Answers are grounded in this paper and cite their sources.
I observed a male Western Bluebird (Sialia mexicana) capture a western fence lizard (Sceloporus occidentalis), the first reported incidence of vertebrate-directed raptorial behavior in the Western Bluebird. There are no previously published reports of Western Bluebirds capturing vertebrate prey, although there is one previous report of a Western Bluebird carrying an unidentified lizard in the manner of a prey item, and a few reports of predation on vertebrates by the congeneric Eastern Bluebird (Sialia sialis).
No takes yet. Share an insight, caveat, or question.
Caitlin A. Stern (2007) studied this question.
Synapse has enriched 10 related papers. Consider them for comparative context: