And you can't be sure what it is until someone has gone and looked. All ATC get from the crew, not unreasonably, is "We think we hit an animal on take-off" or whatever. Occasionally a larger animal can get on to an airfield, I've seen a deer or two in my time, and if an aircraft hits a small deer or something of that elk (sorry, couldn't resist it) at speed it could well leave a few bits of aircraft on the runway too.
A big aircraft at high speed on a runway is carrying a lot of energy and even small bits of debris can cause big problems - this is why runways are regularly inspected and reports of debris are taken seriously.