Xanthocephalus Research Articles

Detection and density of breeding marsh birds in Iowa wetlands.

Accounting for imperfect detection is an important process when obtaining estimates of density or abundance for breeding birds, and this is particularly true when researchers are monitoring birds to assess the success of restored wetlands. Due to the dramatic decline in areal cover and habitat quality, wetland restoration in the Prairie Pothole Region (PPR) is critically important to breeding birds. The Shallow Lakes Restoration Project (SLRP), a partnership between the Iowa Department of Natural Resources and Ducks Unlimited, Inc., aims to restore degraded shallow lakes throughout the Iowa PPR. We conducted unlimited-radius point counts with call-broadcast surveys for breeding marsh birds at 30 shallow lakes in various stages of restoration in 2016 and 2017. Our goals were to assess the impact of covariates on detection probability and estimate density of these species at non-restored, younger (1-5 years since restoration), and older (6-11 years since restoration) restorations. Detection probability ranged between 0.07 ± 0.009 (SE) for Red-winged Blackbird and 0.40 ± 0.09 (SE) for Common Yellowthroat. Percent cattail had a positive quadratic effect on detection probability for four species, with detection decreasing sharply as percent cattail increased and increasing slightly with 100% cattail cover. Wind speed negatively influenced the detection probability of Pied-billed Grebes but had a negative quadratic effect on the detection probability of Marsh Wrens. Both restored shallow lakes had greater densities of breeding Pied-billed Grebes, Marsh Wrens, and Yellow-headed Blackbirds than non-restored shallow lakes, but there was no significant difference between younger and older restorations. Including both habitat and environmental covariates on models for detection probability can improve the precision of estimates for density and should be considered when assessing bird populations pre- and post-restoration of shallow lakes.

Impacts of Shallow Lake Restoration on Vegetation and Breeding Birds in Iowa

The Shallow Lakes Restoration Project aims to restore eutrophic shallow lakes throughout the Iowa Prairie Pothole Region (PPR). We compared the vegetation taxa richness and frequency of taxa in three vegetation groups surveyed in 2016 and 2017 across non-restored sites, younger restorations, and older restorations. We also assessed the impact of these groups on numbers of breeding marsh birds. Restored wetlands had between 2.7 (95% CI 2.3, 3.2) and 4.7 (95% CI 3.9, 5.5) more species than non-restored sites among the three vegetation groups. Typha sp. was the most abundant emergent species. Lemna minor and Ceratophyllum demersum were the most abundant floating-leaved and submersed species, respectively. The frequency of submersed aquatic vegetation increased with years since restoration, while floating-leaved vegetation and Typha sp. peaked at 7 years post-restoration. The frequency of Typha sp. positively influenced Marsh Wrens and Yellow-headed Blackbirds. Water depth negatively affected Marsh Wrens, but water depth positively influenced Yellow-headed Blackbirds. Floating-leaved vegetation positively affected Virginia Rails, while water depth had a negative effect on this species. Our results indicate that after about 7 years further management of the vegetation and water levels may need to occur and should include maintaining appropriate conditions for breeding marsh birds.

Flight Feather Molt in Yellow-Headed Blackbirds (Xanthocephalus xanthocephalus) in North Dakota

ABSTRACT Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) in central North Dakota undergo prebasic molt or prejuvenile molt during late summer. Nestling Yellow-headed Blackbirds initiate a complete prejuvenile molt, grow their primary and secondary regimes in about 40 days, completing molt after they leave the nest by the first week in August. Remiges are not replaced during the subsequent preformative molt, being retained until the second prebasic molt. Nonlinear (logistic) regression of primary remex growth during definitive prebasic molts of Yellow-headed Blackbirds indicated 38 days were required to complete the linear phase of growth (between 10% and 90% of total primary length). Males added 19.5 mm/d and females added 15.7 mm/d to the total length of all primaries during this linear growth phase; an average of 4–5 mm per primary remex per day. Definitive prebasic molting of primary remiges in males and females was initiated in late June, after nesting and brood rearing were completed. Molts ...

