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Direct Estimates of Breeding Site Fidelity and Natal Philopatry in Brood Parasitic Brown-Headed Cowbirds Molothrus ater

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Abstract

Avian obligate brood parasitism is a specialised life history strategy that may impact the dispersal of juvenile and adult parasites when compared with non-parasitic (parental) bird species. In contrast to expectations, however, several brood parasites show a territorial spacing system while breeding, including breeding site fidelity within and across years. In comparison, data are sparse on the extent of natal philopatry in brood parasites. We estimated minimum levels of breeding site fidelity and natal philopatry in the generalist parasitic Brown-headed Cowbird Molothrus ater in two subsequent years of colour-ringing known-sexed adults and nestlings. Adults' breeding site fidelity was moderate and similar to previous reports on this species and on other non-parasitic temperate zone passerines. We recorded lower estimates for natal philopatry compared to adult Cowbird breeding site fidelity, but this still fell within the range typically reported for offspring of other North American, migratory, and parental songbirds. These results suggest that social parasitism as a reproductive strategy does not in itself appear to impact patterns of philopatric behaviours of Brown-headed Cowbirds and perhaps other brood parasites.

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... In avian brood parasitic systems, the bulk of past research has focused on the hosts' and parasites' incubation (time from laying to hatching) and nestling periods (time from hatching to fledging), while neglecting the critical post-fledging period (time from fledging to independence; Soler et al. 2014a; but see Soler 1994;Soler and Soler 1999;De Mársico et al. 2012;Louder et al. 2015;Hauber et al. 2020;Kysucan et al. 2020). Indeed, the post-fledging period is an important ontogenetic stage in a young bird's life, which is often characterized by high rates of juvenile mortality that are thought to play a crucial Communicated by Kathryn E. Sieving. ...
... Indeed, studies on the post-fledging period of passerines have been understudied, in large part, because of these past limitations, which are now being overcome with automated systems (e.g., Jones et al. 2018). Consequently, while biologists have acquired a greater understanding of brood parasite-host systems during the nesting stage, we know comparatively little about young brood parasites once they leave host nests (Woodward 1983;Hauber et al. 2001;De Marsico et al. 2012;Soler et al. 2014a;Louder et al. 2015;Hauber et al. 2020). ...
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... Radiotracking telemetry and molecular methods have revealed that brood-parasitic females overlap territories rather than defend exclusive areas (Bolopo, Roncalli, Canestrari, & Baglione, 2020;Fleischer, 1985;Martínez et al., 1998;Mosk at, B an, Fül€ op, Bereczki, & Hauber, 2019;Nakamura & Miyazawa, 1997;Rühmann, Soler, P erez-Contreras, & Ib añez-Alamo, 2019;Ursino, Strong, Reboreda, & Riehl, 2020; but see . At the same time, both host-specialist and host-generalist individuals exist within various parasite taxa (Alderson, Gibbs, & Sealy, 1999;Ellison, Sealy, & Gibbs, 2006;Nakamura, Miyazawa, & Kashiwagi, 2005;Strausberger & Ashley, 2005;Vogl, Taborsky, Taborsky, Teuschl, & Honza, 2004;Woolfenden, Gibbs, Sealy, & McMaster, 2003) and individual parasitic females may return to the same home range between years (Hahn, Sedgwick, Painter, & Casna, 1999;Hauber, Heath, & Tonra, 2020;Kole cek, Proch azka, Brlík, & Honza, 2020). In addition, there is evidence for nonrandom selection of host nests for parasitism (Mahler, Confalonieri, Lovette, & Reboreda, 2007), which vary according to the particular host community that is being parasitized (De M arsico, Mahler, Chomnalez, Di Giacomo, & Reboreda, 2010). ...
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Attempts to estimate and identify factors influencing first-year survival in passerines, survival between fledging and the first reproductive attempt (i.e. juvenile survival), have largely been confounded by natal dispersal, particularly in long-distance migratory passerines. We studied Prothonotary Warblers (Protonotaria citrea) breeding in nest boxes to estimate first-year survival while accounting for biases related to dispersal that are common in mark-recapture studies. The natal dispersal distribution (median = 1420 m; n = 429) and a distance-dependent recruitment rate, which controls for effects of study site configuration, both indicated a pattern of short-distance natal dispersal. This pattern was consistent with results of a systematic survey for birds returning outside the nest box study sites (up to 30 km in all directions) within a majority (81%) of total available bottomland forest habitat, further suggesting that permanent emigration outside of the study system was rare. We used multistate mark-recapture modeling to estimate first-year survival and incorporated factors thought to influence survival while accounting for the potential confounding effects of dispersal on recapture probabilities for warblers that fledged during 2004-2009 (n = 6093). Overall, the average first-year survival for warblers reared without cowbird nestmates was 0.11 (95% CI = 0.09-0.13), decreased with fledging date (0.22 early to 0.03 late) and averaged 40% lower for warblers reared with a brood parasite nestmate. First-year survival was less than half of the rate thought to represent population replacement in migratory passerines (∼0.30). This very low rate suggests that surviving the first year of life for many Neotropical migratory species is even more difficult than previously thought, forcing us to rethink estimates used in population models.
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Nest predation limits avian fitness, so ornithologists study nest predation, but they often only document patterns of predation rates without substantively investigating underlying mechanisms. Parental behavior and predator ecology are two fundamental drivers of predation rates and patterns, but the role of parents is less certain, particularly for songbirds. Previous work reproduced microhabitat-predation patterns experienced by Yellow Warblers (Setophaga petechia) in the Mono Lake basin at experimental nests without parents, suggesting that these patterns were driven by predator ecology rather than predator interactions with parents. In this study, we further explored effects of post-initiation parental behavior (nest defense and attendance) on predation risk by comparing natural versus experimental patterns related to territory density, seasonal timing of nest initiation, and nest age. Rates of parasitism by Brown-headed Cowbirds (Molothrus ater) were high in this system (49% nests parasitized), so we also examined parasitism-predation relationships. Natural nest predation rates (NPR) correlated negatively with breeding territory density and nonlinearly (U-shaped relationship) with nest-initiation timing, but experimental nests recorded no such patterns. After adjusting natural-nest data to control for these differences from experimental nests other than the presence of parents (e.g., defining nest failure similarly and excluding nestling-period data), we obtained similar results. Thus, parents were necessary to produce observed patterns. Lower natural NPR compared with experimental NPR suggested that parents reduced predation rates via nest defense, so this parental behavior or its consequences were likely correlated with density or seasonal timing. In contrast, daily predation rates decreased with nest age for both nest types, indicating this pattern did not involve parents. Parasitized nests suffered higher rates of partial predation but lower rates of complete predation, suggesting direct predation by cowbirds. Explicit behavioral research on parents, predators (including cowbirds), and their interactions would further illuminate mechanisms underlying the density, seasonal, and nest age patterns we observed.
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En el este de Norte América, la mayoría de los ambientes leñosos en estadíos tempranos de sucesión son efimeros y se transforman en bosques en pocas décadas. En consecuencia, los parches de hábitat son generalmente adecuados para la reproducción de las especies de aves de arbustales sólo por un tiempo corto. Esto ha llevado a algunos autores a sugerir que las aves de arbustales deberían exhibir una baja fidelidad a sus sitios de reproducción previos, y deberían dispersarse frecuentemente a parches nuevos y más adecuados. De modo similar, se esperaría que las aves de bosques maduros presenten mayor fidelidad a los sitios debido a que sus hábitats son más estables. Entender las estrategias de dispersión de las aves de arbustales tiene implicaciones importantes para el manejo y la conservación de estas especies en declive. Por lo tanto, utilicé meta-análisis para comparar las tasas de fidelidad a los sitios de aves paserinas migratorias de arbustales y de bosques que se reproducen en el este de Norte América. Considerando aves adultas, la fidelidad a los sitios de las aves de arbustales (estimado ± EE = 0.353 ± 0.022) y de bosques (0.364 ± 0.036) fue similar. La filopatría de las aves de un año de edad a su sitio natal fue menor, pero similar en las aves de arbustales (0.048 ± 0.014) y de bosques (0.028 ± 0.019). Concluyo que las tasas de dispersión de las aves de arbustales no son inusualmente altas. En cambio, tanto las especies de aves de bosques como las de arbustales parecen seguir reglas para tomar decisiones que son comunes entre los paserinos, de manera que los adultos regresan frecuentemente a los sitios de reproducción y las aves de un año de edad se dispersan alejándose de sus sitios natales. Para propósitos de manejo, estos resultados son esperanzadores ya que las poblaciones de aves de arbustales deberían persistir en ambientes de alta calidad mantenidos en estadíos sucesionales tempranos.
