Communication between parents and dependent offspring is critical not only during provisioning, but also in antipredator contexts. In altricial birds, a top cause of reproductive failure is nest ...predation, and alarm calls both by parents and chicks can serve to alert others and increase the likelihood of offspring escaping predation. Understanding the factors that determine the strength of parental antipredator responses to different nestling alarm calls can provide insight into parent–offspring recognition. The prothonotary warbler (
Protonotaria citrea
), a host of the obligate brood parasite, the brown-headed cowbird (
Molothrus ater
), never rejects cowbird young and raises the parasite together with its own offspring. To determine whether warbler parents learn cowbird nestling alarm calls, we presented experimentally parasitized or non-parasitized parents with playbacks of conspecific warbler, parasitic cowbird, and a harmless heterospecific control, eastern bluebird (
Sialis sialis
), nestling alarm calls. We recorded the latency to respond and the number of chips given by members of the resident warbler pair. We found that parents were most likely to respond to warbler nestling alarm calls, least likely to respond to bluebird calls, with a statistically intermediate likelihood of responding to cowbird calls. Critically, current and past parasitism status did not affect the likelihood of response to any playback or the number of chips given, however, currently parasitized parents had greater response latencies to playbacks than non-parasitized parents. These results suggest that warbler parents do not learn cowbird alarm calls from breeding experiences and, in turn, that cowbirds may employ a generalized, bet-hedging alarm call.
•We tested whether brood parasitism activates the HPA axis in adult accepter hosts.•Incubating female hosts showed no increase in baseline CORT when parasitized.•Females showed no CORT increase when ...parasitized with a non-mimetic vs mimetic egg.•Lack of CORT response is consistent with the “evolutionary lag” hypothesis.
Avian obligate brood parasitism, a reproductive strategy where a parasite lays its egg into the nest of another species, imposes significant fitness costs upon host parents and their offspring. To combat brood parasitism, many host species recognize and reject foreign eggs (rejecters), but others are accepters that raise the parasitic progeny. Some accepter hosts may be unable to grasp or pierce parasitic eggs even if they recognize them as foreign eggs in the clutch, whereas other accepters may not have evolved the cognitive skillsets to recognize dissimilar eggs in the nest. Here we assessed the endocrine responses of an accepter host species to model parasitic eggs to address these two alternatives. We experimentally parasitized nests of a locally common host of the brood-parasitic brown-headed cowbird (Molothrus ater), the prothonotary warbler (Protonotaria citrea; a cowbird-egg accepter), with a mimetic or non-mimetic model cowbird-sized egg. Our goal was to determine whether they perceived the non-mimetic egg as a greater stressor by measuring circulating corticosterone levels. We added eggs to nests during the incubation stage and obtained blood plasma samples from females on the nest 2 h later, using females with unmanipulated clutches as controls. Incubating females showed no differences in baseline plasma corticosterone levels between our different treatments. We conclude that exposure to foreign eggs does not activate the hypothalamic–pituitary–adrenal axis of prothonotary warbler hosts in this experimental paradigm.
Transitions between life history stages are fitness-limiting events that depend on environmental and individual characteristics. For altricial birds, fledging from the nest is a critical shift in ...development with direct impacts on survival, yet it remains one of the most understudied components of avian ontogeny. Even less is known about how brood parasitism affects the fledging process in host nestlings. The prothonotary warbler (
Protonotaria citrea
) is a host of the obligate brood parasitic brown-headed cowbird (
Molothrus ater
). We tested whether the presence of parasitic nestlings negatively alters host fledging by experimentally parasitizing nests with a cowbird (heterospecific parasite) or a warbler (conspecific parasite) egg, comparing them to non-manipulated control nests, and monitoring them using radio-frequency identification (RFID) systems. As expected, in heterospecifically parasitized nests, warblers were smaller, fledged at older ages, and had greater overall fledging latency compared to conspecifically parasitized nests. There was no such impact of conspecific parasitism relative to controls. Warbler nestling size predicted the age and order of fledging, with larger nestlings fledging earlier. Nestlings fledging at earlier ages fledged later during daytime hours. Cowbirds and last-fledged, smaller warbler chicks spent the most time in the nest entrance before fledging. Finally, although male warbler nestlings were larger than females, there were no sex effects or effects of extra-pair status on fledging. Our study shows that while conspecific parasitism has no detectable effect on host nestmates, heterospecific parasitism impacts host size and fledging phenology, which may influence post-fledging survival of parasitized broods.