A Comparison of Natural and Restored Wetlands as Breeding Bird Habitat Using a Novel Yolk Carotenoid Approach

Restored wetlands are commonly assessed as avian breeding habitat using surveys, but data on reproductive parameters are needed to better understand how restoration affects wetland-dependent bird populations. A novel way to assess differences in breeding habitat quality is to compare the maternal allocation of carotenoids (i.e., biologically active yellow, orange and red pigments) to egg yolks, which is both diet and habitat dependent. We compared yolk carotenoid concentrations of red-winged (Agelaius phoeniceus) and yellow-headed blackbirds (Xanthocephalus xanthocephalus) that bred in natural, recently restored (i.e., restored ≤ 6 yrs ago) and restored (i.e., restored 9–16 yrs ago) wetlands to determine if restored wetlands provided critical reproductive resources at levels similar to natural sites. We also measured emerging insect abundances, water chemistry, and landscape-level parameters for study wetlands and related these variables to yolk carotenoid concentrations. Moreover, to understand the importance of damselflies to the diet of blackbirds, we measured the carotenoid content of damselflies and compared this to the yolk carotenoid content found in blackbird eggs. Bird abundances were also measured at each wetland to compare the traditional method of assessing breeding habitat to our novel yolk-carotenoid approach. In 2008, red-winged blackbirds in natural wetlands had higher carotenoid concentrations than birds in recently restored wetlands, suggesting that natural wetlands provided better breeding habitats. In 2009 there was a severe drought and we found contrasting results, with red-winged blackbirds in restored wetlands having higher carotenoid concentrations. Adult damselflies contained five carotenoids commonly found in avian tissues, suggesting they are likely an important source of carotenoids for wetland-breeding blackbirds. In contrast to yolk carotenoids, neither avian species richness nor abundance was significantly related to wetland type. Based on model selection results for water chemistry parameters, variation in concentrations of yolk carotenoids in red-winged blackbirds was best explained by conductivity, whereas variation in avian abundance was explained by total nitrogen, and avian species richness by chlorophyll-a concentration. Model selection results for landscape-level parameters revealed that variation in concentrations of red-winged blackbird yolk carotenoids was best explained by percent water in a 500 m buffer and variation in both avian abundance and avian species richness by wetland surface area. Our novel yolk carotenoid approach allowed us to identify differences in breeding bird habitat quality across wetland types, which we failed to detect with traditional bird survey data; our results highlight the need for additional studies comparing methods for assessing wetlands as breeding habitat for wetland-dependent birds.

Ventilation patterns of the songbird lung/air sac system during different behaviors

Unidirectional, continuous airflow through the avian lung is achieved through an elaborate air sac system with a sequential, posterior to anterior ventilation pattern. This classical model was established through various approaches spanning passively ventilated systems to mass spectrometry analysis of tracer gas flow into various air sacs during spontaneous breathing in restrained ducks. Information on flow patterns in other bird taxa is missing, and these techniques do not permit direct tests of whether the basic flow pattern can change during different behaviors. Here we use thermistors implanted into various locations of the respiratory system to detect small pulses of tracer gas (helium) to reconstruct airflow patterns in quietly breathing and behaving (calling, wing flapping) songbirds (zebra finch and yellow-headed blackbird). The results illustrate that the basic pattern of airflow in these two species is largely consistent with the model. However, two notable differences emerged. First, some tracer gas arrived in the anterior set of air sacs during the inspiration during which it was inhaled, suggesting a more rapid throughput through the lung than previously assumed. Second, differences in ventilation between the two anterior air sacs emerged during calling and wing flapping, indicating that adjustments in the flow pattern occur during dynamic behaviors. It is unclear whether this modulation in ventilation pattern is passive or active. This technique for studying ventilation patterns during dynamic behaviors proves useful for establishing detailed timing of airflow and modulation of ventilation in the avian respiratory system.

