From Lives of North American Birds by Kenn Kaufman:
Besides the thick bill that gives it its name, this tern has a relatively stocky build and broad wings. Typically seen in leisurely flight over marshes, hawking for insects in the air or swooping down to take prey from the water or the ground; unlike typical terns, rarely dives into water for fish. On the ground, walks better than most terns.
Nests mostly on beaches and islands. Reportedly used to nest more often in salt marshes, abandoned those sites because of human persecution.
Feeds on a wide variety of insets caught on the ground, in the air, or at the surface of the water. Also feeds on spiders, crabs, shrimp, mollusks, earthworms, small fish, frogs, etc. Forages by flying slowly into the wind, dipping to surface of land or water to pick up items, or by catching flying insects in the air. Sometimes forages while walking on the ground; rarely plunges into water.
Gull-billed Terns are colonial breeders. Colonies are usually small, not as densely packed as those of many terns. The nest is on open ground. The nest (built by both sexes) is a shallow depression, often with rim of soil and the addition of some plant material and debris. Usually 2-3 eggs. Incubation is by both parents for 22-23 days. The young leave the nest a few days after hatching and move to dense plant cover if nearby. Age at first flight is 4-5 weeks. The young may remain with parents 3 months or more, beginning southward migration with them.
Mainly a summer resident in California and on Atlantic coast; some remain through winter on Gulf coast. Evidently far less numerous on the Atlantic coast today than it was historically. Human disturbance and loss of nesting sites among likely causes. Has begun nesting on rooftops in some Gulf coast areas.
Mark took these pictures at Estero Llano Grande.