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Direct Estimates of Breeding Site Fidelity and Natal Philopatry in Brood Parasitic Brown-Headed Cowbirds Molothrus ater
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.
... Species-speci c drivers of vagrancy: high vagrancy in a brood parasite indicates social route learning in passerines Brown-headed Cowbirds were a notable outlier in my dataset, occurring offshore signi cantly more than other passerines, driven overwhelmingly by juveniles in the fall. Greater vagrancy propensity in juveniles could result from high post-natal dispersal, but this explanation does not t Brown-headed Cowbirds as they show equivalent levels of natal philopatry to other North American passerines (57). Instead, increased vagrancy in Brown-headed Cowbirds may relate to social learning and brood parasitism. ...
... Social route learning is not documented in other passerines (5, 6) but may occur via ight calls in nocturnal migrants or ocking in diurnal migrants (30,31). Cowbirds, and several other members of Icteridae, migrate in ocks during the day (32,57,58), providing opportunities for social route learning. Unlike other members of Icteridae, cowbirds are brood parasites, meaning females lay eggs in other species nests allowing that species to rear their offspring for them. ...
Background
Migratory birds possess remarkable navigational adaptations. Vagrants, few individuals who migrate to incorrect locations, offer a unique opportunity to study how navigation is accomplished. Vagrancy may occur due to external factors forcing birds off course, such as strong wings, or navigational errors. Natural disruptions in the Earth’s magnetic field may cause navigational errors interfering with bird magnetoreception. Failures of other navigation tools like visual landmark recognition and social route learning may also contribute to vagrancy, but these factors are difficult to quantify.
Methods
I used eBird, a community science dataset comprising millions of bird observations, to study the relative likelihood of offshore vagrancy under different external factors including weather, geomagnetic disturbance, and solar activity using mixed effects logistic regression. Then, I studied how variation in species vagrancy propensity is be driven by morphology and migration distance using multiple linear regression, or inheritances of vagrancy alleles with phylogenetic analysis.
Results
High geomagnetic disturbance and low visibility increased offshore vagrancy, and winds did not appear to blow birds off course. Most variation in offshore vagrancy stemmed species specific differences, best explained by an interaction between wing pointedness and migration. A longer migration distance was strongly correlated with increased vagrancy in birds with rounded wings, but the relationship was absent in birds with a pointed wing shape. Brown-headed Cowbirds were notably more prone to vagrancy than any other passerine.
Conclusions
External factors primarily cause vagrancy by interfering with magnetic and visual senses, rather than physically forcing birds off course. Species with longer migrations have more time to encounter these vagrancy causing events, but a more pointed wing shape may allow birds to reorient more efficiently. While migration routes are primarily genetic in passerines, I found evidence of an overlooked role of social learning in route inheritance. I contend that as a brood parasite, solitary juvenile Brown-headed Cowbirds experience more difficulty in joining migratory flocks, thus missing out on social route learning opportunities. These results clarify the relative role of different navigational adaptations in migratory birds and demonstrate the utility of studying vagrants to understand bird migration.
... 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). ...
The antagonistic arms races between obligate brood parasites and their hosts provide critical insights into coevolutionary processes and constraints on the evolution of life history strategies. In avian brood parasites—a model system for examining host–parasite dynamics—research has primarily focused on the egg and nestling stage, while far less is known about the behavior and ecology of fledgling and juvenile brood parasites. To provide greater insights into the post-fledging period of generalist brood parasites, we used handheld and automated telemetry systems to examine the behavior and survival of fledgling brown-headed cowbirds (Molothrus ater). Our host community-wide analysis (data on cowbirds fledged from different host species were pooled) shows that fledgling cowbirds’ follow patterns of movement and survival found across the post-fledging literature on parental passerine species. Cowbird fledgling survival was lowest during the first 3 days post-fledging, whereas daily rates of survival neared 100% after about 16 days post-fledging. Cowbird daytime post-fledging activity rates, perch heights, and distance from the natal area all increased with fledging age and young generally gained independence from host parents at 3–4 weeks post-fledging, with approximately the same latency as has been observed in studies on fledglings of cowbird host species. Our research demonstrates how automated telemetry systems can overcome past methodological limitations in post-fledging research and provides an important foundation for future studies examining adaptations that cowbirds and other brood parasites use to exploit hosts during the post-fledging period.
