Is a bird with venomous claws possible? I want it to jump on people and scratch at their heads while they’re walking as a mating ritual. However, they need to be venomous for plot related reasons.
I give you the Male Platypus
A quick Google search reveals that the male platypus has poison glands on their thighs and a hollow spur near their heels.
The platypus is not a bird, but it's a strange little honker and has, genetically speaking, a lot in common with birds.
The male platypus might be the only critter on the planet with venomous claws that has anything at all to do evolutionarily with birds.
However, one is all you need.
So let it be written, so let it be done.
Toxicity in birds is certainly possible.
Pitohuis use batrachotoxin, the same neurotoxin as several poison dart frogs. They get it from Choresine beetles, and then sequester the toxin in their skin, as well as excreting it to cover the feathers. Your bird would necessarily need some other means of processing the toxins so that they were excreted near the claws, but I don't think that barrier is prohibitive.
I think the larger barrier here isn't the toxicity, it's the "jump on people and scratch at their heads while they're walking as a mating ritual". What is the selective pressure to jump on people's heads??? Popular videos of birds shagging people's heads (like this , or this ) are usually:
a.) Birds that are very confused, and rather than performing a mating ritual, are actively attempting to copulate
b.) Same as a.), but with birds that have imprinted on humans and think that they are mating with a human.
c.) I actually don't have a c.), but the list seemed bare.
Summary: birds in normal circumstances don't shag peoples' heads.
You could maybe have local flora or geologica that vaguely resemble human heads, have the mating ritual center on those, and then have the birds be plausibly confused?
Yes, it would be possible. There don't seem to be any bird which produces venom themselves but there could in your world, there are many types of poisonous.
A slight variant could be that the claws aren't venomous per se, but those birds would also be attacking other (venomous) animals/plants, and their claws keep traces of their venom (or at least they do often enough to be problematic in your world).
I don't see why they would want to jump on people. though. I think that would make humans very motivated into killing any of them they see flying near their cities.
As a mating ritual? Here's a video of what it would look like. And there are these things called hats. People might be inclined to wear them during mating season at least. You might do better making the the bird's saliva venomous and making it a n active hunter the size or a cassowary or similar like terror birds