THE BREEDING OF GRAY HERONS (ARDEA-CINEREA) IN WESTERN SPAIN - THE INFLUENCE OF AGE

In a colony of Grey Herons (Ardea cinerea) studied for 15 years in wes tern Spain, average clutch size was 3.5 +/- 1.0 (1s.d.) and each pair made on average 1.17 breeding attempts per year. The mean brood size w as 2.2 +/- 1.0 nestlings, including totally failed pairs (15.9% of tot al) and 2.6 +/- 1.4 nestlings when these were excluded. Double broods of the same pair were raised, both successfully, by 1.36% of the pairs . Brood size varied with parental age: 1.8 +/- 1.3 nestlings in pairs containing one 2-year-old bird, 2.7 +/- 1.4 in mixed pairs with one 3- year-old bird, and 2.8 +/- 1.4 in pairs of greater-than-or-equal-to 4- year-old birds. Mortality rate was estimated at 66.7% for yearlings (< 2-year-old). From the fledglings produced in a given year, 11.6% retur n to the colony the next year; 5.6% return after three years, and 3.1% return after four years. The population increased at an annual rate o f 34.6% during the first eight years, and then remained stable.