These are some observations about house sparrows (chimnya in Marathi) in Pune, India, and I am sure that it applies to many urban areas too…
I think there are 5 factors affecting small birds and reducing their numbers.
These might be eating sparrow chicks and eggs. I am trying to find this out, but this information is not easy to come by.
Crows are very smart and can outwit other birds easily, and they are very social and operate in packs.
Also, I think they are quite territorial.
Pigeons also could affect small birds, and there could be two factors there.
There are a lot of pigeons in urban areas and buildings (so much so, that they are a menace) and they might not leave any place for sparrows, etc. to perch.
Also, they would eat any food lying around, so sparrows won’t get any food.
I don’t know if you know this, but cuckoos are egg parasites. They lay their eggs in other bird’s nests and even though a sparrow egg might be much smaller than a cuckoo egg, birds do not see the size. They will nurture whatever comes out of the egg.
Also, a strange behavior is that a cuckoo egg hatches earlier and the first thing the cuckoo chick does is put out the other eggs out of the nest.
So, it destroys the other eggs.
Traditionally, cuckoos are revered by people. They have a sweet voice and so people like them.
All these factors have led to exploding numbers of cuckoos.
And they are very noisy.
They have become a nuisance.
That is why I might dislike cuckoos so much.
This is a cause for the loss of wildlife, in general.
At first, I used to think that humans scare sparrows away.
But then, my grandmother and many old people, in earlier times, would feed sparrows by throwing grains for them.
The sparrows were afraid of humans (of course), but did not avoid them totally.
No natural predators
Also, due to habitat loss and human activity, many of the natural predators like owls and hawks and kites , etc. of the above pests have disappeared, and so the pest birds have nothing to fear.
Phone signals and other factors
A common myth that I have heard in Pune, India is that cellphone signals affect birds.
While, I do not rule that out completely, science has yet to prove this, and for now, lets assume that there is no such effect.
Science is researching this, but so far, it has not found anything that we affect sparrows with signals.
With so much urban development, we have created a habitat loss for the sparrows and now they have no real places to build their nests and to breed.
A lot of the birds now are perching birds and are bigger in size, so they then fly up tall buildings and perch on terraces and parapets, etc..
So, they never need to come in human space.
Also, people are not aware that crows and cuckoos (and even pigeons) are pests and people ignore them; they even feed them and so then, their numbers increase.