... Brown-headed Cowbirds were an intriguing outlier in my dataset, occurring offshore more than other passerines, even when correcting for on land frequency. Brooks Movement Ecology (2024) 12:64 Greater vagrancy likelihood in juveniles could result from high post-natal dispersal, but this explanation may not fit Brown-headed Cowbirds as they show equivalent levels of natal philopatry to other North American passerines [52]. Brown-headed Cowbirds are unique in our dataset as brood parasites, meaning females lay eggs in other species nests allowing that species to rear their offspring for them. ...
Background
Migratory birds accomplish remarkable feats of long-distance navigation. Vagrants, few individuals who migrate to incorrect locations, reveal conditions where orientation and navigation fail. Studies of vagrancy on a continental scale reveal the importance of external factors such as strong winds driving birds off course, clouds obscuring migratory landmarks, and natural disruptions in the Earth’s magnetic field interfering with migratory orientation. Species may also possess characteristics that make them more prone to vagrancy. The external drivers of vagrancy on a smaller scale are less understood.
Methods
I used eBird, a community science dataset comprising millions of bird observations, to study land passerines observed over the Pacific Ocean, here termed offshore vagrants. These data present the opportunity to study a particular case of vagrancy: small-scale displacement into highly inhospitable areas. I modeled how season, wind, lack of visibility, interference with magnetoreception, and species differences may predict offshore vagrancy. Then, I modeled how species vagrancy likelihood is predicted by morphological and life history traits.
Results
Vagrancy was more common in the fall and positively associated with stronger tail winds in the spring. Species with greater preference for understory foraging habitat were less likely to occur as vagrants. Species vagrancy likelihood was higher in birds with a longer migration distance and rounded wings, but the relationship was weaker in birds with a pointed wings. Brown-headed Cowbirds were the most common offshore species in terms of absolute number of records and proportional to onshore frequency.
Conclusions
Offshore community science records proved revealing of mechanisms for small scale vagrancy in passerines. Offshore vagrancy can be driven by wind drift in the spring, but not in the fall despite higher overall levels of vagrancy. Life history characteristics like foraging habitat preference and migration duration may make some species more vulnerable to the effects of wind drift. Species with longer migrations may have more time to encounter vagrancy causing events, but greater aerodynamic efficiency may counteract this effect.
... More frequent surveys, along with marking of individual cowbirds, will be necessary to determine if cowbirds are commuting from other habitats. We consider this possibility less likely given the evidence for territoriality and site fidelity in cowbirds (Darley 1982, Rothstein et al. 1984, Strausberger 1998, Alderson et al. 1999, Hauber et al. 2020, but further study is needed to determine the causes of this intriguing trend of persistent parasitism despite decreased parasite abundance. ...
The abundance of a widespread brood parasite, the Brown-headed Cowbird (Molothrus ater), has decreased by »30% in North America over the past 5 decades. Within a community, brood parasite abundance may be expected to positively correlate with host brood parasitism frequency and intensity, but evidence for this correlation is mixed. Few studies have examined if long-term changes in brood parasite abundance have resulted in changes to host parasitism frequency. We measured cowbird abundance, brood parasitism frequency and intensity of 4 riparian songbird species, and host abundance and richness in 2001-2004 and 2012-2014 in riparian vegetation of the south Okanagan Valley of British Columbia, Canada. We compared our data to historical data for the same parameters previously collected between »1960 and the early 1990s in the same area. We found that cowbird abundance decreased by »80% over 2 decades in the Okanagan Valley, mirroring or exceeding regional-scale trends. Host abundance and richness increased as cowbird abundance decreased. However, songbird brood parasitism frequency and intensity either increased or remained relatively high over more than 4 decades. We discuss possible explanations for this apparent disconnect between brood parasite abundance and host parasitism frequency and intensity, which offer opportunity for further study. Temporal changes in brood parasite abundance, such as the decline of Brown-headed Cowbirds in North America and the Common Cuckoo (Cuculus canorus) in Europe, should not be assumed to lead to correlated changes to host parasitism frequency and intensity.
... 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). ...