Responses of Nesting Yellow-Headed Blackbirds and Yellow Warblers to Wrens

Abstract We studied nest defense behavior of Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) and Yellow Warblers (Dendroica petechia) in response to two species of common nest destroyers. We presented freeze-dried models of Marsh Wrens (Cistothorus palustris) at Yellow-headed Blackbird nests and House Wrens (Troglodytes aedon) at Yellow Warbler nests during the incubation stage. We presented a Clay-colored Sparrow (Spizella pallida) as a control for both species. Male Yellow-headed Blackbirds responded more intensively to the Marsh Wren model than the control and female blackbirds responded intensively to both models but were more aggressive toward the Marsh Wren. Most Yellow Warblers did not respond to the House Wren model with their typical predator responses (e.g., alarm calls). Some female warblers were aggressive toward the wren model, whereas others sat in their nest. Sitting in the nest as a defense to deter nest destruction by House Wren needs further investigation. Differences in respons...

Yolk and Feather Carotenoids in Relation to Female Condition and Reproduction in the Yellow-headed Blackbird (Xanthocephalus Xanthocephalus)

ABSTRACT. Carotenoids can be important antioxidants and immunostimulants for developing embryos and adult birds, and they are often incorporated into colorful sexual displays as signals of individual quality. The allocation of carotenoids to egg yolks and feathers can be affected by a female's physical condition, which can in turn affect offspring growth and survival. We examined relationships between yolk and feather concentrations of carotenoids and various indices of female quality (i.e., body condition, heterophil-to-lymphocyte ratio, hematocrit, date of nest initiation, egg mass, reproductive success) in the Yellow-headed Blackbird (Xanthocephalus xanthocephalus), a brightly colored passerine. Measures of physical condition of female Yellow-headed Blackbirds were correlated with the allocation of carotenoids to both eggs and feathers. Specifically, less stressed (i.e., lower heterophil-to-lymphocyte ratio) females had higher concentrations of β-carotene in their eggs than females that were more stres...

Late pleistocene passerine birds from Sonora, Mexico

Songbirds (Passeriformes) have a very limited fossil record in spite of making up more than one-half of the world's 10,000 living species of birds. From the late Pleistocene fossil site of Térapa in east-central Sonora, Mexico, the identifiable fossils of songbirds consist exclusively of species of Icteridae. The seven extant species (Red-winged Blackbird Agelaius phoeniceus, Yellow-headed Blackbird Xanthocephalus xanthocephalus, Brewer's Blackbird Euphagus cyanocephalus, Great-tailed Grackle Quiscalus mexicanus, Bronzed Cowbird Molothrus aeneus, Brown-headed Cowbird M. ater, and Orchard Oriole Icterus spurius) still occur in Sonora, either as residents or migrants. The eighth fossil icterid from Térapa is an extinct genus and species of cowbird, Pandanaris convexa. The genus Pandanaris was known formerly only from late Pleistocene sites in southern California ( P. convexa A. H. Miller) and Florida ( P. floridana Brodkorb). We synonymize P. floridana with P. convexa because their supposed differences either cannot be substantiated or can be attributed to sexual dimorphism in size within a single species. Among the few Pleistocene sites from Mexico or the United States with well-studied and substantial assemblages of fossil passerines, Térapa is unique in its dominance of icterids (which may be common elsewhere but not dominant) and in its absence of corvids. Today, tropical “foothills thornscrub” is the vegetation type at Térapa except for riparian forest along the río Moctezuma. The icterid fossils are compatible with previous evidence (from plant, invertebrate, and other vertebrate fossils) that tropical/subtropical marsh, savanna, and riparian forest existed at Térapa during the late Pleistocene. Just as the two extant species of cowbirds associate today with large grazing mammals, the extinct Pandanaris convexa may have foraged in association with Pleistocene mammals whose extinction led to the demise of P. convexa as well.

The Use of Social Cues in Habitat Selection by Wetland Birds

Abstract. Wetlands are dynamic and can be destroyed and created quickly by natural forces. Therefore, birds inhabiting these wetlands may need to locate new suitable sites quickly. We investigated the cues wetland birds use when selecting a breeding site. Many species may use both habitat (e.g., vegetation structure) and social cues (presence of conspecifics and/or heterospecifics) when selecting a location for breeding. Using a two-species occupancy-modeling approach, we found certain wetland birds more likely to occur with other species, suggesting the presence of heterospecifics may influence settlement. Species that preferred wetlands with a roughly 50:50 ratio of open water to vegetation (hemi-marsh) occurred more frequently than expected with the Pied-billed Grebe (Podilymbus podiceps), suggesting these species may use the presence of grebes when selecting a habitat. Conversely, Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) do not appear to use the presence of other species when selecting...

