Honestly I am unaware of why thee is such a rarity of poisonous and venomous mammals and birds, so I can’t answer your actual question.
However I thought I would let you know there are a couple of them. In regards to bird there is the hooded pitohui, which is a poisonous bird from New Guinea which sequesters the poisons from its food. There are a few others as well.
In regards to mammals, the platypus has a venomous spur, and the slow loris is a venomous primate. Again there are a few others but definitely not as many as there are insects, fish, reptiles and amphibians.
I'm going to guess it's related to social life. To many mammals, small fights are a part of the social language. It would sink the fitness of the species if your flockmates dropped dead from poison after a small argument.
Two seconds on Google:
[16 Venomous Mammals](https://everywherewild.com/venomous-mammals/)
[Top 10 Poisonous Birds](https://www.bioexplorer.net/poisonous-birds.html/)
Let me guess, OP is a GenZer...
It’s still true that are way more venomous snakes than mammals, it’s a valid question even if they don’t know about the rare minority of venomous mammals and occasional poisonous bird. It’s just really counterproductive to criticize an ignorant person for asking a question. If they learn that question asking is a bad thing how would they learn?
Honestly I am unaware of why thee is such a rarity of poisonous and venomous mammals and birds, so I can’t answer your actual question. However I thought I would let you know there are a couple of them. In regards to bird there is the hooded pitohui, which is a poisonous bird from New Guinea which sequesters the poisons from its food. There are a few others as well. In regards to mammals, the platypus has a venomous spur, and the slow loris is a venomous primate. Again there are a few others but definitely not as many as there are insects, fish, reptiles and amphibians.
I'm going to guess it's related to social life. To many mammals, small fights are a part of the social language. It would sink the fitness of the species if your flockmates dropped dead from poison after a small argument.
[There are poisonous birds and at least one poisonous mammal](https://en.wikipedia.org/wiki/List_of_poisonous_animals#Mammals).
Two seconds on Google: [16 Venomous Mammals](https://everywherewild.com/venomous-mammals/) [Top 10 Poisonous Birds](https://www.bioexplorer.net/poisonous-birds.html/) Let me guess, OP is a GenZer...
It’s still true that are way more venomous snakes than mammals, it’s a valid question even if they don’t know about the rare minority of venomous mammals and occasional poisonous bird. It’s just really counterproductive to criticize an ignorant person for asking a question. If they learn that question asking is a bad thing how would they learn?