Calls are short vocalizations distinguished from the longer songs.
(Since songs are generally used for courtship and defending territory, sometimes short vocalizations that serve those purposes
are also defined as songs, so the distinction is a rather fluid one.)
I was fascinating by the variety of different types of calls that I found mentioned, and could never find a glossary that defined
a clear categorization of them. I eventually discovered the excellent book, the 2004
Nature's Music: The Science of Birdsong,
edited by Peter Marler and Hans Slabbekoorn
and in particular chapter 5, Bird calls: a cornucopia for communication by Marler himself.
Often calls are referred to by their sounds, as in "the chink call", both because there may be more than one call of the same category,
and because the same call may have different meanings in different situations.
Here are the categories I've found there and elsewhere.
These are short, soft, quiet, hard to localize sounds used by birds moving in a group to stay in touch with one another.
These are related to contact calls, but used when a bird is out of range for hearing contact calls.
As such they're louder and more complex.
Flight calls. Sometimes this refers to a pre-flight call where individuals essentially make the suggestion that it would be a good time to take off.
If enough others reply, the flock will take off, otherwise the individual will go back to foraging or whatever it was doing.
I've also seen it used to refer to calls made during flight.
Rain calls. One description says they're calls that are made when most other species are silent, which can happen when the weather is bad.
Alarm calls. Species can have many nuanced alarm calls used in slightly different situations.
They're loud, noisy, short, and often high-pitched so as to be hard to localize.
These call on birds, often of mixed species, to mob a predator that may be on the ground and thus easier to attack.
In this case there's no need to make the call hard to localize, as there will hopefully be many individuals responding.
Made by a bird in the grip of a predator.
One hypothesis says the idea is not just to express terror but to attract a second predator who may attack the first one,
giving the victim some chance to escape in the confusion.
These are the calls very young birds make to express hunger.
These are calls made by adult birds to notify others of food.
It seems there are two main contexts. One is when there is a large food source that can be shared by many birds.
The other is when a bird offers a tidbit to a mate or potential mate.
A 2017 article on crossbill calls says:
"Excitement calls are lower pitched sounds that are given when perched, usually in a context of social interacting, such as in pair bonding, or territorial conflicts."
I see the term elsewhere, but mostly in connection with crossbills, so I'm not sure if it's in general use.
Likewise the Peterson field guide of 2019 refers to "Chitter Calls. Are soft conversational calls that are named after their sound" in connection with crossbills.
For this reason, I'm mentioning these last.