Significance statement
In many species, juveniles undergo dramatic transitions in lifestyle as they age and become independent. For most birds, fledging from the nest is an important developmental shift, potentially impacted by their previous growth and early social experiences. One aspect that may affect fledging is brood parasitism, whereby birds lay their eggs into the nests of other birds who care for the unrelated young. Here, we determined experimentally if brood parasitism affects fledging of the prothonotary warbler, a species that always accepts eggs of the larger, obligate brood parasitic brown-headed cowbird. Our results show that while same-species parasitism has no statistical effect on the fledging of host warblers, cowbird parasitism causes delayed fledging in hosts, revealing a previously unappreciated, additional cost of brood parasitism upon host nestlings.
Lay summary Many parasites manipulate the behaviors of their hosts, yet understanding the evolutionary pathways leading to adaptive host manipulation remains unclear. Here, we support a recent ...hypothesis proposing that parasites can exploit host behaviors employed to compensate for the costs of infection. We demonstrate that by reducing the number of host offspring in parasitized nests, cowbirds increase the likelihood that parasitized female warblers will attempt a second brood, thereby increasing the opportunities for parasitism.Host manipulation by parasites is generally regarded as a classic example of the extended phenotype, where selection favors parasite genes that adaptively alter their host's phenotype. However, selection would simultaneously favor both hosts that recoup some fitness lost to infection (i.e., compensation) and the parasites that enhance transmission by exploiting the host's compensatory responses. Using a long-term study (19 years) of an avian brood parasite system, we demonstrate that female prothonotary warblers Protonotaria citrea compensate for partial fecundity reduction during their first brood by initiating a second breeding attempt (i.e., double-brooding). Similarly, in successful nests with naturally reduced fecundity as a result of brood parasitism, we show that being parasitized also stimulates a compensatory double-brooding response, where female warblers raising at least 1 brown-headed cowbird Molothrus ater offspring are more likely to initiate a second brood when compared with successful nonparasitized first broods. In support of the "exploitation of compensatory responses" hypothesis, parasitized females are often parasitized again in their second brood, thereby providing additional fitness benefits to cowbirds while enabling parasitized warblers that double-brood to recoup some fitness lost in the first brood. Experimentally parasitized female warblers exhibited a similar increase in double-brooding behavior, and the double-brooding frequency of parasitized female warblers was not attributable to reduced post-fledgling survival of cowbirds, thereby supplying further support for the role of parasite-induced fecundity reduction in the compensatory double-brooding behavior of a host.
It has been hypothesized that organisms living at different latitudes or in different environments adjust their metabolic activity to the prevailing conditions. However, do differences in energy ...turnover simply represent a phenotypic adaptation to the local environment, or are they genetically based? To test this, we obtained nestling stonechats (Saxicola torquata) from equatorial Kenya (0° N), Ireland (51.5° N), Austria (47.5° N) and Kazakhstan (51.5° N). Birds were hand-raised and kept in Andechs, Germany. We measured their resting metabolic rates (RMR) and locomotor activity at an age of ca. 14 months (July) and 20 months (January), when birds went through postnuptial moult (July), and neither moulted nor exhibited enlarged gonads or migratory activity (January). RMR was generally higher during moult, but differed among populations: RMR was lowest in the resident Kenyan birds, higher in mostly sedentary Irish birds, and highest in migratory Austrian and Kazakhstan birds. Thus our data demonstrate that even in birds kept from early life under common-garden conditions, the 'pace of life', as indicated by metabolic turnover, is lower in sedentary tropical than in north-temperate migratory individuals of the same species. Such intrinsically low energy expenditure in sedentary tropical birds may have important implications for slow development, delayed senescence and high longevity in many tropical organisms.
