One thing about being in palaeontology is that you tend to cover quite a diverse range of subject. Working with the material available means that even specialists in one group or on one method with often dabble quite widely in other areas of research. There’s not too many theropod guys who haven’t at least looked at ornithischians or sauropods or even crocs, pterosaurs and lizards and have probably published on a few of them. So too it is with things beyond bones and again a lot of people will have had some involvement with work on eggs or trackways.
In my case this means that for all my interest in all living things and my tendency to take in local pigeons and squirrels as much as species in zoos and museums, I now also get interested even by things like random footprints in bits of sand. Even this little can provide a bit of food for thought. Here for example the bird tracks are rather deeper than those of the cat and seem to interact with the substrate in a different way. The sand hasn’t changed as such between the two sets being laid down but of course could have been wetter or drier at different times – even a single base can react very differently according to local conditions and affecting how tracks are made and, by extension, how we interpret them.
Dave Hone's Archosaur Musings 
Wow. That’s a really good point. Thank you for posting this – great photo and good example of how to judge fossilized tracks. The cat’s soft pads carry more of its weight than the bird’s narrow toes so it makes sense the bird’s feet sink deeper, also the quadruped’s not putting as much weight per foot proportionally.
Intuitively you would think that the bird’s narrow toes would sink further into the sand than the broad pads of the cat but the difference is less than it might be because a bird of the size that made that track is going to weigh quite a bit less than the cat (it’s pressure rather than surface area that determines how far different objects impress a substrate). Offset against that is the fact that the bird is supporting all of it’s weight on one foot or the other when walking while the cat’s weight is distributed among three (when walking, less for faster gaits).
Things are further complicated by whether the animal suddenly changed its speed or direction (including vertically), which would lead to deeper prints, and whether the two track were made under the same conditions with, as Dave mentioned, moisture content being the main variable.
Interesting stuff.