The Carolina chickadee, Poecile carolinensis Audubon is a relatively small songbird belonging to the tit and chickadee family Paridae. Feces from three P. carolinensis from Polk County, Arkansas, USA, and a single P. carolinensis in McCurtain County, Oklahoma, USA, were collected and examined for coccidia; the latter bird was found to be passing a new species of Isospora. Oöcysts of Isospora oklahomaensis n. sp. are subspheroidal to ovoidal with a smooth to slightly-pitted bi-layered wall, measure (L × W) 32.1 × 28.3 µm, and have a length/width (L/W) ratio of 1.1; a micropyle and oöcyst residuum were absent but a bilobed and refractile polar granule is present. Sporocysts are ellipsoidal and measure 18.4 × 11.8 µm, L/W 1.6; a prominent Stieda body is present as well as a distinct sub-Stieda body. The sporocyst residuum is composed of an irregular mass of granules lying between and dispersed among the sporozoites. This is the first coccidian described from the Carolina chickadee and, most importantly, only the second described from a member of the Paridae, worldwide.
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