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Investigaciones exhaustivas previas han intentado determinar si los dialectos de los cantos representan sistemas sociales aislados reproductivamente, de modo que los individuos tienden a tener un único dialecto durante toda su vida. Estudiamos este tema analizando datos de anillado de Molothrus ater provenientes de la ladera este de la Sierra Nevada de California. Durante 14 años, 1393 juveniles y 2568 individuos maduros fueron anillados a lo largo de una transecta de 40 km que incluyó tres dialectos. De estas aves juveniles y maduras, el 7.9% y el 12.1%, respectivamente, fueron recapturadas en un año subsecuente. Todas las clases de aves maduras (machos del segundo año, machos más viejos y hembras) presentaron tasas de recaptura significativamente mayores que las aves anilladas como juveniles, pero no hubo diferencias entre las clases de aves maduras. En total, el 22.7% de los 110 juveniles recapturados en un año subsecuente fue atrapado en una región con un dialecto distinto al de la región en la que fue anillado, en comparación con el 8.1% de los 310 individuos maduros. Ni las aves juveniles ni las adultas mostraron diferencias relacionadas con el sexo en la proporción de individuos recapturados en los años subsiguientes en regiones con diferentes dialectos. Las aves en todas las clases de sexo y edad presentaron mayores probabilidad de haberse desplazado a un nueva región cuando fueron recapturadas en un año subsecuente que cuando fueron recapturadas en el mismo año, lo que sugiere que los movimientos aparentes entre años fueron casos de dispersión, no viajes de forrajeo de corta duración. Aunque nuestros datos de anillado no pueden confirmar el flujo de genes, los altos niveles de movimiento que evidenciaron concuerdan con estudios genéticos y morfométricos que indican altos niveles de flujo génico entre estos dialectos de M. ater.
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Parents influence the phenotype of their offspring by determining the environment in which early development occurs. The many factors that affect growth in avian brood parasites provide an excellent context in which to examine how ecological variables and sex differences influence plasticity of early development. We used a model-selection approach to determine the most important variable(s) for explaining patterns ill growth rate of the Brown-headed Cowbird (Molothrus ater). Using published growth-rate estimates across various host species, we found that the age-adjusted size of Brown-headed Cowbird chicks increases with increasing hatching synchrony between host and parasite chicks. We also quantified Brown-headed Cowbird growth rates in nests of Song Sparrows (Melospiza melodia) and Yellow Warblers (Dendroica petechia), two common host species at Mono Lake, California, to examine the role of variation in hatching synchrony in broods within host species. Statistical models to explain variation in Brown-headed Cowbird chicks' growth rates were constructed from ecological variables (host species, brood size, multiple parasitism, hatching synchrony between parasite and host chicks) and chick sex. The best model included only sex and there was a 99% chance that this was the best model, given the data set and models compared. Male Brown-headed Cowbird gained an average of 0.7 g day(-1) more than females and weighed 13% more at the same age. The only significant ecological variable, host-parasite hatching synchrony, was found to be sex-dependent, with males more likely than females to hatch earlier than their nest mates. We discuss the possible mechanisms underlying this sex effect and the importance of determining sex when studying nestling growth and competition.
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Although post-fledging care by adult males seems unlikely in bird species that are obligate, interspecific brood parasites, there have been numerous reports of adult male Chrysococcyx cuckoos apparently feeding conspecific young. Most researchers currently view these observations with skepticism, in large part because Chrysococcyx and other cuckoo species engage in courtship feeding, and it is possible that field observers could mistake adult females receiving food from courting males for fledglings, especially given the similar appearances of females and juveniles. Here, we report an observation of an extended provisioning bout by an adult male Klaas's Cuckoo (C. klaas) feeding a conspecific individual with juvenile plumage and behavior, and we summarize our observations of similar occurrences in the Diederik Cuckoo (C. caprius) in Kenya. We suggest that the available evidence indicates that male provisioning, and hence potential parental care, is present in these brood-parasitic cuckoos at a higher frequency than currently recognized. The mechanism that causes males to associate with fledglings is unknown, but warrants further study.
