If you're besieging a city that has a first cohort in it and you hit the auto-combat button, your general doesn't get credit for capturing the eagle, even though there's nowhere for the 1st cohort to retreat to. Maybe they have time to see you coming and melt the eagle down or something.

My Frankish general captured 4 eagles, gaining the attribute "Eagle Collector" and getting the title "...of the Eagles."