Prolactin secretion in a brood parasite, the brown-headed cowbird, Molothrus ater

Blood samples were collected from a free-living population of male and female brown-headed cowbirds, Molothrus ater , during the breeding season and plasma levels of prolactin were measured by radioimmunoassay. Prolactin concentrations in both sexes increased significantly in mid to late May; peak levels were reached by June and were maintained throughout June and into July. Prolactin levels in juveniles were significantly lower than in adults but were still elevated. Prolactin has been implicated in parental care in several avian species; however, cowbirds are brood parasites with complete absence of parental behaviour, suggesting that prolactin must function in another capacity in cowbirds, or that cowbirds have developed target tissue insensitivity to this hormone.     

Low abundance of microsatellite repeats in the genome of the brown-headed cowbird (Molothrus ater).

A cosmid library made from brown-headed cowbird (Molothrus ater) DNA was examined for representation of 17 distinct microsatellite motifs including all possible mono-, di-, and trinucleotide microsatellites, and the tetranucleotide repeat (GATA)n. The overall density of microsatellites within cowbird DNA was found to be one repeat per 89 kb and the frequency of the most abundant motif, (AGC)n, was once every 382 kb. The abundance of microsatellites within the cowbird genome is estimated to be reduced approximately 15-fold compared to humans. The reduced frequency of microsatellites seen in this study is consistent with previous observations indicating reduced numbers of microsatellites and other interspersed repeats in avian DNA. In addition to providing new information concerning the abundance of microsatellites within an avian genome, these results provide useful insights for selecting cloning strategies that might be used in the development of locus-specific microsatellite markers for avian studies.     

Stable isotopes identify the natal origins of a generalist brood parasite, the brown-headed cowbird Molothrus ater

Identifying the natal origins of brood parasites is a major challenge that usually requires labor-intensive searching for nests of host species. Stable isotope analysis of feathers and other body tissues of parasitic young could be a possible tool for determining natal origins if tissues reflect the isotopic composition of the diet fed to nestlings. We measured the carbon (¹³C) and nitrogen (¹⁵N) isotope compositions of feathers for two age-classes of brown-headed cowbirds Molothrus ater at the Konza Prairie Biological Station near Manhattan, Kansas: nestlings raised by five species of songbird hosts in two different habitats, and juveniles captured after independence. Isotope values from cowbird nestlings did not differ among host species and we were unable to assign juvenile cowbirds to their natal hosts. However, nestlings raised in grassland habitat had feathers that contained significantly higher δ¹³C values and lower δ¹⁵N values than nestlings raised in shrub habitats. In addition, independent juveniles had isotopic signatures that were similar to cowbird nestlings raised on shrub habitats. Although dickcissel Spiza americana comprised the majority of samples from shrub habitats, our conclusions reflect the natural pattern of parasitism at the site and should be representative of cowbirds raised at Konza. We conclude that stable isotope analysis of feathers is effective for determining the natal origins of parasitic young if isotope values from nestlings are isotopically distinct among habitats.     

Growth strategies of passerine birds are related to brood parasitism by the brown-headed cowbird (Molothrus ater)

Sibling competition was proposed as an important selective agent in the evolution of growth and development. Brood parasitism by the brown-headed cowbird (Molothrus ater) intensifies sibling competition in the nests of its hosts by increasing host chick mortality and exposing them to a genetically unrelated nestmate. Intranest sibling competition for resources supplied by parents is size dependent. Thus, it should select for high development rates and short nestling periods, which would alleviate negative impacts of brood parasitic chicks on host young. I tested these predictions on 134 North American passerines by comparative analyses. After controlling for covariates and phylogeny, I showed that high parasitism rate was associated with higher nestling growth rate, lower mass at fledging, and shorter nestling periods. These effects were most pronounced in species in which sibling competition is most intense (i.e., weighing over about 30 g). When species were categorized as nonhosts versus old hosts (parasitized for thousands of years) versus new hosts (parasitized the last 100-200 years), there was a clear effect of this parasitism category on growth strategies. Nestling growth rate was the most evolutionarily flexible trait, followed by mass at fledging and nestling period duration. Adjustments during incubation (incubation period length, egg volume) were less pronounced and generally disappeared after controlling for phylogeny. I show that sibling competition caused by brood parasites can have strong effects on the evolution of host growth strategies and that the evolution of developmental traits can take place very rapidly. Human alteration of habitats causing spread of brood parasites to new areas thus cascades into affecting the evolution of life-history traits in host species.     