Understanding egg-laying behaviour of brood parasites in space and time can improve our knowledge of interactions between hosts and parasites. However, no studies have combined information on the laying activity of an obligate brood parasite with detailed information on the distribution of host nests within an area and time period. Here, we used molecular methods and analysis of egg phenotypes to determine maternal identity of common cuckoo, Cuculus canorus, eggs and chicks found in the nests of four species of Acrocephalus warblers in consecutive years. The median size of a cuckoo female laying area (calculated as a minimum convex polygon) was correlated negatively with the density of host nests and positively with the number of eggs assigned to a particular female. Cuckoo female laying areas overlapped to a large extent and their size and location did not change between years. Cuckoo females preferentially parasitized host nests located close to their previously parasitized nests and were mostly host specific except for two that parasitized two host species. Future studies should focus on sympatric host and parasite communities with variable densities across different brood-parasitic systems to investigate how population density of hosts affects fitness and evolution of brood parasites. For instance, it remains unknown whether female parasites moving to new sites need to meet a threshold density of a potential host. In addition, young females may be more limited in their egg laying, particularly with respect to the activity of other parasites and hosts, than older females.
Adaptive responses to ecological uncertainty may affect the dynamics of interspecific interactions and shape the course of evolution within symbioses. Obligate avian brood parasites provide a particularly tractable system for understanding how uncertainty, driven by environmental variability and symbiont phenology, influences the evolution of species interactions. Here, we use phylogenetically-informed analyses and a comprehensive dataset on the behaviour and geographic distribution of obligate avian brood parasites and their hosts to demonstrate that increasing uncertainty in thermoregulation and parental investment of parasitic young are positively associated with host richness and diversity. Our findings are consistent with the theoretical expectation that ecological risks and environmental unpredictability should favour the evolution of bet-hedging. Additionally, these highly consistent patterns highlight the important role that ecological uncertainty is likely to play in shaping the evolution of specialisation and generalism in complex interspecific relationships. Nearly 17% of all bird species are hosts to obligate brood parasites like the common cuckoo. Antonson et al. show that parasite species hedge their reproductive bets by outsourcing parental care to a greater variety of host species when the rearing environment for their young is more unpredictable.
Several hypotheses have been proposed to explain the mechanisms responsible for maintenance of host-specific gentes in the common cuckoo (Cuculus canorus). Some of them expect that when adult cuckoos return to lay their eggs to their natal site (natal philopatry hypothesis) or habitat in which they were reared (habitat-imprinting hypothesis), there is a higher probability of finding nests of the host species by which they were reared. Since published evidence is ambiguous, we here evaluate the natal philopatry and habitat-imprinting hypotheses using information on habitat homogeneity and cross-continental long-term ringing data. We found no evidence for the natal philopatry hypothesis-instead of returning to their natal site, juvenile cuckoos exhibited longer dispersal movements than adults, and the difference was even larger in comparison with a wide array of cuckoo host species. On the contrary, we found support for the habitat-imprinting hypothesis-juvenile cuckoos followed similar levels of natal habitat homogeneity at 5-and 25-km scale when returning to breed in the next years. Our results suggest that preference for the particular habitat structures may help cuckoos to find appropriate hosts.
Females are expected to have evolved to be more discriminatory in mate choice than
males as a result of greater reproductive investment into larger gametes (eggs vs.
sperm). In turn, males are predicted to be more promiscuous than females, showing
both a larger variance in the number of mates and a greater increase in reproductive
success with more mates, yielding more intense sexual selection on males vs.
females (Bateman's Paradigm). However, sex differences in costly parental care strategies
can either reinforce or counteract the initial asymmetry in reproductive investment,
which may be one cause for some studies failing to conform with predictions
of Bateman's Paradigm. For example, in many bird species with small female‐biased
initial investment but extensive biparental care, both sexes should be subject to similar
strengths of sexual selection because males and females are similarly restricted
in their ability to pursue additional mates. Unlike 99% of avian species, however,
obligate brood parasitic birds lack any parental care in either sex, predicting a conformation
to Bateman's Paradigm. Here we use microsatellite genotyping to demonstrate
that in brood parasitic brown‐headed cowbirds (Molothrus ater), per capita
annual reproductive success increases with the number of mates in males, but not
in females. Furthermore, also as predicted, the variance of the number of mates and
offspring is greater in males than in females. Thus, contrary to previous findings in
this species, our results conform to predictions of the Bateman's Paradigm for taxa
without parental care.
