Seagulls gorging themselves on greasy junk food in Hobart, Tasmania, are so fat it is affecting their reproduction, one researcher says.

University of Tasmania researcher Heidi Auman says silver gulls feeding on fatty scraps thrown from seaside cafes has caused them to become overweight.

Auman said the urban gulls were about 10 percent fatter and had higher cholesterol, which was leading to poor quality eggs and a possible nosedive in their populations.

"They are scavenging outside cafes and at tips on things like chips [french fries],'' said Auman, an ornithologist who has been studying human impacts on sea birds for 20 years. "This has led to the Hobart birds laying smaller, lighter eggs, with less yolk.

"It is hard to say what impact this is having on their populations, but their numbers have been dropping significantly over the past 25 years and this could be the reason.''

Auman said the roly poly gulls still were able to fly but the junk food was having the same effect on them as it does on humans.

"Silver gulls have capitalized on our wasteful society and some of the same effects of junk food on people seems to be manifesting in birds as well," she said.

Auman has been comparing city birds to those on the remote Furneaux Islands, off
Tasmania's northeast, as part of her doctorate studies for almost four years.