Brewer's Blackbird

Maximum longevity: 12.5 years (wild)

Brewer’s Blackbirds (Euphagus cyanocephalus) are very common in open country of western North America , including human-built habitats such as parking lots, where they can be seen walking on the ground making jerking movements with their heads. Their breeding range expanded eastward in the Great Lakes region during the 20th century. In the winter, Brewer’s Blackbirds spread eastward through the southeastern United States and south to southern Mexico. These blackbirds feed mainly on insects and seeds, with some berries, usually foraging in flocks outside the breeding season. Nesting is often in loose colonies of up to 20 to 30 pairs. (Kaufman 1996; AOU 1998)

Euphagus cyanocephalus (Wagler)

It was known previously (Friedmann, 1963:134) that Brewer's blackbird was a frequent host of the brown-headed cowbird in Alberta and Montana, but the actual extent to which it is imposed upon in the northwestern prairie areas could hardly be appreciated from the nonquantitative information previously available. The Prairie Nest Records Scheme has kindly sent us copies of all their cowbird data up to mid-1975, and this reveals that of 420 records of cowbird parasitism on 37 species of hosts in Alberta, Saskatchewan, and Manitoba, no fewer than 85 were on Brewer's blackbirds, the most frequently victimized of any of the resident passerine species in terms of total number of nests found to be parasitized, not in percentages. Next in order of total number of parasitized nests came the clay-colored sparrow and the red-winged blackbird, each with 65 instances in the same area. There were, in all, records of 371 nests of Brewer's blackbird in the Prairie Nests Record Scheme, so the 85 instances with cowbird eggs amount to 22.9 percent of the total, a fairly high incidence of parasitism.

Even more numerous and illuminating data are provided by Furrer (1974 and personal communication) who studied this species in the “Potholes Region” (Grant and Adams counties) of eastern Washington in 1969 and 1970. Overall parasitism on 837 nests was 32.0 percent. There was much seasonal variation in the intensity of parasitism. Cowbirds did not become common in the area until late April, and consequently early nests suffered much less parasitism than later ones. In 1969, there was 8.0 percent parasitism of 138 nests in which the first egg was laid between 10 April and 9 May, whereas 50.9 percent of 177 clutches started between 10 May and 20 June were parasitized. Similarly, in 1970, 6.5 percent of 277 clutches started in the early interval were parasitized while 59.6 percent of 245 clutches in the later interval were parasitized. The incidence of parasitism of late nests reached such a high level in 1970 that 87.5 percent of 32 clutches begun after 8 June were victimized. Parasitism at very early nests resulted in total failure for the cowbirds; the cowbird eggs laid in all 10 nests parasitized between 10 April and 3 May (1969, 1970) were laid so late in the host's nesting cycle that they could not have received sufficient incubation for hatching.

In addition to the incidence of parasitized nests, the incidence of multiply parasitized nests also increased as the season progressed. Multiple parasitism peaked between 9 and 14 June. Nests parasitized during this period received an average of 2.67 cowbird eggs. Several times 5 cowbird eggs were laid in the same nests and 1 nest (that was probably abandoned) contained 7 cowbird eggs and 1 Brewer's egg. Some nests subjected to multiple parasitism received cowbird eggs after earlier eggs had already hatched. The unusually intense parasitism of Brewer's in the Potholes Region may not be due to cowbirds in that area having an especially strong preference for parasitizing this species. Furrer points out that the area has a depauperate avifauna; there are few potential hosts besides Brewer's blackbird.

Furrer estimates that about 50 percent of the female cowbirds that parasitized Brewer's nests removed a host egg. Surprisingly, the data showed no decrease in host fledging success attributable to the presence of cowbird nestlings although Furrer suggested that post-fledging success might be reduced among host young reared with cowbirds. It is also worth pointing out that caring for a cowbird can reduce fitness even if the normal number of host young are fledged from an individual nest because the parents' chances of surviving to breed again may be reduced. The extra care devoted to the cowbird produces an additional strain on the parents with no immediate increased output of host offspring and perhaps with a decrease in subsequent output of the breeding pair.

