I have found Great Tits nesting in traffic cones.
The adults must think they are the perfect nesting place but they are death-traps.
The fledgelings cannot get out of the small hole in the top of the cones and I have found dead babies inside.
Someone did suggest cutting a hole to allow the birds to escape but this doesn't seem to be too successful, I think the parent birds try to encourage the babies to fly upwards, also I think heat from the sun could be a cause of the deaths.
Just think of all the discarded cones which are around and all the little birds unable to get out...
PLEASE, PLEASE upturn any cones you see or block up the hole at the top, thanks!