skip to main content

Title: Ontogenetic effects of brood parasitism by the Brown‐headed Cowbird on host offspring

Nest‐sharer avian brood parasites do not evict or otherwise kill host chicks, but instead inflict a range of negative effects on their nestmates that are mediated by interactions between the parasite and host life history traits. Although many of the negative fitness effects of avian brood parasitism are well documented across diverse host species, there remains a paucity of studies that have examined the impacts of parasitism across the entirety of host ontogeny (i.e., from when an egg is laid until independence). More specifically, few studies have examined the impact of brood parasitism on the pre‐ and post‐fledging development, physiology, behavior, and survival of host offspring. To help fill this knowledge gap, we assessed the effects of brood parasitism by Brown‐headed Cowbirds (Molothrus ater) across the ontogeny (incubation, nestling, and post‐fledging period) of nine sympatrically breeding host species in central Illinois, USA; due to sample sizes, impacts on the post‐fledging period were only examined in two of the nine species. Specifically, we examined the impact of brood parasitism on ontogenetic markers including the embryonic heart rate, hatching rate, nestling period length, nest survival, and offspring growth and development. Additionally, in species in which we found negative impacts of cowbird parasitism on host nestmate ontogeny, we examined whether the difference in adult size between parasites and their hosts and their hatching asynchrony positively predicted variation in host costs across these focal taxa. We found that costs of cowbird parasitism were most severe during early nesting stages (reduction in the host clutch or brood size) and were predicted negatively by host size and positively by incubation length. In contrast, we only found limited costs of cowbird parasitism on other stages of host ontogeny; critically, post‐fledging survival did not differ between host offspring that fledged alongside cowbirds and those that did not. Our findings (i) highlight the direct costs of cowbird parasitism on host fitness, (ii) provide evidence for when (the stage) those costs are manifested, and (iii) may help to explain why many anti‐cowbird defenses of hosts have evolved for protection from parasitism during the laying and incubation stages.

more » « less
Author(s) / Creator(s):
 ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Many avian species are negatively impacted by obligate avian brood parasites, which lay their eggs in the nests of host species. The yellow warbler (Setophaga petechia), which is host to the brood-parasitic brown-headed cowbird (Molothrus ater), represents one of the best-replicated study systems assessing antiparasitic host defenses. Over 15 prior studies on yellow warblers have used model-presentation experiments, whereby breeding hosts are exposed to models of brown-headed cowbirds or other nest threats, to test for anti-parasitic defenses unique to this species. Here we present results from our own quasi-replication study of the yellow warbler/brown-headed cowbird system, which used a novel design compared to previous experiments by pivoting to conduct acoustic playback treatments only, rather than presenting visual models with or without calls. We exposed active yellow warbler nests to playbacks of brown-headed cowbird chatters (brood parasite), blue jay (Cyanocitta cristata; nest predator) calls, conspecific “seet” calls (a referential alarm call for brood parasitism risk), conspecific “chip” calls (a generic alarm call), or control wood thrush (Hylocichla mustelina; harmless heterospecific) songs during the incubation stage. Similar to previous studies, we found that female yellow warblers seet called more frequently in response to playbacks of both brood parasitic chatter calls and conspecific seet calls whereas they produced more chip calls in response to the playback of nest predator calls. In contrast, female yellow warblers approached all playbacks to similar distances, which was different from the proximity patterns seen in previous studies. Our study demonstrates the importance of both replicating, and also pivoting, experimental studies on nest defense behaviors, as differences in experimental design can elicit novel behavioral response patterns in the same species. 
    more » « less
  2. Abstract

    Defending offspring incurs temporal and energetic costs and can be dangerous for the parents. Accordingly, the intensity of this costly behavior should reflect the perceived risk to the reproductive output. When facing costly brood parasitism by brown‐headed cowbirds (Molothrus ater), where cowbirds lay eggs in heterospecific nests and cause the hosts to care for their young, yellow warblers (Setophaga petechia) use referential “seet” calls to warn their mates of the parasitic danger. Yellow warblers of both sexes produce this call only in response to cowbirds or seet‐calling conspecifics. Seet calls are mainly produced during the laying and incubation stages of breeding, when risk of brood parasitism is highest, rather than during the nestling stage. On the other hand, general alarm calls (chips) are produced throughout the nesting cycle and are also used in conspecific interactions unrelated to nesting. We hypothesized that context shapes responses prior to breeding as well, such that yellow warblers without a mate and active nest would be less likely to respond to playbacks that simulate brood parasitism risk. To test this hypothesis, we presented playbacks of two nest threats, cowbirds (brood parasite) and blue jays (Cyanocitta cristata; nest predator), on territories of unmated male warblers (unpaired) and male warblers with a known mate (paired). We found that unpaired males were unresponsive toward playbacks indicating nest threats, whereas paired males were significantly more aggressive and vocal toward these playbacks compared to control playbacks. However, both paired and unpaired males were vocally responsive toward chip calls, which are informative for males regardless of pairing status. Male yellow warblers appear to adjust their responses during the earliest stages of breeding depending on the contextual relevance of specific threat stimuli, and together with prior studies, our work further supports that referential seet calls are associated with stage‐specific risk of brood parasitism.

