As they are normally considered salt-water birds, I checked my link for the Delaware River Salt Line. (Check it here: http://www.state.nj.us/drbc/hydrological/river/salt/ ) It's nowhere near Philly, nor should it be. In fact, it's a good 15 miles south of its normal location at this time of year, thanks to all the recent rain.
The first time I saw a Laughing Gull here, years ago, I called the local Audobon Society to report it. I am not sure she believed me, due to that whole salt-water-only thing. But I know what I saw, and I'm seeing it again now.