"Vi plejer at beholde vores fugle."
Translation:We usually keep our birds.
Because vi plejer is the verb in the present tense, followed by at beholde, that is the infinity form. It sounds maybe strange because of the translation of at pleje..in danish is a verb but in english is better translated as an adverb, related to an action that you normally/often do. But in general, every verb after "at" is infinitive, to make it easier.