[snip]

Probably the lack of water. I would wait until the weather starts to cool
off (Autumn) and then seed and water regularly (every evening unless it
rains!). The golden rule is water soft and long so that the water soaks in
instead of heavy and little, which tends to run away or just evaporate.

BTW, if you think lots of cats keep the birds away, you haven't met our
local wood pigeons ;-).

Paul DS.