You maybe have visiting cats, even if you don't have one of your own. There may be a sparrowhawk or similar bird of prey in the area. If your garden is very tidy and you use pesticides and herbicides, there may be nothing much for birds to eat. Do you put food out for them? Do the birds visit your neighbours' gardens or are you near to a park or in a countryside area? - in which case you will need to make a positive effort to attract them with different types of feeders and bowls of water big enough to bathe in. Good luck!