Article
Available estimates of demographic parameters for Brown-headed Cowbirds (Molothrus ater) vary geographically. However, few estimates are based on long-term studies of marked individuals. We conducted a mark–recapture study on the population of cowbirds at Delta Marsh, Manitoba during the 1993–1998 breeding seasons. We estimated annual survival, breeding site fidelity, and sex ratio, and compared those parameter estimates to other populations of Brown-headed Cowbirds. The Delta Marsh population had higher adult survival (male 90.1%; female 69.6%) and breeding site fidelity (males 66.9%, female 59.5%) than reported for other populations, and the sex ratio was significantly different from unity (1.9 males:1 female). We suggest that differences in survival and breeding-site fidelity between the Delta Marsh population and others may be due to differences in methods used to calculate parameter estimates. In contrast, variation in sex ratios is likely real and due to differences in the local ecological conditions. In our population, high survivorship and breeding-site fidelity may lead to low recruitment of new birds into the resident population and intense competition for limited breeding opportunities. The highly male biased sex ratio may result in strong sexual-selection pressure on males competing for the limited breeding opportunities. Those circumstances have implications for the social behavior and mating system of cowbirds.
Article
Common cuckoos (Cuculus canorus) are best known for their simple two-note calls ("cu-coo"), which are uttered only by males during the breeding season. A previous playback study revealed that territorial males were more tolerant toward playbacks of the calls of familiar, neighbouring individuals than toward unfamiliar, stranger simulated intruders, exhibiting the classical "dear-enemy" phenomenon. Here we experimentally assessed whether the acoustic cues for familiarity recognition are encoded in the first and/or second note of these simple calls. To do so, we played mixed sound files to radio-tagged cuckoos, where the first part of the two-note calls was taken from strangers and the second part from neighbours, or vice versa. As controls, we used behavioural data from two-note neighbour and two-note stranger call playbacks. Cuckoos responded consistently to the two types of mixed sound files. When either the first or second note of the call was taken from a stranger and the other note from a neighbour, they responded to these sound files similarly to two-note playbacks of strangers: they approached the speaker of the playbacks more closely and the calling response-latency to playbacks was longer than to familiar controls. These findings point to the importance of both notes in familiarity recognition. We conclude that familiarity recognition in male common cuckoos needs the complete structure of the two-note cuckoo call, which is characteristic for this species.
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Cowbirds and Other Brood Parasites
Article
We tagged 12 adult Common Cuckoos (Cuculus canorus) with non-platform terminal transmitter (non-PTT) GPS-UHF telemetry at their breeding grounds in Hungary. One male and two female Cuckoos (one of them twice) were again observed in subsequent years, and GPS fixes documented their migration routes to and from Africa, as far south as Namibia. All four routes showed the species-typical clockwise loop migration. Although currently non-PPT GPS tracking with remotely downloadable data as an ornithological method is primarily suitable to map home ranges of birds, it could be a complement to PTT technology in migration research, especially for delivery of higher spatial accuracy.
Article
Virulence, the amount of harm a parasite inflicts on its host, is integral to elucidating the evolution of obligate avian brood parasitism. However, we lack information regarding how relatedness is linked to changes in behavior and the degree of harm that brood parasites cause to their hosts (i.e., virulence). The kin competition hypothesis combines theory from offspring signaling and parasite virulence models and states that the begging intensity of co-infecting parasites is driven by their relatedness, with concomitant changes in the degree of virulence expressed by parasitic young. We tested this hypothesis using the Brown-headed Cowbird Molothrus ater, an obligate brood parasitic bird whose virulence at the nestling stage is mediated by vigorous begging displays that are used to outcompete host young during feeding bouts. We found support for both predictions of the kin competition hypothesis: first, the begging intensity of cowbirds was greater in a population where cowbirds typically competed against unrelated host nestmates, relative to a population where they often competed against kin. Second, the greater intensity of begging in cowbirds was positively associated with decreased growth in host offspring during the developmental period. Given the dearth of studies on virulence in avian brood parasites, our results notably extend our understanding of how relatedness is linked to parasite behavior and virulence, and they highlight how spatially-isolated host populations can harbor different levels of virulence that are driven by competitive interactions between co-infecting parasites.