Searching for the functional origins of song in eastern brown-headed cowbirds,Molothrus ater ater

Three experiments were conducted to examine the functional properties of vocal precursors to stereotyped song in socially housed captive cowbirds. Previous studies had shown that eastern male cowbirds developed different song repertoires when housed in different social contexts. This paper reports on the acoustic origins of the different vocal outcomes. In addition, a winter roost of cowbirds was studied to investigate the setting in which males naturally develop song. Analyses of the vocalizations of laboratory-housed subjects revealed acoustic differences from the earliest stages of song ontogeny. The field data indicated that free-living and laboratory-housed males shared the same structural categories of song and that the winter roost provided males with opportunities to interact with females. Taken as a whole, the studies suggest that song ontogeny is a dynamic process involving social and vocal interactions between singers and listeners.     

Sex differences in auditory filters of brown-headed cowbirds (Molothrus ater)

Receiver sensory abilities can be influenced by a number of factors, including habitat, phylogeny and the selective pressure to acquire information about conspecifics or heterospecifics. It has been hypothesized that brood-parasitic brown-headed cowbird (Molothrus ater) females may locate or determine the quality of potential hosts by eavesdropping on their sexual signals. This is expected to produce different sex-specific pressures on the auditory system to detect conspecific and heterospecific acoustic signals. Here, we examined auditory filter shape and efficiency, which influence the ability to resolve spectral and temporal information, in males and females at center frequencies of 2, 3 and 4 kHz. We found that overall, cowbirds had relatively wide filters (lsmean ± SE: 619.8 ± 41.6 Hz). Moreover, females had narrower filters (females: 491.4 ± 66.8, males: 713.8 ± 67.3 Hz) and greater filter efficiency (females: 59.0 ± 2.0, males: 69.8 ± 1.9 dB) than males. Our results suggest that the filters of female cowbirds may allow them to extract spectral information from heterospecific vocalizations. The broader auditory filters of males may reflect limited spectral energy in conspecific vocalizations in this frequency range, and hence, weaker selection for high resolution of frequency in the range of 2–4 kHz.     

The agonistic and sexual functions of vocalizations of male brown-headed cowbirds,Molothrus ater

Functional analyses of three vocalizations of male brown-headed cowbirds, Molothrus ater, are presented and it is shown that two of these function as songs. Perched song (PS) is given by stationary birds and is used over short distances in male-male aggression and female courtship. The ‘flight whistle’ (FW) is a multisyllabic, usually pure tone, vocalization and the ‘single syllable flight call’ (SS) a monosyllabic pure tone. FWs and SSs are given occasionally by perched males and repetitively by flying males. Field playbacks of FWs showed that male cowbirds responded in the agonistic way most passerines respond to playback of conspecific song. Playbacks of the female's chatter call demonstrated that males of all three subspecies use FWs and SSs, but almost never PSs, to communicate with a female as they approach her. Thus FWs and SSs are used to communicate over long distances with both males and females. FWs are also used in one critical short-distance context as they are significantly more likely than PSs to be given just before copulations in nature, at least in our western study area. By contrast, studies of captive eastern cowbirds showed that PSs regularly precede copulations whereas FWs rarely do so. These different results for western and eastern birds may be an artefact of captivity and/or a result of geographical variation in behaviour.     

