Aspects of the biology of the Little Tern (Sterna albifrons) were studied at the Ebro Delta (northwestern Mediterranean) during 1996-1997, including colony site selection, clutch size and egg size. Using a single visit to avoid disturbances, average number of eggs was 2.62 eggs per nest, and was significantly larger in 1996 than in 1997, when the modal clutch size was only two eggs. Mean egg volume was 8.88 cm³ (SD ± 0.57), and mean egg size in a clutch (for both two- and three-egg clutches) was also significantly larger in 1996 than in 1997. Furthermore, average egg volume in three-egg clutches (clutches that were probably completed) was significantly different among the four main breeding areas within the delta. Clutch size was not significantly different among breeding areas in either year. Little Terns bred associated with other colonial Charadriiforms more than in other Mediterranean breeding areas. The terns associated with all the commonest colonial species of shorebirds, but never associated with the Yellow-legged Gull (Larus michahellis), Audouin’s Gull (L. audouinii) or Black-headed Gull (L. ridibundus). The number of breeding pairs decreased from 625 in 1961 to 332 in 2003, this decline occurring mainly since the early 1990s. Mean population declined significantly by 2% per year for the whole period. During the last decade, the number of breeding areas has increased from four to nine, despite the decrease in breeding numbers (rate of decrease 7.1% per year). Nothing is known about the factors driving this decrease, but changes are probably linked to processes in other colonies at a metapopulation level.