we live by the sea and there are loads of baby seagulls nesting on the roofs of our houses. My sister in law rears baby seagulls every year when they fall out the nests, and some return to visit year after year
we have a squab on the roof opposite, it won't fly! and squawks all day long. Presumably a parent is coming back to feed it - but you can hear it thru dg windows!
Seagull chicks are speckly and well-camouflaged, and are often difficult to see. Gulls nest colonially, usually on rocks, shingle or similar, but some kinds on flat roofs.
Once fledged, young gulls look different to adults, although they are the same size and shape. Herring gulls and black-backs, for example, are uniform brown the first year, other kinds have strong brown stripes on the wings. Some don't reach breeding age or full adult plumage for four or more years.
Used to have one nesting on our roof in Torquay. We had a window in the roof and if you even went near it the mother seagull went mad, trying to attack through the glass.
Someone i know climbed a tree to "rescue" a baby seagull that he thought was stuck. For his troubles, the gull (believing it was in danger) regurgitated a large glob of acrid slime into his face, stinging his eyes and making him vomit. Sounds like a good defence mechanism, and a good reason to leave them alone!