According to Armas Hill, of DE/PA birding fame, Silver Gulls were
NOT among those that escaped from the Bronx Zoo as a result of a
roof collapse in 1994.

From:  Delaware RBA, November 15, 1996

A number of people have referred to the birds (particularly the gulls
and terns) that escaped from the Bronx Zoo in New York City, when
part of the birdhouse collapsed from heavy snow, a couple winters
ago. Recalling the incident, and saying the SILVER GULL is probably
one of those birds. It is not. The GULLS and TERNS that escaped a
couple years ago were South American. 33 birds escaped. They
included GULLS of 3 species: ANDEAN, GRAY, and BAND-TAILED.
Also INCA TERNS. There was not 1 SILVER GULL among them.
That was 2 years ago. It has also been said that the National Zoo in
Washington D.C. has had SILVER GULLS. As far as we know, that
zoo is not missing any. They only have one, and they'd know if it weren't
there.

Actually, as far as we know, now, according to the latest published
inventory, there are only 11 SILVER GULLS in reputable captivity in
North America: 1 of them at the National Zoo in Washington D.C., 7 at
a zoo in Saint Louis, and 3 at another zoo in the Midwest. That information,
on the internet, from Bill Smith in Homestead, Florida. Actually, there were
some SILVER GULLS at a zoo in Florida, in  Miami, back in the mid-eighties.
A flock was there in 1985, over 10 years ago. However, the following year,
in 1986, for various reasons, all of the birds  of that flock had to be
killed.
So, if prior to that, one escaped - and were to  appear 10 years later in
Salem County, New Jersey, it would have had to  have lived over a decade
out of captivity, in addition to making, for some strange reason, an odd
migration to the north.

Kurt Schwarz
HowCo