Host recognition and fitness costs of avian brood parasitism

One of the richest opportunities for studying coevolution comes from obligate brood parasitism, a unique reproductive strategy performed by only 1% of the world’s bird species. Brood parasites are birds that lay their eggs into the nest of another species. They never care for their own young, leaving the host to care for the unrelated, parasitic offspring. Assessing and documenting how hosts are affected by and respond to brood parasitism are critical in understanding this coevolutionary process. The brown-headed cowbird (Molothrus ater) is an obligate generalist brood parasite that uses over 200 host species and shares the nest with host nestmates. Using integrative approaches, I characterized two components of coevolution between cowbirds and their hosts – the hosts’ recognition of parasites, and the effects of the parasite on the fitness of host nestlings. In an egg-rejector host, the American robin (Turdus migratorius), I determined whether robin recognition and rejection of parasitic eggs involved lateralized visual processing. Critically, robins were more lateralized in their eye use when viewing mimetic eggs compared to non-mimetic eggs, and more highly lateralized robins rejected more model eggs. In contrast, the prothonotary warbler (Protonotaria citrea) never rejects cowbird eggs, though it is frequently parasitized. It is possible that warblers perceive the cowbird eggs as foreign, but simply do not have the ability to physically remove these eggs from their nest. However, through an experimental egg addition, I documented no difference in the circulating corticosterone levels between females given either a mimetic or a non-mimetic type of model egg or control females given no model egg, which suggests that warblers do not perceive and recognize parasitic eggs, potentially due to evolutionary lag as they are a historically recent cowbird host. To assess recognition at the nestling stage, I compared the antipredator responses of breeding warbler parents through playback of cowbird nestlings’ alarm calls, warbler nestlings’ alarm calls, or bluebird nestlings’ alarm calls as controls. Warbler parents responded most strongly to warbler nestling alarm calls but also responded more strongly to cowbird compared to bluebird nestlings’ alarm calls. These results imply that as a generalist brood parasite, cowbirds employ a general alarm call that elicits host antipredator responses better than other heterospecifics, but not as well as the host’s own species. To investigate physiological effects of cowbirds on host warbler young, nests were experimentally parasitized and physiological measurements of immune (PHA injection) and stress axis (corticosterone) responses were taken from the host nestlings. Parasitized warbler host nestlings exhibited decreased immune responses and higher mortality relative to non-parasitized controls, while their corticosterone levels and body condition were similar. These results suggest that host nestlings incur some adverse effects from parasites but are resilient in other physiological aspects. Finally, I found that when raised with a cowbird, warbler nestlings were smaller, fledged at older ages, and had greater overall fledging latency compared to nests parasitized with an additional warbler chick. Because fledging is such a critical transition between life history stages for birds, these effects could further negatively influence the fitness outputs of parasitized broods. In summary, my dissertation provides a multi-level integrative perspective on host recognition and the fitness costs of brood parasitism. This research explores the physiological understanding of species interactions and has raised exciting questions for future studies on brood parasites and coevolution.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Biology

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2023

Author dc:creator

• Scharf, Hannah

Contributors dc:contributor

• Hauber, Mark E
• Suarez, Andrew V
• Schelsky, Wendy M
• White, David J

• Behavior
• Hormones
• Lateralization
• Breeding Biology
• Fledging
• Host-parasite Interactions
• Coevolution
• Prothonotary Warbler
• Brown-headed Cowbird
• American Robin

Statement dc:rights

• Copyright 2023 Hannah Scharf

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/120203

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F120203