UBC Theses and Dissertations
Habitat selection and time of breeding in the Great Blue Heron, (Ardea herodias) Butler, Robert William
This thesis examines the causes and consequences of habitat selection and timing of breeding of the Great Blue Heron (Ardea herodias). My general hypothesis was that the duration of low tides and seasonal abundance of prey strongly influenced the location of colony-sites; timing of the breeding season; habitat shifts; and the use of space by foraging herons of different age- and sex-classes. I studied Great Blue Herons along the Pacific coast of Canada for five breeding seasons and four winters. Breeding herons were studied at a colony of 85 to 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. At Sidney, I studied the foraging behaviour, food availability, habitat use and reproductive success in detail. At other colonies, I recorded the reproductive success of herons, located their main feeding areas and searched for nests of a predator, the Bald Eagle. In the non-breeding season, I investigated the foraging behaviour, dispersion pattern and habitat shifts of juvenile and post-breeding adult herons in the Fraser River delta. I hypothesized that heron colony-sites were located near food supplies or away from predators. Twenty-nine of 33 colony-sites were located within 6 km of their main feeding site. The number of heron pairs was slightly greater where eagles nested in high abundance than where eagle abundance was low, contrary to the hypothesis that breeding herons avoid areas with active eagle nests. I hypothesized that herons began breeding in spring shortly after females acquired enough food energy to make eggs, or so chicks were in nests when food was most plentiful to their parents. Egg-laying began about 9 days after a female's daily food intake crossed an energy threshold of 1715 kJ/day, whereas the peak availability of food energy to adults occurred about 35 days before the peak food demands of their chicks. Food intake rates by adults increased gradually in March and April with the increasing duration of low tides and the inshore movement of fishes. Adult food intake rates reached a peak in May when sea perch were most abundant, and diminished through June and July. Most juvenile and adult female herons foraged on beaches from February to October and in marshlands and grasslands from November to January. Some males returned to territories along riverbanks in August and remained there until the start of the next breeding season in March. I tested the hypothesis that herons leave foraging habitats in autumn when they can no longer catch enough food or when interference from conspecifics reduced foraging intake rates below a threshold required to maintain their energy balance. In October and November adults moved to marshlands and juveniles moved to grasslands when they could no longer maintain daily energy balance on beaches as a result of declining duration of low tides and food intake rates. Interference competition was too infrequent to explain habitat shifts by adult or juvenile herons in autumn.
Item Citations and Data