Understanding wildlife responses to human disturbance through simulation modelling: A management tool

With conservation awareness and the demand for wildlife preservation increasing, ecotourism and outdoor recreational activities are becoming more popular. If such activities go unmanaged, the disruption to many species may have implications on their breeding success, survival and abundance and these, in turn, may have cascading ecosystem effects. By developing management strategies, through the application of simulation models, to simultaneously maintain recreational opportunities and sustain wildlife populations, these detrimental impacts can be minimised. Simulation of Disturbance Activities (SODA) is a spatially explicit individual-based model designed as a flexible and transferable practical tool to explore the effects of spatial and temporal patterns of anthropogenic disturbance on wildlife. Two case studies were used to illustrate the capabilities of SODA, one explored the effect of potential park designs on a nesting population of yellow-headed blackbirds ( Xanthocephalus xanthocephalus) in Calumet, IL and the other investigated the influence of visitor frequency on the breeding success of barbastelle bats ( Barbastella barbastellus) in the south west of England. For the yellow-headed blackbird, SODA revealed that pathways in close proximity to their main nesting and foraging habitat increased the level of disturbance experienced by the birds, although not to an extent that was detrimental to their breeding success. However, establishing paths beyond the zone of influence may ensure that the yellow-headed blackbird and other avian species continue to use the area in its entirety. In the second case study, SODA demonstrated that female barbastelle bats are significantly disturbed by the unrestricted movement of recreationists at their breeding colony site. However, SODA also revealed a lack of confidence in the critical energetic threshold value applied in the simulation. Subsequently, the overall effect of anthropogenic disturbance on the breeding success of this species could not be fully ascertained.

Double-scratching by Yellow-headed Blackbirds

Some ground-foraging birds, including most New World sparrows in the tribe Emberizini, uncover food items in litter by double-scratching, a backward hop that displaces the litter posteriorly. I report double-scratching by Yellow-headed Blackbirds (Xanthocephalus xanthocephalus), a species not previously known to exhibit this behavior. Double-scratching by one individual that was observed in detail occurred with the bill pointed downward with its tip near or touching the ground, as in other icterines but unlike the behavior in Emberizini. This individual, between double-scratches, also used bill-sweeping to displace litter. Double-scratching has a similar form in at least three of the four icterine species reported to show the behavior, suggesting the trait is homologous among these species. However, phylogenetic relationships imply that double-scratching evolved convergently in these taxa.

Modeling Wetland Blackbird Populations as a Function of Waterfowl Abundance in the Prairie Pothole Region of the United States and Canada

Blackbirds share wetland habitat with many waterfowl species in Bird Conservation Region 11 (BCR 11), the prairie potholes. Because of similar habitat preferences, there may be associations between blackbird populations and populations of one or more species of waterfowl in BCR11. This study models populations of red-winged blackbirds and yellow-headed blackbirds as a function of multiple waterfowl species using data from the North American Breeding Bird Survey within BCR11. For each blackbird species, we created a global model with blackbird abundance modeled as a function of 11 waterfowl species; nuisance effects (year, route, and observer) also were included in the model. Hierarchical Poisson regression models were fit using Markov chain Monte Carlo methods in WinBUGS 1.4.1. Waterfowl abundances were weakly associated with blackbird numbers, and no single waterfowl species showed a strong correlation with any blackbird species. These findings suggest waterfowl abundance from a single species is not lik...

