Note: In the past, most student projects have used the "Realtime Database" rather than the "Cloud Firestore". There are pros & cons. Comparison here: https://firebase.google.com/docs/firestore/rtdb-vs-firestore
As you probably realized, your current code is adding a new document to the collection. This is very useful, say, whenever a user posts something new on a message board, and you want to create a new "post" object with a unique ID to represent that post.
In this case, instead, it sounds like you want to work with the same document, but change its contents, using the set(...) method. Something like this:
Map<String, Object> city = new HashMap<>();
city.put("name", "Los Angeles");
(This example is coming from the Firestore docs -- https://firebase.google.com/docs/firestore/manage-data/add-data )
In your case, you might want a Map<String,Integer> of bird names to counts (of those birds), and do:
Later on, if you need to update a value, you COULD change the map object and call set() again, but it might be more efficient (e.g., if you had thousands of birds?) to use the update() method to update a single field in the document, instead of sending the whole document. See, later on the docs page: https://firebase.google.com/docs/firestore/manage-data/add-data#update-data
There may be other better ways of structuring the data... I would have to think about it, but this is one way that should work!