Phenology of Red-Winged Blackbird Use of Field Corn in Central New York

Crop depredations by red-winged blackbirds (Agelaius phoeniceus) have been a problem since colonial times (Cardinell and Hayne 1945, Weatherhead and Bider 1979, Dolbeer 1980). Today, tens of millions of dollars worth of corn are consumed yearly by blackbirds (Stone et al. 1972, Dolbeer 1980, Tyler and Kannenberg 1980) and the problem appears to be increasing. Most of the initial efforts to reduce blackbird damage to corn and other crops focused on techniques to reduce redwing populations or on chemical and mechanical repellents and frightening devices (Dyer and Ward 1977). The inconsistent effectiveness of these techniques, however, has indicated the need for an integrated approach to understanding the ecological relationships between red wings and corn (Dolbeer 1980, Dolbeer et al. MS)

Successional Changes and Habitat Selection in Hayfield Bird Communities

I examined the effects of successional changes in vegetation on the breeding dispersion of grassland birds in hayfields in New York. I sampled vegetation and counted birds in 90 hayfields of various ages (i.e. number of years since planting) and sizes that were originally planted to a legume-dominated seed mixture. I also resampled a subset of these fields two years later. Over time, these hayfields changed from tall, dense, homogeneous stands of legume-dominated vegetation to short, sparse, patchy stands of grass-dominated vegetation. Fields of all ages were dominated by introduced plant species. Red-winged Blackbirds (Agelaius phoeniceus), the most common bird species breeding in these fields, were most common in fields of intermediate ages, whereas numbers of Bobolinks (Dolichonyx oryzivorus), the second most abundant species, increased logarithmically with field age. Upland Sandpipers (Bartramia longicauda), Eastern Meadowlarks (Sturnella magna), Grasshopper Sparrows (Ammodramus savannarum), and Henslow\u27s Sparrows (A. henslowii) were most abundant in the oldest hayfields, whereas abundances of Savannah Sparrows (Passerculus sandwichensis, the third most common species) did not differ in fields of different ages. Bird-species richness and diversity increased linearly with hayfield age. Three species (Red-winged Blackbird, Bobolink, Savannah Sparrow) accounted for more than 90% of the breeding individuals. Vegetation structure, composition, and patch size were the most important proximate correlates of habitat selection for these species. Hayfield size was positively correlated with abundance for five of the seven most common species. Furthermore, those species that nested late in the season (Bobolinks, Grasshopper Sparrows, and Henslow\u27s Sparrows) had low breeding densities in fields with early mowing dates the previous year

The Effects of Site Quality on Breeding-Site Fidelity in Bobolinks

We compared breeding-site fidelity of Bobolinks (Dolichonyx oryzivorus) at two low-quality sites (Bald Hill and Shackelton Point, BH +SP) with that at one high-quality site (Moore Road, MR) in central New York. Yearly density of fledglings at Moore Road was more than six times the density at Bald Hill and Shackelton Point. Furthermore, individual residents fledged 50-70% more young at the high-quality site. At Moore Road, 70% of the males and 49% of the females returned one or more times, compared with 44% of the males and 25% of the females at the low-quality sites. Successful residents (i.e. those fledging one or more young the previous year) were equally likely to return at all sites. However, the return rates of unsuccessful residents were more than three times higher at the high-quality site. Neither overall return rates nor intrasite dispersal of returning residents differed between individuals that had been successful or unsuccessful the previous year at the high-quality site. At the low-quality sites, individuals unsuccessful the previous year were much less likely to return than were successful individuals (55% vs. 19% for males, 41% vs. 9% for females). Those unsuccessful individuals that did return tended to move farther between nest sites than successful individuals. Overall, site productivity was the best predictor of yearly, site-specific return rates. For females (but not for males), individuals that were resident in one or more years previous were more likely to return than were novice residents (47% vs. 20%). Individuals that had nests experimentally destroyed by hay-cropping returned at similar rates to those of naturally unsuccessful residents. We believe that the patterns of breeding-site fidelity in Bobolinks are primarily the result of experience-based choices by the birds and do not simply reflect patterns of mortality. Bobolinks appear to use both their own reproductive success and that of others at their site to influence their decision to return to a breeding site

EGG REMOVAL BY BROWN-HEADED COWBIRDS: A FIELD TEST OF THE HOST INCUBATION EFFICIENCY HYPOTHESIS

Brown-headed Cowbirds (Molothrus ater) often remove host eggs, usually to the detriment of the host\u27s reproductive success. We tested the hypothesis that host egg size and number influence the incubation efficiency of a parasitic egg. A single House Sparrow (Passer domesticus) or Brown-headed Cowbird egg was placed in each host nest (addition), and in some nests a host egg was removed as well (addition/removal). Hatching success and incubation length were measured to determine whether host-egg removal conferred an advantage in incubation efficiency compared to simple addition of a parasitic egg. Redwinged Blackbirds (Agelaius phoeniceus) and Chipping Sparrows (Spizella passerina) served as medium-sized and small-sized host species, respectively. In Red-winged Blackbird nests, host-egg removal produced smaller clutches, and parasitic eggs had shorter incubation lengths in smaller clutches. However, the parasitic egg\u27s incubation length and probability of hatching did not differ between the addition and addition/removal treatments. Parasitic eggs in Chipping Sparrow nests had shorter incubation periods than in blackbird nests and frequently caused the inefficient incubation of host eggs. Egg removal again did not reduce incubation lengths or increase hatchability of parasitic eggs. Thus, we found little support for the incubation efficiency hypothesis to explain host-egg removal by Brown-headed Cowbirds