Host use strategies of individual female brown-headed cowbirds Molothrus ater in a diverse avian community

Although it is well established that brown-headed cowbirds Molothrus ater are host generalists, the parasitism strategies of individual females are not well documented. Here we use microsatellite genotyping to characterize host use by individual female cowbirds. Of 205 offspring sampled at a site in northeastern Illinois during one breeding season, we identified or inferred 33 mothers of 141 offspring, with up to 13 (mean 4.72) offspring assigned to individual females. Most (83%) females were host generalists and parasitized up to five host species, indicating that community-wide patterns of parasitism do not result from different individuals specializing on different host species. However, some females (17%) parasitized a single species, suggesting that alternative host-use strategies may exist within a single population. For host generalist females, individuals often parasitized host species that differed in body mass, nest-site placement, and quality, indicating that female cowbirds exhibit extremely flexible host ranges. However, female cowbirds consistently discriminated among potential hosts, for example, by completely avoiding some common, but unsuitable species.     

Honesty in host-parasite communication signals: the case for begging by fledgling brown-headed cowbirds Molothrus ater

Nestling parasites typically beg more intensively than do host nestlings yet these exaggerated displays are also honest in that they are modulated by hunger and age. We hypothesized that honesty was also maintained in the food solicitation behaviors of fledgling brood parasites because the benefits and costs of their begging displays are similar to those of nestling parasites. Begging displays of hand-reared 14–32 days old brown-headed cowbirds Molothrus ater that had experimentally manipulated nutritional needs were recorded to analyze variation in the peak frequency, duration, and rate of fledglings' begging bouts. Peak frequency of bouts decreased with greater age and was lower for females. Bout rate was greater with increasing hunger levels of fledgling parasites but did not vary with age. Consistent and predictable variation of acoustic begging displays with age, sex, and hunger-level indicates honesty in host-parasite communication systems through conveying truthful information about the many possible needs of parasitic fledglings.     

Temporal patterns of host availability, brown-headed cowbird brood parasitism, and parasite egg mass

I studied relationships between temporal patterns of host availability, brood parasitism, and egg mass for the parasitic brown-headed cowbird (Molothrus ater). At a study site consisting largely of edge habitat in north-eastern Illinois, I found 834 bird nests from 27 species. A total of 407 cowbird eggs and nestlings were found in these nests over three laying seasons. Nearly all (n= 379; 93%) were found in the nests of seven host species. For these species and all taken together, weekly nest availability generally decreased whereas parasitism frequency generally increased throughout the cowbird laying season, but the proportions of nests parasitized and the mean number of cowbird eggs in them did not. Additionally, no correlation was found between the proportions of nests parasitized and nest availability. Cowbird egg mass generally increased throughout the laying season, indicating that foraging conditions improved and that, early in the laying season, egg mass and quality may be less important than quantity. Consistently high weekly levels of parasitism indicate that cowbird reproduction was less limited by resources needed for egg production and more by the availability of suitable host nests. Fluctuating weekly host availabilities suggest that previously established, constant rates of cowbird egg laying would produce an excess of eggs during periods of low host availability. Further, the low frequency of parasitism (1%) of nests in stages too advanced for successful parasitism, and of abandoned nests, is consistent with the hypothesis that cowbirds' consistently high rate of egg production helps assure an egg is available when an appropriate nest is found. Frequently, nests were parasitized multiple times, raising the possibility that cowbirds were interfering with their own reproduction. A diverse host community increases the possibility that a decline of any one host species is unlikely to affect cowbird reproduction significantly. Received 11 July 1997 / Accepted: 31 March 1998     

Energy demands of migration on red-eyed vireos, Vireo olivaceus

We studied the behavior of red-eyed vireos (Vireo olivaceus)—includingtheir length of stopover, rate of mass change (lipid deposition),and foraging behavior—in relation to energy demand aftertheir spring migration across the Gulf of Mexico. We labeledeach foraging bird as ‘lean’ or ‘fat’based on the average condition of birds captured on that day.Lean birds stayed longer and were more likely to gain mass duringtheir stopover than birds that arrived with unmobilized fatreserves. Several aspects of the birds' foraging behavior variedin relation to mean body mass on observation days: Lean migrants(1) broadened their use of microhabitat but not their verticaldistribution of foraging locations, (2) expanded their feedingrepertoire, (3) moved at a higher mean velocity while averagingthe same frequency of foraging movements, and (4) increasedtheir degree of turning after a feeding attempt. We suggestthat the gain of mass by fat-depleted birds is a compensatoryresponse to energy demand mediated by changes in foraging behavior.     