The two-note call of the male common cuckoo (Cuculus canorus), the so-called “cu-coo”, is well known to people as a natural and cultural signal. However, the so-called “bubbling” call of the female cuckoo is almost unknown to most, and its function in the social organization of cuckoos remains understudied. We carried out a study of a possible intraspecific communication function of female bubbling calls, using playbacks to female cuckoos in their natural environment. Regarding vocal responses, both female and male cuckoos paid attention to the bubbling calls as they consistently responded acoustically by calling but did not so during control playbacks of collared dove (Streptopelia decaocto) calls. Accordingly, in about 63% of trials, females approached the loudspeaker closely and 81% uttered bubbling calls themselves during the experiment. These results are consistent with a function that the bubbling call plays a role in territorial signaling and defense among females. Male cuckoos also showed strong responses to playbacks of bubbling calls, as they approached the speaker and themselves called in 94% of playbacks; this is consistent with a scenario that they are interested in unfamiliar, new females in the area. Specifically, males approached the speaker repeatedly by flight, often flew around it and then perched on a tree, and uttered different call types beside the general “cu-coo” (e.g., quick “cu-cu-coo”, “gowk” call, and “guo” call). Our results represent an illustrative example that a simple female call may have multiple functions, as the cuckoo bubbling call advertises territory need for female cuckoos and attracts males.
Significance statement
Avian brood parasites lay their eggs in nests of other bird species, causing hosts to incubate, feed, and rear the parasitic offspring. Parasitic adult common cuckoos maintain a complex acoustic communication system, but female cuckoo calls are only beginning to be studied. The basic intraspecific functions of females’ sparrowhawk-like “bubbling calls” have not yet been characterized, whereas interspecifically, they use it for reducing antiparasitic attacks by their hosts. Our playback experiments with bubbling calls revealed that both female and male cuckoos responded acoustically to unfamiliar bubbling calls and more males than females approached the speaker, relative to control playbacks. We conclude that bubbling call has dual basic intraspecific functions: mate attraction, and territorial spacing.
Obligate brood parasitic birds have evolved a rare avian strategy for reproduction by laying eggs in the nests of other species. In doing so, their breeding ranges, but not necessarily their foraging habitats, have become intimately related to the nesting territories of their hosts. We studied home range sizes and distribution patterns in Common Cuckoos (Cuculus canorus) on their breeding grounds in central Hungary, where cuckoos parasitize only Great Reed Warblers (Acrocephalus arundinaceus) in channel-side reed-beds at a high frequency (>50%). The geographic coordinates of tagged cuckoos were monitored by high-precision, remotely downloadable non-Platform Terminal Transmitter global positioning system (GPS) loggers, attached to 9 females and 6 males. Our results revealed bimodal use of habitat patches: (1) the home ranges of male and female cuckoos were packed densely along the channels where the hosts breed, and their distribution maps had high overlaps between sexes; (2) ~71% of cuckoos also visited nearby woodland patches, presumably for foraging, where the host species was not present. The size of cuckoo home ranges varied to an unusually great extent: 0.3–185 km2 as calculated by the minimum convex polygon method (85%), or 1–17 km2 when calculated by the more suitable kernel density estimation (KDE) method (Utilization Distribution 85%) for patchy habitats. Male and female cuckoos had similar home range sizes as estimated by the KDE method, consisting of 1–4 areas within the 2 habitat types of channel reed-beds and woodlands. No preference was revealed for night roosting locations between the 2 habitats or sexes. Female cuckoos were more likely to use reed-beds in the afternoons, when females parasitize host nests. Remote downloadable GPS methods offer an effective way of tracking cuckoos across large areas, but the estimation of home range sizes requires caution due to this species’ patchy and disconnected habitat use.
We describe a new collaborative network, the Motus Wildlife Tracking System (Motus; https://motus.org), which is an international network of researchers using coordinated automated radio-telemetry arrays to study movements of small flying organisms including birds, bats, and insects, at local, regional, and hemispheric scales. Radio-telemetry has been a cornerstone of tracking studies for over 50 years, and because of current limitations of geographic positioning systems (GPS) and satellite transmitters, has remained the primary means to track movements of small animals with high temporal and spatial precision. Automated receivers, along with recent miniaturization and digital coding of tags, have further improved the utility of radio-telemetry by allowing many individuals to be tracked continuously and simultaneously across broad landscapes. Motus is novel among automated arrays in that collaborators employ a single radio frequency across receiving stations over a broad geographic scale, allowing individuals to be detected at sites maintained by others. Motus also coordinates, disseminates, and archives detections and associated metadata in a central repository. Combined with the ability to track many individuals simultaneously, Motus has expanded the scope and spatial scale of research questions that can be addressed using radio-telemetry from local to regional and even hemispheric scales. Since its inception in 2012, more than 9000 individuals of over 87 species of birds, bats, and insects have been tracked, resulting in more than 250 million detections. This rich and comprehensive dataset includes detections of individuals during all phases of the annual cycle (breeding, migration, and nonbreeding), and at a variety of spatial scales, resulting in novel insights into the movement behavior of small flying animals. The value of the Motus network will grow as spatial coverage of stations and number of partners and collaborators increases. With continued expansion and support, Motus can provide a framework for global collaboration, and a coordinated approach to solving some of the most complex problems in movement biology and ecology.
