The patient predator: foraging and population ecology of the Great Blue Heron Ardea herodias in British Columbia

This report presents the results of a six-year study (1986-91, including five breeding seasons and four winters) of the diet, food availability, foraging behaviour, use of habitats by age-classes and sex-classes, spacing of breeding colonies, and survival of the Great Blue Heron Ardea herodias on the Pacific coast of British Columbia. The general thesis of this study is that the availability of food energy largely dictates when herons use foraging habitats, the time of year that they breed, and which individuals survive. Breeding herons were studied at a colony of 85-100 pairs on Sidney Island near the town of Sidney, and periodic visits were made to about 40 other colonies around the Strait of Georgia, British Columbia. Field studies on Sidney Island included foraging behaviour, food availability, habitat use, and reproductive success. At other colonies, the reproductive success of herons and the chief foraging sites and locations of a nest predator, the Bald Eagle Haliaeetus leucocephalus, were recorded. In the nonbreeding season, the foraging behaviour, dispersion patterns, and habitat shifts of postbreeding adult and juvenile herons were studied in the Fraser River delta, a few kilometres south of the city of Vancouver. Food was most available to herons in late May and early June, when daytime tides were the lowest of the year and fish prey was most abundant. Adult breeding herons ate mostly small fish caught along beaches. Fish were caught by solitary herons on beaches at an average rate of about one fish every two minutes. Herons reaped 3-4 times more food energy by eating the relatively larger shiner perch Cymatogaster aggregata than by eating other fish species. Adults caught fish about twice as fast as juveniles in autumn. Most juvenile and adult herons foraged on beaches from March to October and in marshlands and grasslands from November to February. The switch from beaches to marshlands and grasslands occurred when the food availability to herons on beaches was below the level required to maintain their energy balance. Herons foraging in grasslands ate mostly Townsend's vole Microtus townsendii in autumn and winter, when vole densities were high. However, the number of herons on grasslands was not related to the time of day or to the diurnal period of vole activity. Herons located colony sites near good foraging habitats, although all good habitats were not used by herons each year. The proximity of nesting eagles did not affect the spacing of colonies along the coastline, the size of heron broods, or the reproductive success of the herons. Herons that hatched eggs in the first two weeks of the breeding season had an 84% chance of successfully raising at least one chick to independence, compared with a 40% chance among nests with chicks hatching in the last 5-11 weeks of the season. It was estimated that 27% of the juveniles and 73% of the older herons present in August were alive the following February. Most chick deaths resulted from traumas and not starvation, as previously believed. However, the greatest mortality rate was in juveniles that became emaciated and were unable to avoid fatal accidents.