Article
Brood-parasitic offspring sexually (mis)imprinting on the foster parents is considered one of the greatest constraints to the evolution of interspecific avian brood parasitism. While most nonparasitic juvenile birds learn the behaviours and mate choice preferences from their own parents, social parasites must avoid misimprinting on their host species' phenotype in order to accurately recognize conspecifics. One possible mechanism to assure accurate species recognition by juvenile parasites is to begin to associate with adult parasitic conspecifics, known as the ‘first contact’ scenario, whereby adult female parasites facilitate the dispersal of their offspring away from hosts, thus providing accurate referents for conspecific recognition. Using an automated radiotelemetry system, we determined the presence or absence (every 1–2 min during three breeding seasons; 516 315 search occasions) of radiotagged parasitic adult female brown-headed cowbirds, Molothrus ater, and compared their departures f
Article
Molothrus ater shows a diurnal cycle in sociality, being relatively asocial in the morning and social in the afternoon. Although cowbirds vary geographically in the extent of their daily movements, all populations share a similar opportunistic approach to their use of space for breeding and feeding. Female cowbirds have a similarly high rate of egg laying throughout the species' range, despite considerable variation in habitats and host species. Yearling males have lower breeding success and body weights than adults in some parts of the US West, whereas such differences may not occur in the East. Cowbirds have highly divergent vocal dialects in some parts of California, but little vocal variation elsewhere in the state. -from Authors
Article
Brood-parasitic Brown-headed Cowbirds (Molothrus ater) in the eastern Sierra Nevada of California, breed and feed in almost totally disjunct areas that reflect local optima for finding host nests and food, respectively. Radio-tracking showed that five females and four of eight males spent mornings in host-rich habitats such as forests and then commuted 2.1-6.7 km to one or more prime feeding sites such as horse corrals and bird feeders for the rest of the day. The four noncommuting males, which were all yearlings and possibly socially subordinate, also showed high mobility bud did not occupy the same area each morning. Since cowbirds lay eggs in the morning and were rarely seen feeding then, the disjunct areas visited by commuters can be characterized as morning-breeding (egg-laying) and afternoon-feeding areas. We found little evidence of territoriality on morning ranges, nor did we find evidence of prolonged pair bonds. The morning ranges of commuters averaged 68 ha, and their total home ranges, including afternoon-feeding areas, averaged 442 ha. These are among the largest breeding home ranges described for passerines, and they equal those of certain raptors. Raptors require large areas to provide a sufficient prey base, and cowbirds require similarly large areas to provide a sufficient number of host nests. The cowbird's commuting pattern, which is unique among passerines, involves a shift from largely asocial behavior in the morning to extreme sociality in the afternoon. Unlike related nonparasitic icterids that disperse from central breeding sites to feed, cowbirds do the reverse, dispersing from centralized feeding sites to breed. The cummuting behavior of cowbirds is yet another example of the flexibility of a species' behavioral ecology in response to the dispersion of resources essential for maintenance and reproduction.
Article
We studied brood parasitism by Brown-headed Cowbirds (Molothrus ater) in a strongly fluctuating island population of Song Sparrows (Melospiza melodia). One to three cowbird females visited the island daily to search for host nests in 13 of 16 study years. Individual cowbirds visited on more days and laid more eggs at high sparrow densities, but some factor, perhaps aggression by territorial female cowbirds, limited cowbird numbers at high sparrow densities. Female cowbirds exhibited natal and breeding philopatry. Individual cowbirds laid about every other day on average, began to lay well after sparrows in most years, and avoided laying two eggs in the same nest. Cowbirds rarely laid after hosts had completed laying, and their eggs hatched as often as host eggs. Parasitism rates increased with host age, probably because of age-related differences in host behavior. Cowbirds removed a host egg from two-thirds of parasitized nests, and damaged two per cent of remaining host eggs. Parasitism reduced production of fledgling sparrows per nest by 0.79, but only by 0.27 fledglings in years when food was added experimentally to some territories. Nests failed more often during incubation in years with than without parasitism, supporting our earlier suggestion that cowbirds destroy eggs in host nests to enhance future laying opportunities. Colonization of Mandarte Island by cowbirds had little effect on sparrow numbers because of: (1) incomplete overlap in laying seasons; (2) multiple broods in sparrows; (3) functional and numerical responses of cowbirds to host density; (4) the sparrows' ability to rear their young with young cowbirds; and (5) density-dependent host reproductive success.