Multiple Paternity in a Territorial Passerine: The Bobolink

Electrophoretic and behavioral evidence corroborate the conclusion that Bobolink (Dolichonyx oryzivorus) females occasionally copulate with more than one male, which results in the insemination of a single clutch of eggs by those males. In 2 of 12 families from which blood samples were obtained from the mother, the putative father, and the nestlings, at least 2 males were responsible for fertilization of the clutch. We believe this is only the second bird species in which multiple paternity has been documented conclusively in the wild. Our speculation that it may not be uncommon leads to a caution for evolutionary biologists who use apparent reproductive success in birds to address questions about fitness

EXPLANATIONS FOR THE INFREQUENT COWBIRD PARASITISM ON COMMON GRACKLES

We determined the factors responsible for the lack of parasitism on Common Grackles (Quiscalus quiscula) by Brown-headed Cowbirds (Molothrus ater). 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 beyond 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 female 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 infrequent 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 whiwhich would increase the chances of them mistakenly rejecting their own discordant eggs

Responses of Nesting Bobolinks (Dolichonyx oryzivorus) to Habitat Edges

ABSTRACT.-Avoidance of habitat edges may be contributing to reduced densities of grassland birds in small habitat patches. Nest densities for grassland-nesting Bobolinks (Dolichonyx oryzivorus) were much lower than expected within 25m of forest edges at three sites in New York, and that pattern ( edge avoidance ) extended to 100m at one site. Nests located within 50 m of forest or wooded hedgerow edges had lower daily survival rates, compared with nests \u3e100m from any habitat edge. Bobolinks tended to move away from forest edges when renesting after nest failure; that pattern was especially evident in females that placed their first nest within 50 m of a forest or wooded hedgerow edge. Second nests of all seven of those females were farther from that edge type than their first nests. However, nest placement in relation to wooded edges did not vary significantly between years for those philopatric females nesting at our study sites for more than one year. Bobolinks also avoided nesting near road edges, even though nest survival rates were not lower near that edge type. However, Bobolinks did not appear to avoid nesting near edges adjacent to old fields or pastures. Nest survival near those edge types was higher than near wooded edges and similar to or higher than survival of nests \u3e100m from any edge. Thus, responses of Bobolinks to habitat edges were inconsistent, and nest success was dependent on type of edge

Predation Rates on Real and Artificial Nests of Grassland Birds

We estimated nesting success at real and artificial nests of grassland birds to test the influence of nest type, nest position, and egg size on predation rates. We distributed wicker nests and realistic woven-grass nests baited with a clay egg and either a Northern Bobwhite (Colinus virginianus) egg or a House Sparrow (Passer domesticus) egg in four grasslands that were part of the Conservation Reserve Program in east-central Illinois. Nesting success averaged 86.5% for 12 days of exposure for artificial nests. For real nests, nesting success was markedly lower, averaging 39% over the entire nesting cycle and 59% during approximately 12 days of incubation. Wicker nests were depredated more often than wovengrass artificial nests (18% vs. 8%), and nests baited with House Sparrow eggs were depredated more often than nests baited with Northern Bobwhite eggs (22% vs. 9% ). Elevated and ground nests were depredated at the same rate. Patterns of nest predation on wicker nests were markedly different from depredation patterns on real nests over time and among fields. In contrast, patterns of nest predation on realistic woven-grass nests corresponded much more closely with predation rates of real nests over time and among fields. We suggest that future artificial nest studies use nests and eggs that mimic as closely as possible the real nests and eggs of target species. Use of unrealistic artificial nests and eggs, at least in grasslands, may result in patterns of predation that do not accurately reflect those of real nests. Artificial nests of any type appear to underestimate predation rates on nests of grassland birds, possibly because of a lack of snake predation on artificial nests

Rejection of Cowbird Eggs by Mourning Doves: A Manifestation of Nest Usurpation? Brian D. Peer and Eric K. Bollinger

This article was printed in The Auk, Volume 115, No.4 (1998)

Multiple Paternity in a Territorial Passerine: The Bobolink

Electrophoretic and behavioral evidence corroborate the conclusion that Bobolink (Dolichonyx oryzivorus) females occasionally copulate with more than one male, which results in the insemination of a single clutch of eggs by those males. In 2 of 12 families from which blood samples were obtained from the mother, the putative father, and the nestlings, at least 2 males were responsible for fertilization of the clutch. We believe this is only the second bird species in which multiple paternity has been documented conclusively in the wild. Our speculation that it may not be uncommon leads to a caution for evolutionary biologists who use apparent reproductive success in birds to address questions about fitness