In addition, special collective nouns may be used for particular cases: for example a flock of geese, if not in flight, is sometimes called a gaggle. However, in theoretical discussions in behavioural ecology, the term "herd" is used for all these kinds of assemblage. A herd may also refer to one that tends and cares for such groups (e.i. shepherds tend to sheep, and goatherds tend to goats, etc.).