I can see this is preliminary for now, so my strong recommendation as you're putting together future versions is to show detailed source and biblio information on the image itself. It's essential to making a professional, usable visualization.

Regarding what you have so far, you obviously will have to find some way to indicate or adjust for the obvious skew people will have toward reporting larger mammals. As a basic adjustment to the data (or rather, to the size of colored blocks in your visualization -- keep the numbers the same) you could divide each species by their average body weight. This may actually have to be a power of the weight, or even the log. if you plot for yourself and then fit the reported deaths versus weight, you'll get an idea of what function to try for the adjustment. For anything like insects in which the entire category has only one report, you might consider omitting that block entirely, noting "insufficient data".

This would seem to me to be the most beneficial adjustment to make, but there might be more adjustments or indications to consider in future, such as for nocturnal species. (They would definitely be hit more frequently, and if they're large I'd guess they'd be reported more, and if small I'd guess they'd be reported less.)