Is timing everything? Examining the importance of the timing of parasitism to cowbird reproductive success

Brood parasitism is a reproductive strategy where a parasitic parent leaves its offspring to be raised by an unrelated host. This has resulted in specialized adaptive behaviors selected to optimize the success of their progeny. The timing of when to leave a propagule with a host can be paramount to parasite success, and erring in this decision could have dire fitness consequences that strongly select for accurate parasitism timing. Brown-headed cowbirds (Molothrus ater) are North America’s most widespread avian brood parasite and successfully parasitize 170+ host species, but female cowbirds are cryptic and difficult to monitor, and much of how they make parasitism decisions is not well understood. Despite such diversity in hosts, cowbirds nearly always parasitize nests during the narrow window of the host’s egg-laying period before the start of incubation, resulting in their offspring typically hatching before the host’s. How important is it to get this timing right, and are there reproductive costs to parasitizing nests “late” (i.e., after the onset of host incubation)? I simulated three different parasite timing decisions by adding cowbird eggs to the nests of a common host, the prothonotary warbler (Protonotaria citrea), at different stages of the host’s laying and incubation period and monitored the effects on cowbird hatching success, hatching asynchrony relative to host eggs, and nestling size, health, and survival. I found that late egg placement produced cowbird nestlings that hatched at the same time or later than their host nestmates, resulting in smaller body size and markedly lower survival than those placed in the pre-incubation window, suggesting that accurate parasitism timing is an essential and likely strongly selected for behavior in brood parasites.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Natural Res & Env Sciences

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Boldrick, Julia

Contributors dc:contributor

• Schelsky, Wendy
• Hoover, Jeffrey P
• Ward, Michael P
• Hauber, Mark E

• brood parasitism
• behavior
• bird
• cowbird
• breeding behavior
• avian biology
• avian ecology
• parasitism

Statement dc:rights

• Copyright 2025 Julia Boldrick

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/129299

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F129299