In case you didn't find the answer to your question, those feathers are called 'filoplumes' [1]. Relatively obscure anatomical detail like this is more easily d

In case you didn't find the answer to your question, those feathers are called 'filoplumes' [1]. Relatively obscure anatomical detail like this is more easily discoverable in the scientific literature than on the web. I found the answer by searching Google Scholar for "marabou stork", browsing the results for a paper about the "gular sac" (although it has other names), then reading the most promising looking paper - the answer was in the second paragraph [2]. 1. https://en.wikipedia.org/wiki/Feather#Classification https://en.wikipedia.org/wiki/Feather#Classification 2. Akester, A. R., Pomeroy, D. E. and Purton, M. D. (1973), Subcutaneous air pouches in the Marabou stork (Leptoptios curmeniferus). Journal of Zoology, 170: 493–499. doi:10.1111/j.1469-7998.1973.tb05063.x https://sci-hub.io/http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.1973.tb05063.x/full https://sci-hub.io/http://onlinelibrary.wiley.com/doi/10.111...