“While some earlier rishonim required waiting between eating beheima meat and eating dairy, Rambam was the first to state that one must wait after eating poultry, as well”

While some earlier rishonim required waiting between eating beheima meat and eating dairy, Rambam was the first to state that one must wait after eating poultry, as well. (Beheima meat refers to meat from cows, sheep or goats.):

מִי שֶׁאָכַל בָּשָׂר בַּתְּחִלָּה, בֵּין בְּשַׂר בְּהֵמָה בֵּין בְּשַׂר עוֹף–לֹא יֹאכַל אַחֲרָיו חָלָב עַד שֶׁיִּשְׁהֶה בֵּינֵיהֶן כְּדֵי שֵׁעוּר סְעוֹדָה אַחֶרֶת, וְהוּא כְּמוֹ שֵׁשׁ שָׁעוֹת: מִפְּנֵי הַבָּשָׂר שֶׁלְּבֵין הַשִּׁנַּיִם, שְׁאֵינוּ סָר בְּקִנּוּחַ
רמבם משנה תורה מאכלות אסורות פרק ט’ הלכה כז

This contradicts the simple reading of the Gemara Chullin 104b:

תנא אגרא חמוה דרבי אבא עוף וגבינה נאכלין באפיקורן הוא תני לה והוא אמר לה בלא נטילת ידים ובלא קינוח הפה

which states plainly that poultry and cheese (even in that order) may been eaten באפיקורן (in one kerchief)) or without concern (לשון הפקר) even without washing one’s hands or mouth in between their consumption. See, for example, Ritva (Chullin 104b):

פירש”י שאם אכל זה ובה לאכול זה א”צ לקנח פיו ולא ליטול. ונראה מלשונו אפי’ בשאכל בשר עוף בתחילה…

Earlier rishonim who required a long wait between beheima meat and dairy did not require this waiting between poultry and dairy. R. Chananel and R. Yitchak Alfasi required a waiting period only between beheima meat and dairy, but not between poultry and dairy. Rishonim in the years close after Rambam’s lifetime challenged his innovation.

Tzvi H. Adams, “Waiting Six Hours for Dairy – A Rabbanite Response to Qaraism”, Seforim Blog (11 August 2015) [http://seforim.blogspot.com/2015/08/waiting-six-hours-for-dairy-rabbanite.html]