XKCD Isn't Funny - #1879 - Eclipse Birds
I like this one! It's not perfect by a long shot, but the joke works, and that's the most important thing.
The way the characters react as if the birds aren't acting out of the ordinary (for the eclipse, I mean) really sells it. If they were acting scared, I think it'd take away from the comic. It'd just be people reacting the way that, y'know, people would react. That's not funny, that's just people being people, but in a situation that's too 'out there' to be a satire on how people act.
Where the comic falls down is really in the presentation area. I usually have my brightness all the way down, so the words in the top of the last comic were hard to make out. That's not a huge problem, but if I was XKCD's editor, it would be something I'd fix. Maybe the gradient could have the darker side on the bottom of the panel, or maybe it could just be lightened a little.
It's also very static from panel to panel. I'm not asking to see a whole flock of birds or the construction of the blood cauldron, but if we saw like, a sparrow pecking around in the first panel, then it looks up in the second and flies away in the third, it'd make the comic more visually interesting. Or show the eclipse happening! As it is, it's just two people standing there.
Also, very minor nitpick, but the two people should be wearing eclipse glasses. Stay safe!