There is a druid in my group and they will often use their ability to speak with wild animals to foresee incoming threats in the woods (thanks to birds) or to spy on people in towns (thanks to rats or pets).
I would like to better understand the dialogue abilities of wild / tamed animals. How much information can a bird or a rabbit give to the party ? Should they understand complex requests ? How much is a wild animal willing to help strangers ? Should they always be friendly with druids ?
There are three common approaches people take, and which one you take depends on your whims.
One common approach is to model them as very basic instinctual creatures. The birds will have a vague awareness of food and threats and such, but won't be able to communicate any complicated information due to their low intelligence scores. Complex requests won't be understood. This approach is good if you don't want to voice NPCs, want a more realistic approach, and want to avoid powergaming.
The second common approach is to model them as excitable toddlers. They have a decent amount of intelligence, can carry out a basic conversation, but can't understand complicated instructions and are pretty instinctual.
The third common approach is to model them as full NPCs, give them complicated histories and emotions, and use it as a chance to show off your roleplay skills. Baldur's Gate 3 does this famously.
I've done all three, depending on how much the player's powergame, how fun they find it, and how much I want to pretend to be a talking rat. There's no right answer, just what is right for your game. You can decide how intelligent animals are since you control their stat blocks, so you can vary their responses based off your whims.
The ability to speak to random entities does not give you control or even rapport with them.
Make the druid do the legwork of earning the creature's trust and friendship. Don't let them automatically gain minions just because they picked this or that class. Would a bard become instantly able to ask NPCs for favors upon arriving at a new village? Probably not, and so shouldn't the druid.
Then, the animal has its own agenda. Pets like their owners and most of them are naturally as loyal as any tyrant would wish their henchmen to be. Wild animals are 80% focused on their own survival and 17% focused on the survival of their offspring (the remaining 3% is the influence of the chaos gods).
Next, beasts are not people. They don't think like people, they don't remember stuff like people. They don't pay attention to people like people, and they don't understand people's languages.
A rat may spy on a pair of rogues doing shady business, but its report of what happened should not include any details a sapient, language-wielding being would notice. What the rat might notice, remember, and report is left as an exercise for the reader.
Key wording from the spell:
The knowledge and awareness of many beasts is limited by their intelligence
Don't be afraid to roleplay the beast like an idiot savant. Force the player to spend time and resources training their would-be minions. The rat spying on the rogues might not even notice that money was exchanged. If the druid wants the rat to recognize gold coins, the rat needs to learn what a gold coin is and pick it apart from the other types. And I doubt a rat would be able to count more than two or three.
There might be some pressure to treat these animal NPCs as a mage would its familiar. They are completely different entity types, though.
If the player attempts to abuse the use of animals as an information gathering network, remember that the beast's life is a short one and fraught with perils. The rat spy might have a deadly encounter with Mr. Mittens, the miller's prized granary guardian and never be heard of again. Likewise, a bird messenger might unwittingly enroll in a tactical air superiority class taught by a hawk at the cost of a pound of flesh (and the bird weighed less than that, regrettably). This allows the DM to curtail any abuse and withhold information the DM doesn't want the players to have.
So, make the player work for the benefits they want, make sure they are limited, and don't be afraid of throwing Curveballs of Fate at their attempts. Just don't be an ass and turn this into a source of grief. Just challenge.
Ease is a close friend of boredom.
Rats in a town don't care much about human level issues. In fact, if you wanted to know if what someone was up to, asking a human would be far better! They understand human behavior, and can recognize one human as different from another far better than a rat would.
So when the druid asks a local rat what a humanoid in a house is up to, how is this different than asking the humans in the neighbourhood?
I'd treat it as a different kind of contact. Like, asking the underworld about the comings and going of a noble, or the police about merchants. For the most part they'll only know of some minor information, and they have little reason to share it with some random stranger from out of town.
Now, it may be possible to bribe the rats, but they aren't known to be the most faithful of bargainers, nor do they actually know that much. Most city rats know big people as predators or rivals, and even if they are speaking rat they aren't going to trust them -- so finding a rat to have a chat with is going to be the first challenge.
In the woods, the birds will be mainly concerned about the hawk, and not the humans traveling through the area. Here, taking to the birds will be like asking a random street urchin about the lives of rats. Sure, they see rats all the time, and rats are somewhat dangerous and important, but that isn't what the kids mind is focused on. Unless you find that one urchin who has figured out how to hunt rats for food, or has trained rats to steal for her; similarly, most of the birds will be aware of humans, but not that knowledgable. That rare bird will be obsessed with human-spotting, counting how many colours of human plumage and eyes they see, keeping track on pieces of bark.
Still not the information you want, but better than "I dunno".
As I wouldn't actually want to roleplay every single interaction, I'd throw out ability checks to gather information – here, I might make it a Wisdom (Nature) or even Charisma (Nature) check to use Speak with Animals to canvas the fauna in an area and gather useful information. And then roleplay a selection of encounters depending on the pace of the game – maybe only roleplay if the result is really good, or roleplay one example interaction.
"You spend the much of the day trying to find a rat, cat or street dog that is interesting in having a chat and knows something." is a good tool here.