An effective nest trap for female Yellow-headed Blackbirds

Capturing breeding female blackbirds using mist nets and food-baited traps can be challenging because nets are difficult to set in wetland habitats and females feed preferentially on insects. Given the problems associated with other trapping methods, we designed a highly effective nest trap that allowed us to successfully capture targeted female Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) at our wetland study sites in North Dakota. From 15 May to 16 July 2004, we captured 56 of 59 (95%) targeted females during incubation and eight of 18 (44%) targeted females with nestlings, and were also able to recapture 12 of 16 females (75%). Based on these results, we recommend using our trap to capture female blackbirds during incubation rather than during the nestling period. The nest trap is easy and inexpensive to construct and is light and easy to carry through dense wetland vegetation. In addition, the trap's large diameter and wire construction provide the flexibility needed to trap at sites where the density of vegetation varies. This trap has also been used to capture female Red-winged Blackbirds (Agelaius phoeniceus) and could potentially be used to trap females of other species of wetland-breeding icterids, including Common Grackles (Quiscalus quiscula). SINOPSIS El capturar hembras en estado reproductivo de individuos de Xanthocephalus xanthocephalus utilizando redes de niebla y trampas con cebo, pudiera ser un ejercicio de gran reto, dado el caso de que las aves se alimentan de insectos y las redes son dificiles de montar y manejar en anegados. Dados los problemas asociados con otros metodos disenamos una trampa efectiva que nos permitio capturar con exito individuos particulares en un estudio que llevamos a cabo en Dakota del Norte. Del 15 de mayo al 16 de julio de 2004, capturamos 56 de 59 individuos deseados (95%) durante el periodo de incubacion, ademas a 8 de 18 hembras (44%) con pichones. Tambien recapturamos 12 de 16 (75%) de las hembras. Basandonos en estos resultados, recomendamos el uso de nuestra trampa durante la fase de incubacion, en vez de la epoca de cria. La trampa es facil de construir, liviana, facil de transportar (a traves de un anegado denso) y de bajo costo. Ademas, el diametro de la trampa y su construccion en alambre ofrecen la flexibilidad necesaria para colocarla en sitios en donde la densidad de la vegetacion resulte variable. Este tipo de trampa tambien ha sido utilizada para atrapar hembras de Agelaius phoeniceus y puede ser, potencialmente, utilizada para atrapar hembras de otras especies de icteridos que anidan en anegados, incluyendo a Quiscalus quiscula.

LAYING-SEQUENCE VARIATION IN YOLK CAROTENOID CONCENTRATIONS IN EGGS OF YELLOW-HEADED BLACKBIRDS (XANTHOCEPHALUS XANTHOCEPHALUS)

Abstract Asynchronous hatching creates a size hierarchy among siblings and a survival disadvantage for last-hatched nestlings. Female birds can influence this disadvantage by differentially investing maternal resources, such as carotenoids, across the laying sequence. We studied intraclutch variation in carotenoid concentrations in Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) and predicted that yolk concentrations would decrease across the laying sequence, because nestling mortality is inversely related to hatching order in this species. We quantified intraclutch variation in the concentrations of total and individual identifiable carotenoids (β-carotene, β-cryptoxanthin, lutein, and zeaxanthin) in Yellow-headed Blackbird eggs collected from five breeding colonies, and correlated these concentrations with egg mass, yolk mass, and yolk water content. Carotenoid concentrations were not related significantly to any of the egg metrics measured. The concentration of total identifiable carotenoids i...

Identification, Antimicrobial Resistance Profiles, and Virulence of Members from the Family Enterobacteriaceae from the Feces of Yellow-Headed Blackbirds (Xanthocephalus xanthocephalus) in North Dakota

