This answer contains spoilers… (view spoiler)[In the interlude called The Fireworkers Part 1 Carmine talks about how the gulls are attracted to the area surrounding The Lab by the buildup of nitrate in the soil. But I attributed their appearance at the Hamilton-Sweeney building as being more symbolic than an attraction to the contents of the duffle bag. (hide spoiler)]
Julia Charlie also said something about Sol's birds being gone and now they were circling the building. And I Charlie was even having Pulaski follow where he saw the birds when they were trying to figure out where the bomb was, which lead them to the Hamilton-Sweeny building.
This answer contains spoilers… (view spoiler)[The birds had been fed by Sol, ostensibly as they provided cover for the work being done at the shed and their filth discouraged anybody from poking around. Presumably once they moved the bomb out of the shed the birds followed; this is all part of a Veddy Symbolic usage of birds throughout the book -- if you are looking for a plausible explanation I guess you could assume the birds had associated the smell of the chemicals in the shed with home. (hide spoiler)]
Stephanie The birds were cared for by the guy who owned the backpack. They were abandoned by him, and somehow managed to locate his aura(smell? sighted him?). It was because of that guy that the pigeons were circling the building, at any rate.