Gemara Beitzah Daf 11a
More on Rebbe Chanina from the mishnah discussing when you designate 2 birds in the nest
and found them on the ledge in front of the nest on YomTov. Mishnah says forbidden.
Are they from the huge number of birds on the outside (Rov) or were they originally
in the nest (kirov).

Abaye - this ledge is a frequent meeting place for birds, therefore this Mishnah is not a proof for Rebbe Chanina.
The Rov Is Karov because the ledge is closer to the outside (rov & kirov) than the nest

Rava - we're dealing with a 2 story dovecot: If I designate birds on bottom, it's easy for top birds to get down
However if I designate birds on TOP, it's hard for the little birds to climb up, so I would think top designated
birds are permitted...however Rava's chiddush is: no way, we are still afraid the bottom birds can climb!

Mishna: Find birds in different place, and no other birds around....then muttar
Gemara: No hopping birds within 50 amot of this nest. Non-flying birds dont travel more than 50 amot from the nest.
remember Medad = HOP, not FLY!

Why is this not obvious? Why does Mishnah bother with this?
Because this is a case where the nests are positioned on opposite sides of the corner.
Therefore the nests are not visible! Birds will not move to something that is invisible.

To be continued....