A number of colonial waterbird species have been documented nesting on roof-tops throughout Europe and North America. The most common hypothesis explaining why gulls (Laridae) select roof-tops for nesting has been that population growth rates are higher than territory vacancy rates in traditional (island) habitat, suggesting that roof-tops are a non-preferred habitat. Roof-top habitat may actually be equal to or higher quality than island habitat as anthropogenic food is abundant and lower nest density may lead to lower intraspecific aggression and predation. During 2011–2012, reproductive effort and success was monitored in a regionally declining population of Herring Gulls (Larus argentatus) nesting on roof-tops in Portland, Maine, USA, and a nearby island-nesting colony on Appledore Island, Maine. Clutch size was lower in the roof-top colony, but egg volume did not differ between sites. Herring Gulls in the roof-top colony had lower hatching success but greater survival to day 30 for chicks that hatched than for those breeding in the island colony. The average number of chicks per nest to reach day 30 was 0.72 on the roof-tops and 0.84 on the island. This shows, therefore, that roof-top nesting may be an adaptive reproductive strategy even under scenarios with reduced competition for nesting territories on traditional nesting islands.
Perlut, Noah G.; Bonter, David N.; Ellis, Julie C.; and Friar, Margaret S., "Roof-Top Nesting In A Declining Population Of Herring Gulls (Larus Argentatus) In Portland, Maine, USA" (2016). Environmental Studies Faculty Publications. 23.