Abstract
Researchers often examine symbiont host specificity as a species‐level pattern, but it can also be key to understanding processes occurring at the population level, which are not as well ...understood. The specialist–generalist variation hypothesis (SGVH) attempts to explain how host specificity influences population‐level processes, stating that single‐host symbionts (specialists) exhibit stronger population genetic structure than multi‐host symbionts (generalists) because of fewer opportunities for dispersal and more restricted gene flow between populations. However, this hypothesis has not been tested in systems with highly mobile hosts, in which population connectivity may vary temporally and spatially. To address this gap, we tested the SGVH on proctophyllodid feather mites found on migratory warblers (family Parulidae) with contrasting host specificities,
Amerodectes protonotaria
(a host specialist of
Protonotaria citrea
) and
A. ischyros
(a host generalist of 17 parulid species). We used a pooled‐sequencing approach and a novel workflow to analyse genetic variants obtained from whole genome data. Both mite species exhibited fairly weak population structure overall, and contrary to predictions of the SGVH, the generalist was more strongly structured than the specialist. These results may suggest that specialists disperse more freely among conspecifics, whereas generalists sort according to geography. Furthermore, our results may reflect an unexpected period for mite transmission – during the nonbreeding season of migratory hosts – as mite population structure more closely reflects the distributions of hosts during the nonbreeding season. Our findings alter our current understanding of feather mite biology and highlight the potential for studies to explore factors driving symbiont diversification at multiple evolutionary scales.
St. John, U.S. Virgin Islands, is one of the most forested islands in the West Indies and provides an opportunity to conserve both resident birds and wintering neotropical migrants. We conducted ...double-observer point counts of landbirds in December 2005 and 2006 in Forest Inventory and Analysis (FIA) plots and National Park Service (NPS) trails in Virgin Islands National Park (VINP) to assess population trends of birds in subtropical dry and moist forests. We recorded 2,270 individual birds representing 35 species at 150 point count stations in 2005, and 3,092 individuals of 32 species at 143 of these stations in 2006. The increase in birds per point from 2005 (15.1) to 2006 (21.6) was due to resident species, 17 of which were recorded more frequently in 2006. The 17 species of neotropical migrants composed 11.8% of all registrations in 2005 and 2006. Subtropical moist and dry forest habitats differed strongly in vegetation characteristics and plant species, but no species of birds exhibited a strong affiliation with either habitat type on FIA plots. Data from NPS trails showed that most migrant species were detected more often in moist, mature forest. The resident Bridled Quail-Dove (Geotrygon mystacea) also was correlated with mature forest. Plant and bird species co-occurrence with positive correlations that may carry a signal of preferred frugivory included Guettarda odorata (Rubiaceae) with Bridled Quail-Dove, and Myrciaria floribunda (Myrtaceae) with Pearly-eyed Thrasher (Margarops fuscatus). Migrant species did not exhibit strong long-term changes in relative abundance since founding of VINP in 1957, but four open-country resident species declined significantly between 1957 and 2006 as the forest matured. Forest maturation should continue on St. John, yielding a bright future for most of its landbirds barring catastrophic hurricanes, pathogens, or invasive plants.
St. John, U.S. Virgin Islands, is one of the most forested islands in the West Indies and provides an opportunity to conserve both resident birds and wintering neotropical migrants. We conducted ...double-observer point counts of landbirds in December 2005 and 2006 in Forest Inventory and Analysis (FIA) plots and National Park Service (NPS) trails in Virgin Islands National Park (VINP) to assess population trends of birds in subtropical dry and moist forests. We recorded 2,270 individual birds representing 35 species at 150 point count stations in 2005, and 3,092 individuals of 32 species at 143 of these stations in 2006. The increase in birds per point from 2005 (15.1) to 2006 (21.6) was due to resident species, 17 of which were recorded more frequently in 2006. The 17 species of neotropical migrants composed 11.8% of all registrations in 2005 and 2006. Subtropical moist and dry forest habitats differed strongly in vegetation characteristics and plant species, but no species of birds exhibited a strong affiliation with either habitat type on FIA plots. Data from NPS trails showed that most migrant species were detected more often in moist, mature forest. The resident Bridled Quail-Dove (Geotrygon mystacea) also was correlated with mature forest. Plant and bird species co-occurrence with positive correlations that may carry a signal of preferred frugivory included Guettarda odorata (Rubiaceae) with Bridled Quail-Dove, and Myrciaria floribunda (Myrtaceae) with Pearly-eyed Thrasher (Margarops fuscatus). Migrant species did not exhibit strong long-term changes in relative abundance since founding of VINP in 1957, but four open-country resident species declined significantly between 1957 and 2006 as the forest matured. Forest maturation should continue on St. John, yielding a bright future for most of its landbirds barring catastrophic hurricanes, pathogens, or invasive plants.
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