Furrer's work makes it evident that numerous cowbirds are reared by this species. About one-third of all cowbird eggs resulted in fledglings. The success rate of cowbird eggs in nests in bushes over land was higher than that in nests over water or on the ground. Furrer found several nests in each year of his study that fledged only cowbirds. In 1970 he found 6 nests, each of which contained 3 cowbirds that reached ages of at least 8 days and therefore probably survived long enough to fledge. That Brewer's rear large numbers of cowbirds is also shown by Gordon H. Orians' studies in the Potholes Region (pers. comm.). Despite its large size, Brewer's blackbird is evidently a good host.

We have compared Furrer's data from 1969 and 1970 with data Henry S. Horn (pers. comm.) collected in the same region in 1964, 1965, and 1966. The general trends Furrer found are also shown by Horn's data. The only major difference between the 2 bodies of information is that Horn found less parasitism. In Horn's study 2.6 percent of 234 clutches started on or before 10 May were parasitized, whereas Furrer's study showed 6.5 and 8.0 percent parasitism for the period. Similarly, 26.8 percent of 56 clutches started after 10 May were parasitized according to Horn's study whereas Furrer found 50.9 and 59.6 percent parasitism for this period. Quantitative comparisons are difficult but it is evident that the incidence of multiple parasitism was higher in Furrer's study than in Horn's. This increase in parasitism over a 7 year period, revealed by a comparison of Horn's and Furrer's studies, is almost certainly a reflection of the fact that the cowbird is a relative newcomer to the Potholes Region and that it increased in population between the two studies. We are indebted to Drs. Furrer, Horn, and Orians for making their data available to us. Dr. Furrer will publish more detailed analyses of his information elsewhere.

All the data discussed above refer to the northwestern race of the cowbird (M. ater artemisiae). Previously (Friedmann, 1963:134–135) there was but a single instance on record of Brewer's blackbird as a victim of the dwarf or southwestern race of the cowbird (M. ater obscurus). A second record, from near Buckeye, Maricopa County, Arizona, is in the collections of the Western Foundation. During May 1975, one of us (S.I.R.) found 4 additional cases of parasitism by M. a. obscurus, 1 near Tupman and 3 near Shandon in Kern and San Luis Obispo counties, California, respectively. The 3 Shandon nests constituted 3.6 percent of 83 nests found in 1974 and 1975. The Tupman nest was 1 of 2 found on 20 May 1975. Cowbirds were abundant at Tupman, but uncommon near Shandon. An interesting facet of parasitism by M. ater obscurus is that the size differential between it and Brewer's blackbird is even greater than is the case for M. ater artemisiae. In marked contrast to Furrer's experience in Washington is the situation in western Ontario, where Peck (1975) reported not a single one of 171 nests recorded in the Ontario nest records files was parasitized.

Strangely enough, in earlier records, Brewer's blackbird was never mentioned as having actually reared young cowbirds. This omission has now been corrected by Furrer's and Orians' numerous observations. In a similar vein, Dr. J. B. Tatum sends us a record from Saanich, near Victoria, Vancouver Island, 23 July 1973, of a Brewer's blackbird feeding 2 fledged cowbirds. Also, 1 of the parasitized nests S.I.R. observed near Shandon produced a fledgling cowbird. Lastly, on 3 August 1975, S.I.R. observed a female Brewer's feeding a fledged cowbird near Crowley Lake, Mono County, California at an altitude of 7200 feet. This is an unusually late date and an unusually high altitude for cowbird parasitism.

COMMON GRACKLE

Female Brewer's blackbird calls

Brewer's blackbird (Euphagus cyanocephalus) is a medium-sized New World blackbird. It is named after the ornithologist Thomas Mayo Brewer.