wren
Rail
A swan is an aquatic bird starting with S and having 4 letters.
Gull.
loon
Bird
Cuckoo
Teal
A three letter bird is an owl.
Canary is a bird. Cassowary is a bird.
Hare and dove are four letter animals. Additional four letter animals include nene (bird) and mule.
A duck. It fits the description perfectly.
BEAK