Probably not, because being a seagull would be a physical property that other people can notice. So, I'd expect a human woman giving birth to a seagull would likely be a notable event that could make the local news, or at the very least be noted in the birth certificate. In other words, there'd be evidence of it.
Or they could be relying on their own personal experience. Those I've spoken to on the subject have said as much, so I tend to believe them.
Maybe it's a personal quirk of mine, but I tend to rely more on a person's own experience about themselves than some other random person's opinion about what their experiences were. Especially when that other random person never met them nor knew anything about their experiences.
-- A2SG, but you do you, dude....