos estudios. Because more than 140 passerine species in North America are known to have raised Brown-headed Cowbirds (Molothrus ater; hereafter cowbirds; Friedmann and Kiff 1985), generally at some expense to the host's own reproductive output (Rothstein 1990, Robinson et al. 1995, Ortega 1998, Lorenzana and Sealy 1999), many investigators have attempted to quantify the cost of such parasitism. In 95% of the studies where the cost of brood parasitism was estimated, cost was measured as the difference between the number of nestlings that fledged from parasitized and unparasitized nests (Lorenzana and Sealy 1999). This measurement may underestimate the cost, however, because at nests where hosts also fledge with cowbirds (Table 1), fledgling cowbirds may out-compete host young in the days prior to independence (Smith 1981). Furthermore, brood reduction may occur during the fledgling period because the period of dependence on parental care of offspring after leaving the nest may be twice as long as the nestling period (Smith 1978). Because it is difficult to study fledglings, the possible effect of cowbird parasitism on survival of host fledglings has been studied infrequently (Woodward 1983) even though the cost of parasitism may continue to rise (May and Robinson 1985, Sealy et al. 1997). Only two attempts have been made to quantify the survival of host young in the first year after fledging. Indigo Buntings (scientific names are provided in Appendix) that fledged from parasitized nests were only 18% as likely as those that fledged from unparasitized nests to return the following season (Payne and Payne 1998). These authors did not determine whether the reduced survival of buntings from parasitized nests was due to deaths during or after the fledgling period. They did, however, determine that parasitism was more costly than previously believed when host brood reduction after fledging is considered. By contrast, the presence of cowbird young did not affect the survival of host fledglings in an insular population of Song Sparrows (Smith 1981). Our review was sparked by the compilation of Sealy et al. (1997) of 28 observations of 22 host species feeding fledged Bronzed Cowbirds (M. aneus), with no cases of host fledglings observed being fed. Many more records were available in the vast literature on hosts of the Brown-headed Cowbird, and we have compiled them here. We compiled anecdotal observations of hosts feeding cowbird fledglings. Only species known to fledge cowbirds along with their own young were considered in our study (Ortega 1998, Davis and Sealy 2000). However, we included observations of host species feeding young cowbirds for which fledging success of host nestlings when fledging a cowbird was not known. Published estimates of the reproductive success of hosts parasitized by cowbirds are available for all host species in Table 1. For most estimates, however, authors did not specify whether hosts fledged young and cowbirds from the same nests or whether host young only fledged from parasitized nests when the cowbird egg hatched late or did not hatch. Observations of host young and cowbirds that fledged from the same nests were available for nine species (Table 2). We included only observations where feeding of a fledgling cowbird by a host and the number of host fledglings, if any, were mentioned. For each observation, we recorded the species, number, and sex (if known) of the host adults feeding the fledgling(s), and number of cowbird and host fledglings being fed. Observations of cowbird fledglings fed by species that reject cowbird eggs and those with diets incompatible with cowbirds were not included. A pair was assumed to be a male and female in a sexually dichromatic host species. The presence or absence of host fledglings was noted when an author specifically mentioned it. We did not analyze data statistically because observations were anecdotal and likely biased due to the disproportional number of observations of host species caused by (1) the relative abundance of each host species, (2) conspicuousness of young of each species, and (3) amount of research conducted on each species (Skutch 1996, Sealy et al. 1997, Sealy and Lorenzana 1997). The nomenclature used follows the American Ornithologists' Union (1998, 2002). We compiled 102 records of hosts, representing 45 species from nine families, with fledgling cowbirds in their broods (Table 1). In 97 cases, only cowbird fledglings were being fed, and in 11 of these cases observers stated that no host young were being fed. Host fledglings also were present in five records. In one case, hosts never fed the cowbird fledgling, but fed their own fledglings (Table 1; Stewart et al. 1977). One or more than one host adult fed fledgling(s) in 72 and 30 cases, respectively. Feedings by species other than those assumed to be the hosts were observed in five cases. Of the species observed feeding cowbirds, 39 (87%), 4 (9%), and 2 (4%) have lower, similar, or greater body mass than a cowbird, respectively. Ninety-two cases involved hosts smaller than cowbirds. The observations reported in Table 1 suggest that host broods are reduced further during the fledgling period because most observers reported hosts feeding only cowbird fledglings and not their own young. If host young die after leaving the nest because they have been outcompeted by a cowbird during the nestling stage or early in the fledgling stage, then the reproductive cost of hosts does not end after fledging. If the cost of parasitism to hosts increases during the fledgling stage, as these observations suggest, the cost of parasitism has been underestimated in most studies where cost has been estimated. However, because the evidence is anecdotal and certain biases may be present in the results, further research on fledgling biology and the effects of brood parasitism during this stage is needed. Where hosts were not being fed, we cannot know whether host nestlings fledged and then died before they could be observed or whether they died in the nest. Therefore, it is not known whether brood reduction occurred during the nestling stage or fledgling stage, or both. Body size has been positively correlated