William Post

Reproductive Interactions of the Shiny Cowbird and the Yellow-Shouldered Blackbird

1967) and in the Caribbean (Bond 1973). with deciduous trees located along the mangroves, and on offshore mangrove cays. The climate around The race M.b. minimus, aided by man-made La Parguera is arid tropical, with a mean annual introductions (Post and Wiley 1977)) has been rainfall of 56 cm (12-year average recorded at La expanding its range NW through the CaribCueva Island, 1962-1973). The Ceiba study site

Conservation of the yellow-shouldered blackbird Agelaius xanthomus, an endagered West Indian species

Abstract The yellow-shouldered blackbird Agelaius xanthomus, endemic to Puerto Rico and Mona Island, is endangered, mainly because of brood parasitism by the shiny cowbird Molothrus bonariensis, which reached Puerto Rico at least 30 years ago. The yellow-shouldered blackbird populations have since declined, about 770–1200 remaining (470–900 on Mona Island) by 1982–1986 compared to a population of about 2400 in 1975. Nearly all nests of blackbirds in most of its habitats are parasitized by cowbirds. This significantly reduces nesting success, but blackbirds have evolved no defenses against brood parasitism. Removal of cowbirds from the yellow-shouldered blackbird nesting grounds, modeled after similar programmes for the brown-headed cowbird Molothrus ater on Kirtlands warbler Dendroica kirtlandii nesting areas, significantly increased blackbird production. Blackbirds readily accept nest boxes, and breeding populations can be established in otherwise unusable sites and can be concentrated in mangrove habitats, were they are more easily protected by cowbird removal. Furthermore, yellow-shouldered blackbird pairs using cavities, including nest boxes, fledged more young per nest than pairs using open nests.

Nesting Associations of Least Bitterns and Boat-Tailed Grackles

Interspecific nesting assemblages may result from independent settlement by individuals or groups of each species in the same limited habitat, or association with other species may be actively sought. Although Least Bitterns (Zxobrychus exilis) often nest solitarily, in a South Carolina impoundment they frequently associated with colonies of Boattailed grackles (Quiscnlus major). Bittern nests inside and outside colonies occupied the same microhabitats. However, more bitterns nested in grackle colonies than in equal-sized areas of equivalent habitat in the same marsh. In one of two years, colonial bitterns had higher reproductive success than bitterns nesting outside colonies. We conclude that Least Bitterns actively associate with grackles. Possible reasons for the association of bitterns with grackles are as follows (1) Critical density effect: at high population densities, the advantages of dispersed (cryptic) nesting are lost. Bitterns switching to nest in groups may experience little reduction in reproductive success, relative to those nesting solitarily. (2) Vigilance and mobbing: grackles provide early warning, and actively exclude predatory birds from colonies. Bitterns nesting among grackles appeared to incur few costs, such as may result from competition for food or from intracolony predation.

Extra-pair paternity in Seaside Sparrows

Abstract Seaside Sparrows (Ammodramus maritimus) nest at varying population densities, and breeding pairs may occupy either large, all-purpose activity spaces or small nesting territories, foraging in undefended areas separate from the nest site. We determined the prevalence of extra-pair paternity in a large, socially monogamous population of Seaside Sparrows nesting in small, overlapping territories. We used six microsatellite DNA markers and a likelihood-based approach to paternity assignment. Five of 47 chicks (11%) in three of 18 broods (17%) in this population were sired by extra-pair males. Although this is the first study of the genetic mating system in the genus Ammodramus, the rate of extra-pair paternity we observed is lower than in most other New World emberizines. As the first measurment of extra-pair paternity in Seaside Sparrows, this study provides a baseline for comparative studies of how extra-pair paternity is influenced by the wide variation in nesting density and territoriality found in Seaside Sparrows. These results, from a socially monogamous sparrow may also provide a context for studies of unusual mating systems in other salt-marsh nesting birds.

REVIEW OF THE OCCURRENCE OF VAGRANT CAVE SWALLOWS IN THE UNITED STATES AND CANADA

Abstract We review the extralimital occurrence of Cave Swallows (Petrochelidon fulva) in the United States and Canada since the 1960s. Nineteen (29%) of 65 occurrences not associated with an incursion in late autumn 1999 have been verified with photographs or specimens. Spring specimens of the West Indian subspecies P. fulva fulva are from Alabama and Nova Scotia; the subspecies of a Cave Swallow salvaged in Mississippi in spring is uncertain. Autumn specimens of the southwestern subspecies P. fulva pallida are from North and South Carolina. Cave Swallows measured and photographed at Cliff Swallow (P. pyrrhonota) breeding colonies in Arizona and Nebraska were P. f. pallida, as well as one bird photographed at a Cliff and Barn Swallow (Hirundo rustica) colony in Louisiana along the Mississippi line. Photographs of Cave Swallows in spring at California and in late autumn at New York were P. f. pallida. Subspecies could not be determined for six other records verified by photographs. Most of 46 sight observations of Cave Swallows have been reported at similar locations, seasons, and times as the verified records. Extralimital Cave Swallows in the United States and Canada have occurred along the Atlantic and Gulf coasts and in the interior, especially during spring and late autumn, and extralimital occurrences in the 1999 incursion matched the pattern in other years. The majority of birds on the Gulf coast occurred before the 1990s, mainly in spring, and on the Atlantic coast and in the interior during the 1990s, mainly in late autumn. The pattern of extralimital occurrence is consistent with changes in the breeding and winter status of the two subspecies. In Texas, breeding and overwintering populations of P. f. pallida have rapidly increased since the mid 1980s, in contrast to the slight increase of breeding populations of P. f. fulva in southeastern Florida.

The head-down display in shiny cowbirds and its relation to dominance behavior

Brood parasitic cowbirds (Icterinae) often give the headdown display when they approach other birds (Chapman 1928; Selander and La Rue 1961; Rothstein 1977, 1980). The display sometimes results in individuals of the same or other species preening the giver of the display. The function of the head-down is not fully understood, although some have suggested that it may appease a potential aggressor (Selander and La Rue 1961, Robertson and Norman 1976). However, experimental and naturalistic evidence now favors the opposite explanation, that the head-down display is an aggressive display which enables birds giving it to assess at close range the relative dominance of other individuals (Rothstein 1980). In addition, the donor often usurps the space occupied by the recipient of the display, while the probability that the recipient will attack may be reduced (Rothstein 1977, 1980). That some individuals preen the displaying cowbird may be a result of behavioral mimicry, as the head-down display resembles postures shown by birds engaged in body maintenance (Harrison 1965, Rothstein 1980). The head-down display and associated allopreening have frequently been recorded for captive cowbirds (Selander and La Rue 1961; Selander 1964; Rothstein 1977, 1980), but they are less often seen in nature (Darley 1968). This study reports the occurrence of the display among free-living and captive Shiny Cowbirds (Molothrus bonariensis). We also report the frequency of the display, the species to which cowbirds displayed, the responses of recipients, and the contexts in which the head-down occurred. Our objective is to clarify the function of the head-down display in the Shiny Cowbird and to compare our data with similar information for the Brown-headed Cowbird (M. ater).