Cultures, genes, and neurons in the development of song and singing in brown-headed cowbirds (Molothrus ater)

In brown-headed cowbirds, Molothrus ater, as in many songbird species, vocalizations are fundamental to reproduction. In our studies, experiments utilizing different social housing regimes and geographic comparisons have indicated the social learning of males' vocalizations and associated abilities to use vocalizations effectively during the breeding season. Here, we describe studies indicating roles of cultural and genetic background, and of social influences from females, on male vocal development. These influences can interact with neural regions, including song learning and song control nuclei, but also visual-processing nuclei, in the development of signaling. We argue that a developmental systems approach to the study of vocal behavior provides a structure to organize these different influences and how they may interact with one another over development. A systems approach requires that researchers study the social context in which signals and signalers develop - both the ontogenetic arena in which young animals learn their signals from older animals, and the functional arena in which young and older animals socially interact with one another.     

Temporal patterns of circulating LH and steroid hormones in a brood parasite, the Brown-headed cowbird, Molothrus ater.

The reproductive cycle of monogamous passerines is characterized by a sequence of events leading from courtship and pair formation through nest building, egg laying and incubation, and ending with the care of young. Each of these stages is accompanied by equally predictable changes in the pattern of hormone secretion. This report describes endocrine changes in the plasma of free-living Brown-headed cowbirds, Molothrus uter , during a breeding season. Changes in some morphological parameters are also presented.
Cowbirds are brood parasites that show no parental behaviour and lay eggs almost daily for approximately eight weeks. Males are not territorial, but they mate guard extensively. Hcncc, their breeding biology is quite different from that of the nesting species described above. They are hormonally distinct, as well. Luteinizing hormone rises in the spring and remains elevated throughout May and most of June. By July, when breeding has ceased, plasma luteinizing hormone levels are at pre-breeding values. Plasma androgens are also maintained at peak levels for an extended period of time, although they decline briefly between the establishment of dominance hierarchies and the onset of breeding. Circulating levels of corticosterone are elevated until the middle of the breeding season, when they begin to fall, although here, too, there is a transient depression just prior to breeding. In the light of the studies on nesting species, these data suggest that hormone profiles are closely correlated with the breeding and social activities that characterize the species under investigation.     

Genetic analysis of brown-headed cowbirds Molothrus ater raised by different hosts: data from mtDNA and microsatellite DNA markers

Ecological and behavioural data suggest that female brood parasitic brown-headed cowbirds Molothrus ater are host generalists; this predicts that there should be no genetically differentiated host races within cowbird populations. We tested this hypothesis by comparing differentiation in two rapidly evolving DNA markers (mtDNA control region sequence and nuclear DNA microsatellite loci) among unrelated cowbird chicks raised by two ecologically distinct hosts. No differentiation was observed in either marker supporting the absence of host race hypothesis.     

Non-invasive corticosterone treatment changes foraging intensity in red-eyed vireos Vireo olivaceus

Corticosterone is thought to play an important role in food caching and foraging behaviour. However, the direct influence of increased plasma corticosterone on feeding behaviour is still unclear. In this study the effect of increased corticosterone on feeding behaviour in migratory active red-eyed vireos Vireo olivaceus was investigated. We hypothesized that if increased corticosterone levels facilitate foraging behaviour, an increased number of visits to the food bowl by corticosterone treated birds would be seen. In addition to ad lib food during the experiment, the vireos were fed every full hour between 09.00–13.00 h with one meal worm injected either with 4 μg corticosterone dissolved in 20 μl DMSO, or with DMSO only as a control treatment. The presence or absence of a bird in the food bowl was recorded by a motion detector between 09.00–15.00 h. The non-invasive corticosterone treatment increased plasma corticosterone levels and caused vireos to visit the food bowl more frequently compared to control treated individuals between 11.00–12.00 h and 13.00–15.00 h. Our data indicate that corticosterone has an effect on feeding behaviour in birds.     