Article
The social behavior of 154 banded male Brown-headed Cowbirds (Molothrus ater) was studied at London, Ontario in 1966 and 1967. Aggressive encounters between them appeared to involve either the establishment and maintenance of a dominance hierarchy or the guarding of resident females. Dominant males mated with resident females in mostly monogamous relationships. Males occupied specific ranges but did not defend them. Dominant males, however, defended their mates, and females sometimes defended their males from other females. Excess males were present and in breeding condition. They attended any female during her mate's absence, courting and guarding her from advances by other males and occasionally copulating with her.
Article
The Monitoring Avian Productivity and Survivorship (MAPS) programme is a cooperative effort to provide annual regional indices of adult population size and post-fledging productivity and estimates of adult survival rates from data pooled from a network of constant-effort mist-netting stations across North America. This paper provides an overview of the field and analytical methods currently employed by MAPS, a discussion of the assumptions underlying the use of these techniques, and a discussion of the validity of some of these assumptions based on data gathered during the first 5 years (1989-1993) of the programme, during which time it grew from 17 to 227 stations. Ageand species-specific differences in dispersal characteristics are important factors affecting the usefulness of the indices of adult population size and productivity derived from MAPS data. The presence of transients, heterogeneous capture probabilities among stations, and the large sample sizes required by models to deal effectively with these two considerations are important factors affecting the accuracy and precision of survival rate estimates derived from MAPS data. Important results from the first 5 years of MAPS are: (1) indices of adult population size derived from MAPS mist-netting data correlated well with analogous indices derived from point-count data collected at MAPS stations; (2) annual changes in productivity indices generated by MAPS were similar to analogous changes documented by direct nest monitoring and were generally as expected when compared to annual changes in weather during the breeding season; and (3) a model using between-year recaptures in Cormack-Jolly-Seber (CJS) mark-recapture analyses to estimate the proportion of residents among unmarked birds was found, for most tropical-wintering species sampled, to provide a better fit with the available data and more realistic and precise estimates of annual survival rates of resident birds than did standard CJS mark-recapture analyses. A detailed review of the statistical characteristics of MAPS data and a thorough evaluation of the field and analytical methods used in the MAPS programme are currently under way.
Article
We live-trapped and banded Brown-headed Cowbirds (Molothrus ater; n = 1,722 individuals) to evaluate sex ratios and survival probabilities during the breeding season at the San Juan Basin Research Center, La Plata County, Colorado, from 1992 to 1999. For adult Brown-headed Cowbirds, sex ratios varied significantly among years, and sex ratios were always male-biased within a year. In hatch-year (HY) cowbirds, and within years, sex ratios were similarly male-biased, but there was no difference among years in HY sex ratios. Using the program MARK, for adult females, second-year (SY) males, and after-second-year (ASY) males, the most parsimonious model, phi((g*a2-t/.))p((t)) suggested that some variation occurred among these groups in individual survival probability (phi) to the next year after the initial year of capture and in all following years. However, this model indicated no differences among these groups to best describe the probability of recapturing an individual (p). For HY individuals, analyses in MARK indicated that the most parsimonious model, phi((a2-./.))p((t)) did not include any differences in survival probability between the sexes, and survival probability was constant for the cohorts and periods examined. We also found some evidence that the residency status of adult Brown-headed Cowbirds potentially affected their frequency of recapture in the following year. In adults, the male-biased sex ratios that we observed were very likely the result of decreased Survival probability of adult females compared with both SY and ASY males. Additionally, the male-biased sex ratios of adults may have already begun to develop in HY individuals. Received 6 September 2007 accepted 15 October 2008.
Article
Identifying the factors used by an avian brood parasite to select host nests is important in understanding the determinates of individual reproductive success, yet such factors are poorly known for most parasitic species. Insights into these factors may come from understanding the conditions under which female parasites lay more than one egg per host nest (multiple parasitism). Using genetic techniques we examined patterns of multiple parasitism on a preferred host, the song sparrow (Melospiza melodia), to determine some basic patterns of multiple parasitism. Multiple parasitism involved equal frequencies of the same female parasitizing the same nest again and two or more females parasitizing the same nest. The frequency of multiple parasitism increased as the season progressed. We also documented a high frequency of parasitism that was not synchronized with host laying. These laying patterns may be the result of cowbirds "making the best of a bad situation" or of suboptimal host choice by inexperienced, nonselective females.
