How to make audio files with different lengths the same duration?

How far you can go with this depends entirely on the length, and to an extent the content of the files. It's possible to expand or shrink the duration of files by up to about +/- 5% without too much in the way of noticeable artifacts by using Audition's stretch algorithm (or the Radius version of it, anyway). Alternatively you can copy and paste bits of the same file to itself to get the lengths the same, and with animal sounds you may well be able to get away with this. I find that it works better with non-musical sounds, so I would have thought that as long as you aren't being unreasonable, you should be able to do this.