Southern countries in Europe are hot like Greece, Spain, Italy an so on.
If you come to noth like Baltic sea it can be cool, but chance of really hot weather is rather low near the coast.
In north and even middle of Europa, different birds left their area in August e.g. waders.
for example
http://www.oulu.fi/northnature/english/englanti/elaimetmulin.html
"The end of July is already busy time for returners. Anyhow there aren’t big flocks of migratory birds, they just disappear from the inland. They gather in to certain places to form bigger flocks. Many broods are already able to fly and they leave too. Swallows gather in to flocks, last starlings fly restlessly in big flocks and first white wagtail groups appear to the roadsides.
Most shorebirds disappear in August and perching birds seek for food everywhere in bushes and in courtyards. Big birds of prey and first cranes start their migration, although the main migration is in September. In September the migration can also be noticed the best. Thrushes and chaffinches come to courtyards and cranes, wood pigeons, pipits and buntings are found from fields. Thrushes fly after rowan tree berries to courtyards and to parks, cranes gather in to big flocks to unthreshed fields."
I like middle of August, because many migration waders appear and other interesting birds still there like Lesser spotted Eagle, Osprey, swallows, terns.