Article
For generalized brood parasites, which use many host species, it is assumed that the population dynamics of the parasite are unaffected by any one host species and that the hosts accept the eggs of parasites. Females of many parasitized species must renest several times within a season in order to replace themselves. Several species are in danger of extirpation as a result of brood parasitism by cowbirds Molothrus spp., which are increasing in population and have expanded their ranges and come into contact with many species that have not evolved a resistance to brood parasitism. One-host-one-brood-parasite associations represent a generalization of insect host-parasitoid associations, which have been extensively studied. These associations have a stable equilibrium point, in contrast with the oscillatory behavior exhibited by the simplest (Nicholson-Bailey) host-parasitoid models. In general, the longer the host lives, the more likely the system is to be stable rather than oscillatory. Most specialized brood parasites are too rare to have an impact on the overall host population, although some local populations may be affected. These models can be modified to include the effects of host resistance (individuals of some host species reject the eggs of parasites), the effects of spatial heterogeneity (whereby some host populations are more vulnerable to brood parasites than others), and renesting following parasitism. -from Authors
Article
Descriptions of the laying behaviours of obligate brood parasites such as the brown-headed cowbird,Molothrus ater , typically fall into one of two categories. These categories are ‘shotgun behaviour’, where females are predicted to distribute their eggs apparently at random between available host nests, or ‘host selection behaviour’, where females lay their eggs discriminately, placing them into the most suitable host nests. To test these hypotheses, data on individual female fecundity and host use patterns are required, but until recently, such information has been largely unavailable. We used field observations and genetic parentage analysis to describe the fecundity and laying behaviours of individual female cowbirds over six breeding seasons at Delta Marsh, Manitoba, Canada. Although some females used more than one host species both within and between breeding seasons, other females parasitized specific host species regardless of their availability. This nonrandom host use suggests some degree of host selection in this population. We also found that realized female fecundity was low (X±SD=2.3±0.6 eggs per female) compared with previously published estimates. Our results suggest that female laying behaviour lies on a gradient between the two extreme categories of pure ‘shotgun’ or ‘host selection’ laying behaviours, and females may optimize their reproductive effort by varying their behaviours as environmental conditions dictate. When high-quality hosts are not available for parasitism, cowbirds may switch to parasitizing lower-quality hosts. Flexibility in cowbird laying behaviour may be an adaptation that enables individuals to successfully exploit local and temporal variation in the availability of different hosts. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.
Article
Summary In brood parasites, knowledge of spacing behaviour, habitat use and territoriality may reveal cues about how parasites find and use their hosts. To study the use of space and habitat of European cuckoos, Cuculus canorus, we radio-tagged 16 females during four consecutive reproductive seasons. We hypothesized that during the laying period cuckoo females should (1) use habitats selectively, and (2) attempt to monopolize potential egg laying areas to reduce competition for host nests. Our data are consistent with the first hypothesis: the use of pond edges compared to forest and transitional habitats was significantly greater than expected from the habitat availability in the total area and within individual female home ranges. All 26 directly observed egg layings and 27 nest visits without laying occurred at pond edges in nests of Acrocephalus spp. Females spent significantly more time at pond edges on egg- laying days than on non-laying days. The second hypothesis was not supported: female home ranges overlapped similarly in all three major habitat categories of the potential egg laying areas, and only little aggression was observed between females. We discuss whether female
Article
The reproductive success of obligate brood parasitic birds depends on their ability to seek out heterospecific nests. Some nests are more suitable for parasitism than others and, for example, parasitic females may benefit from laying eggs preferentially and repeatedly at safer sites. Observations on patterns of Brown-headed Cowbird (Molothrus ater) parasitism on Eastern Phoebes (Sayornis phoebe) across 2 years suggested that parasitism occurred at above chance levels during the first rather than the second nesting attempts and at nests located under eaves rather than bridges. Previously parasitized nests were more likely to be parasitized again in the subsequent breeding season. Sites under eaves and bridges did not differ in whether Brown-headed Cowbirds could be detected in the proximity of the nest. However, nests from first nesting attempts and nests under eaves were less likely to be lost as a result of structural failure of the Eastern Phoebe's mud nest. These data suggest that site discrimination by Brown-headed Cowbirds leads to adaptive patterns of parasitism among available Eastern Phoebe nests.