Hybrid pairing between a Cerulean Warbler and a Black-throated Blue Warbler

Detailed nest observations confirm the first case of this hybrid combination.

When it comes to hybridization, wood-warblers are the bird family to study (see the Parulidae page for an overview). In 2014, Pamela Willis and her colleagues counted 24 species (out of 45) that are known to hybridize. With all these crosses, it is no surprise that some species hybridize with several other species. This became clear when I visualized the hybridization patterns in my review paper on multispecies hybridization in birds (see figure below). However, the resulting network is already outdated. A recent paper in the Wilson Journal of Ornithology reported on a new hybrid cross between two wood-warblers: the Cerulean Warbler (Setophaga cerulea) and the Black-throated Blue Warbler (S. caerulescens).

A hybrid network displaying the incidence of hybridization between different members—genera Geothlypis, Mniotilta, Oreothlypis, Setophaga, and Vermivora—of the Parulidae family. Thin, black edges indicate uncommon hybridization, while thick, red edges indicate extensive hybridization. From: Ottenburghs (2019) Avian Research.

Nest Observations

On 7 July 2017, birdwatcher Matt Wistrand discovered a nest that was being visited by a male Cerulean Warbler and a female Black-throated Blue Warbler in Brown County, Indiana. He took several pictures and made some audio recordings. This observation caught the attention of Clayton Delancey, Garrett MacDonald and Kamal Islam, who were allowed access to the nesting site on 12 and 13 July 2017. They monitored the nest and collected extra information on the behavior of the birds. The nest contained four nestlings that were fed by both parents. The researchers noted that they “did not observe any aggressive interaction between the male Cerulean Warbler and the female Black-throated Blue Warbler, and both individuals were observed multiple times at the nest simultaneously.”

All in all, these observations suggest that we are dealing with a hybrid pairing between these two species. To remove all doubt, the researchers returned to the nest on 13 July 2017 to collect the nestlings and take blood samples for genetic analyses. Unfortunately, they found the nest on the ground with no sign of the nestlings. It seems that it had been predated.

Pictures of the hybrid nest. (a) View of the Cerulean Warbler–Black-throated Blue Warbler nest in mixed live and dead foliage. (b) Closeup of 4 nestlings. (c) Male Cerulean Warbler with a beak full of green caterpillars. (d) Male Cerulean Warbler feeding nestlings. (e) Female Black throated Blue Warbler at the nest. (f) Female Black-throated Blue Warbler feeding nestlings. © Clayton Delancey

Rare Occasion

The final piece of (genetic) evidence to confirm this hybrid mating is thus missing, but it is likely that these birds paired up due to a scarcity of partners. The male Cerulean Warbler might have been unsuccessful in attracting a female, causing it to settle with a Black-throated Blue Warbler that happened to be around. Black-throated Blue Warblers are not known to breed in Indiana, making this hybrid pairing even more special. We will probably not see this species combination in the near future, but you never know… A perceptive birdwatcher might discover an unusual nesting situation.

References

Delancey, C. D., MacDonald, G. J., & Islam, K. (2019). First confirmed hybrid pairing between a Cerulean Warbler (Setophaga cerulea) and a Black-throated Blue Warbler (Setophaga caerulescens). The Wilson Journal of Ornithology, 131(1), 161-165.

Featured image: Cerulean Warbler (Setophaga cerulea) © Mdf | Wikimedia Commons

This paper has been added to the Parulidae page.