Lateralization and motor stereotype of song production in the brown-headed cowbird

Song production in adult brown-headed cowbirds(Molothrus ater ater) is lateralized, with a slight right syringeal dominance. The left size of the syrinx produces low-frequency (200–2000 Hz) notes within the introductory note clusters, while the right side produces the higher-frequency (1500–6000 Hz) introductory notes, the interphrase unit (10–12 kHz), and the final high-frequency whistle (5–13 kHz). Cross-correlation analyses reveal that individual cowbirds produce each of their four to seven song types with a distinct stereotyped motor pattern–as judged by the patterns of syringeal airflow and subsyringeal pressure. The acoustic differences across song types are reflected in the differences in the bronchial airflow and air sac pressure patterns associated with song production. These motor differences are particularly striking within the second and third introductory note clusters where there is a rapid switching back and forth between the two sides of the syrinx in the production of notes. These motor skills may be especially important in producing behaviorally effective song. 1994 John Wiley & Sons, Inc.     

Shiny cowbird Molothrus bonariensis egg size and chick growth vary between two hosts that differ markedly in body size

In birds, egg size affects chick growth and survival and it is an important component of reproductive success. The shiny cowbird Molothrus bonariensis is an extreme generalist brood parasite that uses hosts with a wide range of body masses. Survival of cowbird chicks decreases with host body mass, as competition for food with nestmates is more intense in large than in small hosts. We studied variation in shiny cowbird egg size and chick growth in two hosts that differ markedly in body size: the chalk‐browed mockingbird Mimus saturninus (70–75 g), and the house wren Troglodytes aedon (12–13 g). We analyzed: 1) if females parasitizing mockingbirds lay larger eggs than those parasitizing wrens, and 2) the association between egg size and chick growth. We experimentally controlled for time of parasitism and number of host chicks and evaluated growth rate of male and female parasite chicks. Shiny cowbirds parasitizing mockingbird nests laid larger eggs than those parasitizing wren nests. Chick body mass after hatching was positively associated with egg size until chicks were five days of age, but there was no association between egg size and growth rate, or asymptotic mass. There were no sexual differences in egg size or body mass at the time of hatching, but growth rate was higher in males than in females leading to sexual dimorphism in asymptotic mass. Differences in egg size between hosts and the effect of egg size on body mass after hatching support the hypothesis that different females are specialized in the use of hosts that differ in body mass.     

Sex-specific variation in brown-headed cowbird immunity following acute stress: a mechanistic approach

There is some discrepancy in the literature regarding whether acute stress is immunostimulatory or immunosuppressive. Studies of domesticated (laboratory and food) animals and humans typically indicate that acute stress is immunostimulatory, whereas studies of non-domesticated species document both immunostimulatory and immunosuppressive results. Few studies have examined the mechanisms responsible for changes in immune activity in species other than those classically used in laboratory research. We examined the effect of both acute stress and exogenous corticosterone (CORT) on the bactericidal capacity (BC) of blood plasma from captive, wild-caught brown-headed cowbirds (Molothrus ater) to determine if CORT is responsible for changes in levels of immune activity. We conducted "stress tests" in which we handled birds to elicit a stress response and then measured the birds' total CORT and BC at 30 or 90 min post-stressor. We also conducted non-invasive tests in which we administered exogenous CORT by injecting it into mealworms that were fed to the cowbirds remotely. Total, free, and bound CORT levels, corticosteroid binding globulins (CBGs), and BC at 7 or 90 min post-mealworm ingestion were measured. Both males and females exhibited significant increases in total CORT following handling stress and the administration of exogenous CORT. Experimental males and females also exhibited a significant increase in CBG capacity at 7 min post-mealworm ingestion compared to controls. Male cowbirds exhibited a significant decline in their BC following both handling stress and the administration of exogenous CORT whereas female cowbirds exhibited no decline under either condition. Female CBG levels were not different than those of males, suggesting that differences in BC could be due to differences between the sexes in the number of corticosteroid receptors which, along with CBGs, regulate the stress response. Female cowbirds may modulate their stress response as an adaptive life-history strategy for maximizing current reproduction.