Being an obligate parasite, juvenile common cuckoos Cuculus canorus are thought to reach their African wintering grounds from Palearctic breeding grounds without guidance from experienced conspecifics but this has not been documented. We used satellite tracking to study naïve migrating common cuckoos. Juvenile cuckoos left breeding sites in Finland moving slowly and less consistently directed than adult cuckoos. Migration of the juveniles (N = 5) was initiated later than adults (N = 20), was directed toward the southwest–signifi-cantly different from the initial southeast direction of adults–and included strikingly long Bal-tic Sea crossings (N = 3). After initial migration of juvenile cuckoos toward Poland, the migration direction changed and proceeded due south, directly toward the winter grounds, as revealed by a single tag transmitting until arrival in Northwest Angola where northern adult cuckoos regularly winter. Compared to adults, the juvenile travelled straighter and faster, potentially correcting for wind drift along the route. That both migration route and timing differed from adults indicates that juvenile cuckoos are able to reach proper wintering grounds independently, guided only by their innate migration programme.
Migratory species are in rapid decline globally. Although most mortality in long-distance migrant birds is thought to occur during migration, evidence of conditions on migration affecting breeding population sizes has been completely lacking. We addressed this by tracking 42 male Common Cuckoos from the rapidly declining UK population during 56 autumn migrations in 2011-14. Uniquely, the birds use two distinct routes to reach the same wintering grounds, allowing assessment of survival during migration independently of origin and destination. Mortality up to completion of the Sahara crossing (the major ecological barrier encountered in both routes) is higher for birds using the shorter route. The proportion of birds using this route strongly correlates with population decline across nine local breeding populations. Knowledge of variability in migratory behaviour and performance linked to robust population change data may therefore be necessary to understand population declines of migratory species and efficiently target conservation resources.
The timing and ecological circumstances under which individual songbirds acquire memorized copies of their species-typical songs can vary significantly within a population. Males that hatch later in the breeding season are likely to hear less conspecific song as juveniles than earlier-hatched individuals. In addition, late-hatched males will experience shorter days and decreasing photoperiods during their song acquisition phase, factors known to affect vocal development. We tested the prediction that yearling Brown-headed Cowbirds (Molothrus ater) that hatched earlier the previous season are more advanced in their development of repertoires of local songs than those hatched later. We recorded perched songs from 17 yearling and 20 adult males trapped at two adjacent sites in New York state and found that yearling perched song repertoires were smaller and contained few of the perched song types common to the repertoires of local adults (adult-shared types). As found in previous field studies of cowbirds, yearlings did not alter the content or size of their repertoires during the season. We used underwing juvenal-feather retention as a measure of relative hatching date in a subset of 15 yearlings and found that perched song repertoires of earlier-hatched yearlings contained more local adult-shared types than repertoires of younger, later-hatched birds. We also investigated flight whistles of males from one site and found that only 4 of 10 yearlings produced the flight whistle type typically given by local adults. Evidence linking flight whistle development and the plumage character we used as an indicator of hatching date was inconclusive.
