There is also concern that whatever is killing the birds could spread to other wildlife or domesticated animals, scientists warn as they look for answers.

The bird population in the United States and Canada was probably around 10.1 billion nearly half a century ago and has fallen 29 per cent to about 7.2 billion birds, according to a study in Thursday’s journal Science.

Wildlife Haven Rehab Centre still unsure what caused dozens of birds to become ill at a retention pond.