This shows that/how the heron was able to make use of a tool.

Well, "that" means we've just shown that the heron was able to use a tool, and "how" means we've explained how the heron used a tool. But since the description of what the heron did speaks for itself, there's effectively little or no difference in meaning. Even with "that", it's still obvious to the reader that the description also explains how.

Angliholic

By any chance, could "how" be thought of as "interjection?" Therefore, is it also correct to view "how the heron was able to ... tool" as an interjectional clause? If yes, does this interjectional clause add more emphasis to the base sentence than the "that clause?"

In this sentence, "how" can't be an "interjection" in the everyday sense that I understand the word. Unfortunately, though, I have no idea what an "interjectional clause" is, and I can't seem to find any clear explanation on the web, so I can't answer your question.

Can anyone else help?

I'm still not very confident I understand this, so apologies if this reply is irrelevant.

In contexts similar to your original text, it is possible, informally/conversationally, for "how" not to really mean how, but just to mean that it happened. (Actually, I put a mention of this in my original post, but then I deleted it because I thought it was over-complicating matters!)

For example, if your computer suddenly broke for an unknown reason, you might say "This just shows how computers break all the time", even though it hadn't really shown how they break, just that they break.

This usage is, as I say, informal, and I can't immediately think of an example where "how" would be a good choice of word to use in this sense in a more formal piece of writing, such as your original.

Peaceblinkfriend

Mr Wordy, when you said 'speaks for itself', did you mean that the description of what the heron did can be understood easily? I am sorry but I don't understand why there would effectively be little of no difference in meaning if 'the description speaks for itself'. 

What I mean is that the nature of the account referred to by "This shows that/how the heron was able to make use of a tool" is obvious to the reader. This is partly because it can be readily understood, but also simply because it's there in front of you and you've just read it. If the nature of the thing you're describing is not so obvious or accessible to the reader, then the difference between "how" and "that" may become more important.