Correlación Entre el Desarrollo Vocal y un Indicador de Tiempo de Eclosión en Molothrus ater
Resumen. El tiempo y las circunstancias ecológicas bajo las cuales las aves canoras adquieren copias memorizadas de los cantos típicos de su especie pueden variar entre poblaciones. Los machos juveniles que eclosionan tarde en la temporada tienen (1) menor probabilidad de escuchar cantos coespecíficos que los individuos que eclosionan antes y (2) experimentarán días más cortos y una disminución del fotoperiodo durante la fase de adquisición del canto, factores que afectan el desarrollo vocal. Probamos que juveniles de Molothrus ater que eclosionaron tempranamente durante la temporada anterior presentan un mayor avance en el desarrollo de repertorio de cantos locales que aquellos que eclosionaron más tarde. Registramos los cantos de percheo de 17 machos juveniles y de 20 machos adultos capturados en dos sitios adyacentes en el estado de Nueva York. Los repertorios de canto de juveniles fueron menores y contenían sólo unos pocos tipos de canto de percheo típicos de adultos locales (tipos compartidos entre adultos). Los juveniles no cambiaron el contenido ni el tamaño del repertorio durante la temporada. Utilizamos la retención de plumas juveniles bajo el ala como una medida relativa del tiempo de eclosión en un subconjunto de 15 juveniles. Los repertorios de canto de percheo de los juveniles que eclosionaron tempranamente contenían más tipos adultos que los de aves más jóvenes. En machos de uno de los sitios encontramos que sólo 4 de 10 juveniles produjeron el silbido al vuelo típicamente emitido por los adultos locales. La evidencia que conecta el desarrollo del silbido al vuelo y el indicador del plumaje que utilizamos como indicador del tiempo de eclosión no fue conclusiva.
We analyze geographic variation in morphology for Sierra Nevadan populations of Brown-headed Cowbirds (Molothrus ater) in relation to two levels of geographic structure of populations: 1) subspecies and 2) vocal dialects. Two morphologically distinct subspecies occupy opposite slopes of the Sierra Nevada: M. a artemisiae on the east slope is larger than M. a. obscurus on the west slope, and its juveniles have predominantly white versus yellow rictal (bill) flanges. Populations of obscurus moved into California from the lower Colorado River around 1900 and invaded the western Sierra during the 1930's. A relatively constant type of flight whistle occurs in obscurus populations up to 700 km apart, but east-slope artemisiae are divided into many distinct dialects. The means of seven morphological measurements and of principal component 1 (i.e., body size) for 2,287 individuals show similar clinal patterns for adult males, yearling males, and females over a 60-km north-south transect along the eastern Sierra Nevada: large sizes occur at the southernmost study site at Bishop, decrease clinally to the north to Mammoth Lakes, and then increase clinally to the north from Mammoth Lakes to the northernmost site at Lee Vining. This reversal in cline slope is reiterated for variation in the frequency of white-flanged juveniles. Our data do not support adaptive explanations for the clines. Rather, the most parsimonious explanation is that there has been flow of obscurus genes into artemisiae from the west slope across the crest of the Sierra Nevada into the Mammoth Lakes area. Concordantly, the Sierran crest at Mammoth Lakes is considerably lower than any other point within about 80 km to the north or 180 km to the south. Two other findings strongly support this interpretation. First, wing lengths of our longest-winged samples (at Bishop and Lee Vining) are significantly shorter than those of cowbirds collected in the same region between 1912 and 1922. Second, the wing lengths of Mammoth Lakes adult males and females during 1978-1981 are significantly larger than those of the 1982-1985 period. This putative hybridization must have been rapid, as there have been fewer than 50 years since secondary contact could have occurred. Calculation of the gene-flow parameter l of Endler (1977) indicates that the gene-flow rate reported here is higher than for nearly all of the species he surveyed. This gene flow occurs between two subspecies with different flight whistles and across several cowbird flight-whistle dialects within artemisiae. Therefore, vocal differences among these dialects do not appear to be a strong deterrent to gene flow.
Background
Many potential hosts of social parasites recognize and reject foreign intruders, and reduce or altogether escape the negative impacts of parasitism. The ontogenetic basis of whether and how avian hosts recognize their own and the brood parasitic eggs remains unclear. By repeatedly parasitizing the same hosts with a consistent parasitic egg type, and contrasting the responses of naïve and older breeders, we studied ontogenetic plasticity in the rejection of foreign eggs by the great reed warbler (Acrocephalus arundinaceus), a host species of the common cuckoo (Cuculus canorus).
Results
In response to experimental parasitism before the onset of laying, first time breeding hosts showed almost no egg ejection, compared to higher rates of ejection in older breeders. Young birds continued to accept foreign eggs when they were subjected to repeated parasitism, whereas older birds showed even higher ejection rates later in the same laying cycle.
