Isler, M. L., P. R. Isler, B. M. Whitney, K. J. Zimmer, and A. Whittaker. “Species Limits in Antbirds (Aves: Passeriformes: Thamnophilidae): an Evaluation of Frederickena Unduligera (Undulated Antshrike) Based on Vocalizations”. Zootaxa, vol. 2305, no. 1, Dec. 2009, pp. 61–68, doi:10.11646/zootaxa.2305.1.5.