Public pressure to reduce or eliminate antimicrobials as ingredients of feed for poultry and other agricultural animals is mounting, primarily due to the fear of multidrug-resistant bacteria in clinical infections in both animals and humans. Exploration of the occurrence of antibiotic resistance in the gut flora of wildlife avian flocks that presumptively do not receive antimicrobials will determine the rate of resistance in a naïve population. Fecal samples collected from a healthy population of the yellow-headed blackbirds (YHB) (Xanthocephalus xanthocephalus) in North Dakota were cultured to determine what genera and species of gram-negative facultative anaerobic bacteria these wild birds carry in their intestinal flora and to evaluate the antimicrobial susceptibility profiles. Isolates of Escherichia coli were further characterized for the presence of putative virulence factors and for pathogenic potential using the chicken embryo lethality assay (ELA). The ELA was performed in chicken embryos with challenges at both 12 days and 16 days of incubation to determine whether the 16-day-old embryos were better able to fight the infection and subsequent disease and also to determine whether the ELA could distinguish between primary and secondary avian Escherichia coli pathogens. After screening 33 isolates from the 21 fecal samples, only two E. coli isolates were identified. The predominant genus and species of bacterium identified was Pantoea agglomerans. Collectively, 12 of the 33 isolates (36%) exhibited no resistance to any antimicrobial tested. However, several multidrug-resistant isolates of varying genera were identified. Among the antimicrobial resistances observed, the most common was to ampicillin (60%), followed by cephalothin (33%). Neither E. coli isolate belonged to serogroups that are notorious for causing major outbreaks of colibacillosis in poultry, and only one E. coli isolate retained resistance to any antibiotics; nevertheless, the ELA results indicate that at least one of these E. coli may be a primary pathogen of chickens. This study demonstrates that antibiotic resistance occurs in the gut flora of natural populations of YHB despite the absence of antibiotic pressure. In addition, these results indicate that YHB will harbor E. coli isolates that are potentially pathogenic in poultry. However, these E. coli isolates are not a significant reservoir for multiple antibiotic resistances nor are they widespread in the population of YHB surveyed in North Dakota.

Egg color as an adaptation for thermoregulation

Avian embryos are incubated at temperatures only 2-6 ! C below that at which hyperthermia beginstoinfluencesurvival.Inhabitatswheresunlightdirectlystrikestheeggs,evenforshortperiods,heatgainmay beasubstantialthreattosurvival,andreflectivepigmentationmayreducetherateofheatgain.Theresultsofprevious studies suggest that light-colored eggs acquire heat slower than dark eggs, but artificial pigments were used to create differences in egg coloration. This approach is problematic because natural eggshell pigments have low absorbance inthenear-infraredwavebandthatencompassesabouthalfofincidentsolarradiation.Weusednaturally-pigmented eggs to measure the influence of egg coloration on heat gain. Triads (N = 18) of eggs from Brewer's (Euphagous cyanocephalus), Red-winged (Agelaius phoeniceus), and Yellow-headed (Xanthocephalus xanthocephalus) blackbirds were crossed with six nests of each species and either exposed to full sunlight or placed under a diffusing umbrella. Thermisters recorded internal egg temperature every minute until an asymptotic temperature was reached. Eggs in full sunlight acquired heat more rapidly than eggs in the shaded environment, but heat gain did not vary with egg color in either environment. Eggs placed in Yellow-headed Blackbird nests took longer to reach asymptotic temperature, but there was no significant egg-by-nest interaction. Thus, it appears that differences in reflectivity of eggshell pigments in the visible range (400-700 nm) do not result in different rates of heat acquisition. The thermoregulation hypothesis was not supported.

Sex-specific differences in site fidelity and the cost of dispersal in yellow-headed blackbirds

Male migratory birds tend to be more faithful than females to previous breeding sites, suggesting sex differences in costs or benefits of dispersal. In Illinois, greater site fidelity by male yellow-headed blackbirds (Xanthocephalus xanthocephalus) was associated with reduced reproductive success the following year for dispersers relative to non-dispersers. Dispersing females suffered no reduction in reproductive success the following year. Males that attracted few social mates, and thus had low reproductive success, were more likely to disperse, whereas females dispersed in response to low-patch reproductive success, regardless of their individual performance. Males that dispersed appeared to be successful acquiring territories because none was observed as a floater. The rate of dispersal by males in this low-density population was greater than in more dense populations where dispersing males may be less successful at acquiring territories. Despite success at obtaining territories, males that dispersed acquired territories on the periphery of wetlands where fewer females nested, resulting in lower reproductive success. In the second year after dispersing, however, males moved onto more central territories where they acquired larger harems. Thus, dispersal by males may be a long-term strategy requiring at least 2 years for benefits to be realized. Long-term success was enhanced because dispersing males moved to wetlands on which reproductive success was higher than on the wetlands they left. In addition to demonstrating that both individual and patch reproductive success affect dispersal decisions, these data indicate that when evaluating costs and benefits of dispersal, researchers should use a time frame beyond 1 year.