Conclusions
Our results are consistent with the hypotheses that (i) naïve hosts need to see and learn the appearance of their own eggs to discriminate and reject foreign eggs, whereas (ii) experienced breeders possess a recognition template of their own eggs and reject parasitic eggs even without having to see their own eggs. However, we cannot exclude the possibility that other external cues and internal processes, accumulated simply with increasing age, may also modify age-specific patterns in egg rejection (e.g. more sightings of the cuckoo by older breeders). Future research should specifically track the potential role of learning in responses of individual hosts between first and subsequent breeding attempts by testing whether imprinting on a parasitized clutch reduces the rates of rejecting foreign eggs in subsequent parasitized clutches.
One of the best known outcomes of coevolution between species is the rejection of mimetic parasite eggs by avian hosts, which has evolved to reduce costly cuckoo parasitism. How this behavioral adaptation varies along the life of individual hosts remains poorly understood. Here we identify for the first time, lifetime patterns of egg rejection in a parasitized long-lived bird, the magpie Pica pica and show that, during the years they were studied, some females accept, others reject, and some others modify their response to model eggs, in all cases switching from acceptance to rejection. Females tested in their first breeding attempt always accepted the model egg, even those individuals whose mothers were egg rejecters. A longitudinal analysis showed that the probability of egg rejection increased with the relative age of the female, but was not related to the risk of parasitism in the population. We conclude that ontogeny plays a fundamental role in the process leading to egg rejection in magpies. This article is protected by copyright. All rights reserved.
The temporal and spatial patterns of avian brood parasitism can critically influence host fitness, the coevolution of parasitic strategies and host resistance, and their reciprocal effects on population dynamics. This study examined spatial patterns of Brown-headed Cowbird (Molothrus ater) parasitism on Eastern Phoebe (Sayornis phoebe) hosts in 2009-2011 near Ithaca, NY, USA, and compared them with published data from 1999-2002 at the same nest sites, encompassing a period long enough for a turn-over of most of the breeding adult host and parasite populations. Relative to non-parasitized nests, host nests, that were more likely to be parasitized annually in 2009-2011, had also been parasitized more often during years 1999-2002. These findings are consistent with the hypothesis that Brown-headed Cowbirds, even across generations, show a consistent preference for particular Eastern Phoebe nest sites. It remains unclear what the relative roles of the biological traits of the hosts occupying those sites versus the ecological and physical characteristics of the actual nest sites may be in yielding these patterns. Nonetheless, the results imply that knowing the site-specific histories of parasitism of reusable nests and breeding sites can be used to predict variation in the risk and future impact of cowbirds on this host species.
We characterized several equivocal aspects of the breeding biology of the brood parasitic Brown-headed Cowbird (Molothrus ater) at a study site in northeastern Illinois. A total of 175 offspring and a partial sample of parents were sampled and genotyped at six microsatellite loci. A combination of sibling-group and parentage assignment enabled us to characterize mating behavior and area used for reproduction by breeding adults, even for those not sampled. We assigned a mean of 4.7 (range 1-13) and 4.4 (range 1-16) offspring to 33 female and 32 male parents, respectively. Adults typically reproduced with a "primary partner" but up to three partners were common. Offspring females and males were spread over 9 and 12 ha, respectively. Half of the polygynous males produced offspring with an additional mate that parasitized nests near to or within areas overlapping those of their primary partner. That suggests that mate choice takes place at females' egg-laying areas as opposed to more remote social areas. Multiple females frequently parasitized a single host nest and areas used for reproduction overlapped extensively for individuals of either sex. High frequencies of parasitism and superparasitism indicate a high cowbird density relative to that of hosts. Frequent promiscuity by cowbirds at our site contrasts with other studies reporting monogamy and may be due to higher densities of breeding cowbirds at our site.
