A Comparative Study of Cowbird Parasitism in Yellow-headed Blackbirds and Red-winged Blackbirds

--We compared Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) and Red-winged Blackbirds (Agelaius phoeniceus) to determine possible explanations for the lack of Brown-headed Cowbird (Molothrus ater) parasitism on Yellow-heads. Twenty-three cowbird eggs were cross-fostered from Red-winged Blackbird nests to Yellow-headed Blackbird nests. Eleven of the cross-fostered cowbird eggs hatched, and seven fledged. Of the four that hatched but failed to fledge, three disappeared and one starved as did all except one of its Yellowheaded Blackbird siblings. Of the 12 cowbird eggs that did not hatch, 8 were preyed upon and 4 were inviable. Cowbirds grew at the same rate in Yellow-headed and Red-winged blackbird nests. Differences in breeding characteristics between Yellow-headed and Redwinged blackbirds may help explain the lack of cowbird parasitism of Yellow-headed Blackbird nests. For Yellow-headed Blackbirds the breeding season had an earlier peak of nest initiation, the abandonment rate of active nests was dramatically higher at the end of the season, and nestlings were more heavily parasitized with mites than were Red-winged Blackbird nestlings. Received 18 December 1990, accepted 4 July 1990. AVIAN brood parasitism is a reproductive strategy where parasitic birds lay their eggs in nests of other birds and depend on their hosts to incubate the eggs and raise their offspring. In this dynamic parasite-host relationship, there are constraints upon the parasite, antiparasite defenses by the host, and counter adaptations by the parasite (Payne 1977). Parasites may be constrained by the host's breeding season, egg size, incubation period, diet, acceptance of the parasitic egg and nestling, host growth rate, and postfledging (foster) parental care (Friedmann 1963, Payne 1977). Antiparasite defense mechanisms commonly employed by hosts are aggression toward intruding parasites (Robertson and Norman 1976) and rejection of parasitic eggs through ejection, damage, desertion, or building a new nest floor over the parasitized clutch (Rothstein 1975a, b). The marsh-nesting Yellow-headed Blackbird (Xanthocephalus xanthocephalus; hereafter Yellow-head) is a locally abundant bird in western North America. Its colonial breeding habits provide an opportunity to experiment on large numbers of individuals. Brown-headed Cowbird (Molothrus ater) parasitism on Yellow-heads appears to be only "accidental" (Friedmann et al. 1977, Ortega and Cruz 1988), yet Yellowheads accept cowbird eggs (Ortega and Cruz 1988). The relationship between cowbirds and Yellow-heads is of particular interest because 16 Yellow-heads can readily be compared with the related Red-winged Blackbird (Agelaius phoeniceus; hereafter Red-wing). Red-wings also nest in marshes and have similar breeding biology, and they are parasitized frequently by cowbirds (Hergenrader 1962, Friedmann et al. 1977, Ortega and Cruz 1988). Although extensive egg manipulation experiments have been performed on species that reject parasitic eggs (Victoria 1972; Rothstein 1974, 1976, 1977, 1982; Finch 1982; Cruz and Wiley 1989), and some cowbird mount experiments have been performed on species with low or zero rates of parasitism (Folkers 1982), most studies of brood parasitism have focused on species that commonly serve as hosts. Our purpose was to investigate possible explanations for the lack of cowbird parasitism on Yellow-heads. We attempted to experimentally test two hypotheses: first, that Yellow-heads are above the upper size limit of successful hosts (i.e. the host siblings may outcompete cowbird nestlings for food), and second, that there is an aggressive behavioral response by Yellow-heads which deters cowbirds. STUDY AREA AND METHODS We investigated Red-winged and Yellow-headed blackbird nests from 1985 through 1986 in nine cattail (Typha latifolia and T. angustifolia) marshes and two The Auk 108: 16-24. January 1991 January 1991] Blackbirds and Cowbirds 17 flooded willow (Salix sp.) stands in Boulder County, Colorado. All study sites were surrounded by open fields, primarily pastoral and agricultural. The two willow stands and four marshes were on the shores of Boulder Reservoir (approximately 103 surface hectares at low water level). Within the breeding season, the water fluctuated as much as 1 m; the low point occurred during the nest-building phase. Two sites were along the periphery of small ponds (both approximately 1.2 surface ha). No Yellow-heads nested in the above sites, whereas Red-wings nested in all. Both Yellow-heads and Red-wings nested in three additional marshes. One of these marshes (Walden Ponds) is a reclaimed gravel pit (9.3 surface ha) a?•d was surrounded by a patchy distribution of cottonwoods (Populus argentii) and willows as well as weedy fields. Approximately 100 Yellow-heads and 30 Redwings nested during both 1985 and 1986 in this marsh. Two of the three marshes (Olive east and Olive south, approximately 1.6 and 4 surface ha), divided by a small road, supported at least 800 nesting Yellowheads and 50 Red-wings in 1986. These two marshes, studied only in 1986, were surrounded by weedy fields and agricultural land. Red-wing and Yellow-head nests were found and identified with individually numbered tags. We visited each nest every 1-3 days and recorded nest contents. Nestlings were marked individually by colorcoding their tarsi with permanent ink. They were weighed and measured at each visit. Experimental parasitism.--We cross-fostered 23 cowbird eggs from Red-wing nests to 23 Yellow-head nests during the egg-laying stage and early incubation. Eggs transferred between nests were, to the best of our knowledge, the same age within a day or two of the host eggs. Measuring eggs and nestlings.--Eggs were weighed to the nearest 0.1 g on a 10-g Pesola spring scale. Egg width and length were measured to the nearest 0.01 mm with a Mitutoyo metal dial caliper. In 1986, nestlings were weighed to the nearest 0.1 g on 10-g, 50g, and 100-g Pesola spring scales. We measured to the nearest 0.01 mm tarsus length, culmen length, width of bill at loral feathering (gape width), and length of ninth primary, according to Baldwin et al. (1931). Breeding season analysis.--For visual display, the breeding season (1 May to 30 July) was divided arbitrarily into 1-week periods. For statistical analysis, each day was considered as a category. To calculate the number of nests initiated, we used only nests with known date of initiation (i.e. nests that were found during the egg-laying stage or that we followed through to the hatching stage and could estimate dates of initiation). Nest initiation was considered the date of the first egg laid. For the analysis of number of active nests, we included all nests that were followed for at least two 1-week periods (two visits or more), whether or not we knew the initiation date. Aggression experiments.--To test aggression toward intruders at nest sites, we placed models 1 m from Yellow-head and Red-wing nests for 5 min. Only birds in the phases of egg laying or early incubation were tested. Models were birds mounted in a lifelike perching position, presented on poles, slightly above and facing the nest. At each nest tested, we used an adult female cowbird and a control model that was either an adult White-crowned Sparrow (Zonotrichia leucophrys) or an immature Western Meadowlark (Sturnella neglecta). These species were selected because both occurred in the surrounding area; therefore, blackbirds may have been as familiar with them as they were with cowbirds. The controls were considered appropriate because they posed relatively little competition for food and nesting resources. For half the experiments, cowbirds were used first; and for the remainder, control models were used first. We allowed 10 min between cowbird and control tests. Observations were made as far away as possible, usually >40 m. In a few cases, however, we observed the test birds from as close as 20 m because of the density and height of the cattails. Both blackbird species were initially disturbed by our approach and left their nest sites, but most returned quickly after we left the immediate nest vicinity. We did not use a blind. In all cases, the test bird returned to the nest site within the 5-min period. Because colonial nesting may aid in providing defense against brood parasites, our observations included all birds responding to the