Habitat selection by dispersing yellow-headed blackbirds: evidence of prospecting and the use of public information

In migratory birds individuals may prospect for potential breeding sites months before they attempt to breed and should use the cues most predictive of future reproductive success when selecting a breeding site. However, what cues individuals use when prospecting and which cues are used in selecting a breeding site are unknown for most species. I investigated whether yellow-headed blackbirds (Xanthocephalus xanthocephalus) prospect for future breeding sites and whether they select breeding habitats based on food availability, male or female density, or the average number of young produced per female in the previous year. Although it is often assumed that migratory birds prospect for potential breeding sites at the end of the breeding season, I investigated this by recording all visits to sites early and late in the breeding season. I found that males and females who visited sites other than the site at which they bred were more likely to disperse than individuals only observed at the site where they bred, and that males and females were more likely to prospect late in the breeding season. Both food availability and density in year(x) were not predictive of the number of young per female in year(x+1); however, the number of young produced per female at a site in year(x) was predictive of the number of young per female in year(x+1). As expected, dispersers used the most informative cue, the number of young per female and moved to sites with a relatively high number of young per female. This study suggests that individuals prospect for potential breeding sites late in the breeding season when they can use information gathered from the reproductive success of other individuals (i.e., public information) to select a breeding site.

Consequences of rainfall variation for breeding wetland blackbirds

Annual variability in abiotic factors can be pronounced, especially in systems that rely on precipitation, such as arid regions and prairie potholes. We report how annual variation in precipitation from 1999 to 2002 in the Prairie Pothole Region of Iowa, USA, affected both density and reproduction of two interspecific competitors: yellow-headed blackbirds, Xanthocephalus xanthocephalus (Bonaparte, 1826), and red-winged blackbirds, Agelaius phoeniceus (L., 1766). During dry years, yellow-headed blackbirds, an obligate wetland-breeding species, showed a marked reduction in density and a complete reproductive failure in which none of the nests we monitored fledged young. The reproductive failure was attributed primarily to nest predation, which was negatively correlated with water levels in wetlands. Conversely, red-winged blackbirds, a facultative wetland-breeding species, showed little variation in density and nest success. Both species exhibited similar patterns of reduced clutch size and later nest initiation dates in dry years, measures often tied to bottom-up effects of food availability and (or) age of individuals. Yet top-down effects of nest predation had a stronger influence, because lower clutch size did not result in fewer young fledged per successful nest. Incorporating how rainfall variation can affect wetland songbird demography will be critical for understanding population and community dynamics in changing environments.

IMPACT OF BLACKBIRD DAMAGE TO SUNFLOWER: BIOENERGETIC AND ECONOMIC MODELS

We constructed bioenergetic and economic models to estimate the potential impact of Red-winged Blackbirds (Agelaius phoeniceus), Common Grackles (Quiscalus quiscula), and Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) on production yields of sunflower in the northern Great Plains of North America. The amount of sunflower consumed annually by males and females, after considering field metabolic rates, energy value and moisture content of achenes, and percentage of sunflower in diets was, respec- tively: Red-winged Blackbirds 277 g and 168 g; Common Grackles 267 g and 230 g; and Yellow-headed Blackbirds 248 g and 139 g. The per capita annual economic damage was: male Red-winged Blackbirds $0.09 (U.S. dollars), females $0.05; male Common Grackles $0.09, females $0.07; and male Yellow-headed Blackbirds $0.08, females $0.05. Annual loss was $5.4 + 1.3 X 106 for all three species in aggregate, with Red-winged Blackbirds accounting for 52% of the loss. Blackbird damage represented 1.7% of the dollar value of the 1999 sunflower harvest in the northern Great Plains. This loss would be inconsequential if damage were distributed evenly; however, bird damage is often localized around wetlands and can be economically debilitating to individual producers. Although our model was based on regional population estimates, it should perform well at local scales, provided that a local population can be defined, accurately estimated, and remains stable in size over the six-week length of the damage period. Because of the large numbers of blackbirds that congregate in the region during August and September prior to migration, sunflower pro- ducers should expect some crop losses. The solution to the conflict appears to be one that focuses not on eliminating all damage, but on preventing it from exceeding 5% per field.