    more » « less
  3. Abstract

    The role of species interactions, as well as genetic and environmental factors, all likely contribute to the composition and structure of the gut microbiome; however, disentangling these independent factors under field conditions represents a challenge for a functional understanding of gut microbial ecology. Avian brood parasites provide unique opportunities to investigate these questions, as brood parasitism results in parasite and host nestlings being raised in the same nest, by the same parents. Here we utilized obligate brood parasite brown‐headed cowbird nestlings (BHCO;Molothrus ater) raised by several different host passerine species to better understand, via 16S rRNA sequencing, the microbial ecology of brood parasitism. First, we compared faecal microbial communities of prothonotary warbler nestlings (PROW;Protonotaria citrea) that were either parasitized or non‐parasitized by BHCO and communities among BHCO nestlings from PROW nests. We found that parasitism by BHCO significantly altered both the community membership and community structure of the PROW nestling microbiota, perhaps due to the stressful nest environment generated by brood parasitism. In a second dataset, we compared faecal microbiotas from BHCO nestlings raised by six different host passerine species. Here, we found that the microbiota of BHCO nestlings was significantly influenced by the parental host species and the presence of an inter‐specific nestmate. Thus, early rearing environment is important in determining the microbiota of brood parasite nestlings and their companion nestlings. Future work may aim to understand the functional effects of this microbiota variability on nestling performance and fitness.

    more » « less
  4. Abstract

    Brown‐headed cowbirds (Molothrus ater) are generalist obligate brood parasites, laying in the nest of nearly 300 avian species, and successfully parasitizing well over 100 host species. Cowbird eggs are generally considered non‐mimetic, although some have suggested that cowbird eggs resemble several of their host species’ eggs. To date, no investigation has examined the level of avian‐perceived similarity between cowbird and diverse host eggs in the contexts of light characteristics at the nest and the visual system of the relevant viewer. Because the cowbird exploits a wide range of species that lay in a variety of nest types, hosts view these eggs under an array of light conditions which could facilitate or hinder egg discrimination. When considering the visual system of the relevant viewers and the light conditions at their nest, we found that the coloration of cowbird eggs was more similar to host than non‐host species’ eggs. Host responses (whether they accept or reject cowbird eggs) were not statistically different when hosts perceived a large chromatic difference between their own eggs and the cowbird's eggs. Instead, we found that host responses were predicted by the degree to which nesting light conditions facilitated color similarity between host and cowbird eggs, such that hosts typically nesting under light conditions where this color discrimination task was more challenging were more likely to reject cowbird eggs. This suggests that the nesting light environment may have selected for increased coevolved egg recognition abilities in a suite of cowbird host species, even in the absence of parasitic egg color mimicry.

    more » « less
  5. Referential alarm calls that denote specific types of dangers are common across diverse vertebrate lineages. Different alarm calls can indicate a variety of threats, which often require specific actions to evade. Thus, to benefit from the call, listeners of referential alarm calls must be able to decode the signaled threat and respond to it in an appropriate manner. Yellow warblers ( Setophaga petechia ) produce referential “seet” calls that signal to conspecifics the presence of nearby obligate brood parasitic brown-headed cowbirds ( Molothrus ater ), which lay their eggs in the nests of other species, including yellow warblers. Our previous playback experiments have found that red-winged blackbirds ( Agelaius phoeniceus ), a species also parasitized by brown-headed cowbirds, eavesdrop upon and respond strongly to yellow warbler seet calls during the incubation stage of breeding with aggression similar to responses to both cowbird chatters and predator calls. To assess whether red-winged blackbird responses to seet calls vary with their own risk of brood parasitism, we presented the same playbacks during the nestling stage of breeding (when the risk of brood parasitism is lower than during incubation). As predicted, we found that blackbirds mediated their aggression toward both cowbird chatter calls and the warblers’ anti-parasitic referential alarm calls in parallel with the low current risk of brood parasitism during the nestling stage. These results further support that red-winged blackbirds flexibly respond to yellow warbler antiparasitic referential calls as a frontline defense against brood parasitism at their own nests. 
    more » « less