"They are ducks."
You use a form of "ser" if something is always like that. And you use a form of "estar" if something is only like that at the moment. Therefore you say "ellos son patos", because they are always ducks. If you´d say "ellos estan patos" it would mean, that they are only patos at the moment, which would´nt make sense.
"La iglesia es grande" - (form of ser) the church is big , because she always is.
"La comida está caliente" - (form of estar) the food is warm, because it is only at the moment and might be already cold in some minutes