Around 800 pheasant chicks have been stolen by burglars, police said.
The three-week-old birds need access to heat and there is concern for their welfare if they are not suitably cared for, police said.
They were stolen from brooder units in Colemore near Alton in Hampshire on Wednesday.
Pc Lee Coleshill said: "Birds of this age are very vulnerable and if they are not cared for correctly they will die.
"They need suitable lighting and heating equipment which wasn't stolen, which could either mean the thief already has the necessary items needed, has an outlet for the chicks or has stolen them without realising what is needed.
"It is likely the thief did know what they were doing as to catch and handle the chicks would require some degree of confidence and knowledge of the task, so it is unlikely this has been an opportunistic burglary."