For what it’s worth, this article cites a longitudinal study done by German researchers.

They basically installed a bunch of bird feeders outside nursing homes to see if watching birds improves the health of the elderly people. The researchers concluded that it did:

    Nursing home staff were the first to see the difference the birds made. "For weeks, they haven't seen a smile on [a resident's] face, and then they see the smile," Kals says.
    Some of the memory and concentration improvements are evident after just three to four weeks, Kals says.

https://www.lbv.de/umweltbildung/fuer-seniorenheime/ausstatt...