Acknowledgments: The late L. Richard Mewaldt was the first among,equals in setting high standards and maintenance of accurate records. His contributions are detailed by Ralph (1992). We take great pleasure in dedicating this handbook,to him. This handbook,is a direct outgrowth,of the landbird program,started more than 25 years ago at the Point Reyes Bird Observatory to monitor landbird populations in coastal California, and much of this handbook is the result of the methods developed and adapted there. Over this period, many people contributed to the development,of this landbird monitoring program. The handbook benefitted greatly from discussions and correspondence by Bruce Bingham, Grant Ballard, Danny Bystrak, Barbara Carlson, Brenda Dale, Sam Droege, John Faaborg, Kevin J. Gutzwiller, Denise Hardesty, Kimberly Hollinger, Bill Howe, David W. Johnston, Stephanie Jones, Cherry Keller, Kathy Klimkiewicz, Rolf R. Koford, Karin Kozie, Borja Milá, Sherri Miller, Michael Morrison, Barry
We observed brood parasitism by brown-headed cowbirds (Molothrus ater) on indigo buntings (Passerina cyanea) and estimated the impact of parasitism on the success of the individual buntings in their current nests and in their future survival and reproduction. Rates of parasitism over 8 years were 26.6% in 1040 nests and 19.8% in 693 nests in two areas in southern Michigan. Risk of parasitism was high early in the season; half the bunting nests were begun after the end of the cowbird season. Risk was independent of female age, plant containing the nest, or habitat. The immediate cost of parasitism was 1.19 and 1.26 fewer buntings fledged per nest. Bunting success was lower in parasitized nests with cowbird eggs (nests were more likely to be deserted or predated), lower when the cowbird nestling failed (nests were more likely to be predated), and lower when the cowbird fledged (fewer buntings fledged) compared to nonparasitized nests. Costs were due to removal of a bunting egg when the cowbird laid its own egg and to competition for parental care of the cowbird and bunting nestlings. Buntings that fledged from nests where a cowbird also fledged were only 18% as likely to survive and return to their natal area in the next year as buntings from nests where a cowbird did not fledge. Long-term effects of cowbird parasitism on adult breeding later in the season, survival to the next season, and reproductive success in the next season were negligible when compared between birds that reared a cowbird and birds that reared only a bunting brood, or between birds that were parasitized and birds that escaped parasitism. The results indicate little long-term cost of brood parasitism on individual fitness of adult buntings beyond the impact on the current nest and the survival of buntings that fledge from it; nearly all cost is to the parasitized brood.
The Brown-headed Cowbird (Molothrus ater (Boddaert, 1783)) is a generalist brood parasite that often lays into nests that contain conspecific eggs. Although it has often been assumed that this multiple parasitism reduces Cowbird survival, this has rarely been evaluated. We measured Cowbird survival in nests of the Blue-gray Gnatcatcher (Polioptila caerulea (L., 1766)), Plumbeous Vireo (Vireo plumbeus Coues, 1866), and Western Tanager (Piranga ludoviciana (A. Wilson, 1811)) in New Mexico, USA. Our objectives were to measure the costs of intraspecific competition on Cowbird survival in multiply-parasitized nests, evaluate if these costs were related to host size, and to compare the costs of multiple parasitism relative to other mortality sources that occur over the entire nesting cycle. Intraspecific competition reduced Cowbird survival during the nestling period in nests of all three hosts, and was of particular importance in nests of the two smaller hosts. When all sources of egg mortality were considered, however, the costs of multiple parasitism were small compared with the large effects of predation and nest desertion. Given that multiple parasitism reduces Cowbird egg survival, it is unclear why Cowbirds multiply-parasitize. Possible explanations depend on an improved understanding of fecundity and level of host nest selectivity by female Cowbirds.
Surveys in the 1990s did not find the Willow Flycatcher along Rush Creek, a
tributary of Mono Lake, Mono County, California. In 2001 and 2002 we located nine Willow
Flycatcher nests along lower Rush Creek, in a riparian corridor currently in its 15th year of
long-term rehabilitation after decades of livestock grazing and water diversion for municipal,
hydroelectric, and irrigational use. The mated pairs' habitat differs from that reported for the
Willow Flycatcher elsewhere in California. Males selected territories in tall thickets of of Woods' Rose (Rosa woodsii), and Woods' Rose was the substrate of all nine nests. In addition, the flycatchers territories and
nests were located farther from water than reported elsewhere in California, averaging 129 m for nine nests and 86 m for seven territories.
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.
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.
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.
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.
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.
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
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
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.
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.
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.
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.
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.
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.
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
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.
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
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.
The territorial behavior of the female brown-headed cowbird (Molothrus ater) was studied at London, Ontario, in 1966 and 1967. The cowbirds arrived in the study area during the last week of March and the first 2 weeks of April. Adult birds arrived about 2 weeks before the yearlings. Forty-six resident and 56 nonresident females were observed in the study area; these nonresidents appeared to use the area for feeding. The breeding residents established home ranges mainly through the use of threat displays. The defence of these home ranges suggest that they might be classified as territories. Thirty-nine breeding resident females, 17 yearlings, and 22 adults had home ranges ranging from 0.9 to 13.4 ha (average 4.5 ± 0.4 ha).
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.