with the ability to provision young and defend nests (Rothstein 1975, Trine 2000). Larger hosts are expected to be able to fledge more of their own young when raising a cowbird and smaller hosts are not expected to fledge any (Peer and Sealy 2004). However, because some cowbird hosts fledge one or more of their own young along with cowbirds (Table 2), it is possible that at least some host young died during the fledgling period. Furthermore, hosts as small as the Wilson's Warbler (7.7 g) have fledged cowbirds and their own young (Table 2), although, for the rest of the species in Table 1, the number of hosts that fledged with a cowbird(s) is not known. Despite the size distribution of hosts in Table 1 being skewed heavily toward species that are smaller than cowbirds (i.e., 92 of the observations are of hosts that are smaller than a cowbird), it is representative of natural cowbird host use because 16 of the 17 hosts parasitized most frequently by cowbirds weigh less than a female cowbird (Lowther 1993). A lack of effort on the part of observers searching for host fledglings could potentially lead to biased observations mentioning only cowbirds being fed (Skutch 1996). The greater conspicuousness of fledgling cowbirds compared to most host fledglings may facilitate detection of fledgling cowbirds, whereas host fledglings may be overlooked (Smith and Merkt 1980, Woodward 1983, Skutch 1996). In addition, we do not know whether the lack of a comment on the feeding of host young meant that the author did not observe host fledglings or simply did not record them, with the latter being more likely in instances where recording the cowbird as a breeding species for a particular area may have been of more concern than detailed observations of the host species feeding young. The results also may be biased if cowbirds are better able to survive as fledglings than hosts. Although fledgling survival rates of host species have been studied in few species, host fledglings in the absence of cowbirds have equal or greater survival rates than cowbird fledglings for most host species with known fledgling survival rates. Fledgling survival rates are known for cowbirds and five host species: 50% for White-crowned Sparrows (Barker et al. 1981), 55.7% for Ovenbirds (Hann 1937), 70% for Song Sparrows (Nice 1937), 86% for Golden-crowned Kinglets (Ingold and Galati 1997), 36% for Seaside Sparrows (Post and Greenlaw 1982), and 47.6% for Brown-headed Cowbirds (Woodward and Woodward 1979). With the exception of the Seaside Sparrow, observations of cowbird fledglings being fed in the absence of host fledglings are likely not attributable to host or cowbird survivorship through the fledgling period. Instead, these observations suggest that competition in the nest lowers the survivorship of fledged host young, perhaps because they weigh less or because of competition for parental care between host and cowbird fledglings. In support of this suggestion, previous studies have revealed that cowbird fledglings elicit disproportionately more parental care than host fledglings (Skutch 1996, Payne and Payne 1998). Both Eastzer et al. (1980) and Woodward (1983) found that cowbirds were more successful as fledglings than other passerines when fed by Barn Swallows, but fared no better as nestlings. Hosts fed cowbird fledglings at a higher rate than they fed their own fledglings per unit mass (Woodward 1983), suggesting that they compete better than fledglings of other species. Woodward (1983), however, reported that cowbird fledglings were fed in the absence of host young. As only one host parent fed the cowbird in 71% of the observations (Table 1), the brood division and care for certain young, potentially involving one parent feeding the cowbird and the other feeding its own young, may explain why hosts observed feeding cowbirds were not observed feeding their own young (Skutch 1976). Of the species in Table 1, brood division has been observed in 14 (Table 3) and remains undocumented in the rest. As host species that divide their broods are involved in 29 of the 72 observations involving only one parent, brood division is a possible explanation for the observations of cowbird fledglings being fed by one parent in the absence of host young. Nevertheless, brood division does not explain the absence of host young in observations where two host parents fed a cowbird fledgling. Despite these possible biases, the observations compiled here suggest that measuring the cost of parasitism as the difference between the number of host young fledged from parasitized and unparasitized nests probably overestimates the reproductive output of many hosts (see also May and Robinson 1985). A complete estimate of the cost of parasitism requires quantification of the cost also incurred during the fledgling period (May and Robinson 1985). Because of the inherent difficulties of studying young that have left their nests (Skutch 1976, Smith 1978), few determinations of the cost of cowbird parasitism incurred during the fledgling stage are available. Smith (1981) suggested that the cost of parasitism did not increase during the fledgling stage. Other studies (Sealy et al. 1997, Payne and Payne 1998, this study) suggest that the cost increases during the fledgling stage, but this requires confirmation. Because of the uncertainties concerning the cost of parasitism incurred during the fledgling stage, quantitative studies of the possible effect of cowbird parasitism on the survival of host fledglings are needed. Furthermore, research concerning the cost on the total reproductive output per season for individual females is needed because many hosts have multiple broods per season (May and Robinson 1985). This information will improve our understanding of the effects of cowbird parasitism on host population demographics and the evolutionary dynamics involved. We thank C. Ortega, B. Peer, and an anonymous reviewer for comments that improved this manuscript. This review was funded by a grant from the Natural Sciences and Engineering Research Council of Canada to S. G. Sealy, and a NSERC Undergraduate Summer Scholarship to J. L. Rasmussen.

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