Not being particularly bird knowledgeable and not living in an area that seems to have many cuckoos (outside of clocks) - what is the connection between Cuckoos and July?   Is it just the normal matter of birds returning in the Spring and thus being a harbinger of the coming summer or is there something more unique to the cuckoo.

The line "She never says 'cuckoo' til the 4th day of July " in the version I know best always seemed a bit puzzling if cuckoos normally show up in the spring when most other birds do. Do cuckoos not show up until later? Do cuckoos that come earlier say something different before July? The line "And when she sings 'Cuckoo' the summer is near" in other versions would suggest that cuckoos can and do show up with their usual call in spring - but perhaps different cuckoos do different things in different places?