Wing, Leonard W. (1956) Natural History of Birds. The Ronald Press Company. [1]
doi.org
Peter R. Stettenheim (2000) The Integumentary Morphology of Modern Birds―An Overview. American Zoologist 2000 40(4):461-477; doi:10.1093/icb/40.4.461.
Piersma, Theunis; Renee van Aelst, Karin Kurk, Herman Berkhoudt and Leo R. M. Maas (1998). “A New Pressure Sensory Mechanism for Prey Detection in Birds: The Use of Principles of Seabed Dynamics?”. Proceedings: Biological Sciences265 (1404): 1377–1383. doi:10.1098/rspb.1998.0445.