Where have all the birds gone?

 I've been waking up early in the summer months for years. Normally, my eyes open at around 5 a.m. and I get up, make some coffee and then sit in bed, reading, with the curtains open and one of the balcony doors on a tilt to let in some fresh air.

In the past, the dawn chorus was almost deafening. The corkscrew willow in front of the balconies on my side of the building was full of small birds, chirruping away and hopping around: blue tits, coal tits, sparrows (whatever happened to them?), sometimes even finches. It's been years since I saw the jay and the thrush that often visited my balcony (not at the same time, though). Occasionally, I still see spotted woodpecker, but not for the last few months. 

Now when I sit in bed in the early hours, I hear nothing but the trains pass by in the distance, the trams trundle along the road, the cars... but no birds. 

Where have all the birds gone?