Where are the birds??
I had an email from the RSPB which threw some light on the fact.
This one I had never heard of before but the second one is common knowledge.
They’re keeping themselves safe
The first reason that birds are less visible in the summer is that they are growing a new set of feathers, which can be very draining and make it harder for them to fly. This means that for a few weeks they are much more vulnerable to predators and territorial birds. To stay safe, they keep as quiet as possible and hide themselves away.
Secondly, in late summer grain, berries, and fruit are more readily available so the birds don’t need to rely on your bird feeders quite as much.
In areas where farmland is nearby, for example, house sparrows, starlings and many finches move out to fields to feast on the abundance of grain before it is harvested, and on the spilt grain after the harvest. Even in cities, sparrows and finches are attracted to any piece of rough ground that provides a good crop of weed seeds, and blackbirds and song thrushes tend to move to where there is an abundance of fruit or berries.
Hopefully it won't be long before blackbird and robin return to my garden.