Nestling Development of Green Herons at San Blas, Nayarit, Mexico

Abstract

Nestling development of the Green Heron was studied at San Blas, Nayarit, Mexico, July- September 1964. Daily measurements were made on marked young, including weight, and lengths of the culmen from nostril, culmen from tear duct, tarsus, and primaries 3 and 8. In Butorides sexual dimorphism in size is less than 3 percent and did not constitute a significant variable. Weight measurements among young of the same age varied by as much asnearly 400 percent. On hatching, young weighed about 16 g and gained an average of about 8 g per day during their first two weeks. Minimal adult weight of 160 g was reached by some young 13-14 days old. Bill measurements varied less within an age group than any other measurement. The basal portion of the bill grew more rapidly during the first 12-13 days, after which both portions grew at the same rate. The bill continued to grow steadily for an extended period, in contrast to the tarsus, the growth rate curve of which leveled off sharply at day 16-17. Minimal adult tarsal lengths were reached by some young by day 16-17; however, apparently slow tarsal growth continued beyond the age for which data were obtained. Growth of primaries 3 and 8 was similar. They erupted between the third and eighth day and grew an average of 4 mm per day for the first four weeks. Based on these measurements, a table was compiled which will permit the aging of a nestling Green Heron within a range of two days. Caution should be used when applying these aging criteria because of variation in food supplies and/or other factors that affect growth rates from one area to another, especially between tropical and temperate .climates.