. The Canadian field-naturalist. Distribution and Biology of Black-crowned Night Herons in Alberta James W. Wolford and David A. Boag Department of Zoology, University of Alberta, Edmonton, Alberta Abstract. Black-crowned Night Herons have expand- ed their range in Alberta since the first record in 1958. They have been found nesting at several loca- tions at the latitudes of Edmonton and Calgary, primarily in emergent vegetation of marshes. Three colonies in southern Alberta were studied in 1964 and 1965. Most of the initial egg-laying took place in late April and early May, and average initia
. The Canadian field-naturalist. Distribution and Biology of Black-crowned Night Herons in Alberta James W. Wolford and David A. Boag Department of Zoology, University of Alberta, Edmonton, Alberta Abstract. Black-crowned Night Herons have expand- ed their range in Alberta since the first record in 1958. They have been found nesting at several loca- tions at the latitudes of Edmonton and Calgary, primarily in emergent vegetation of marshes. Three colonies in southern Alberta were studied in 1964 and 1965. Most of the initial egg-laying took place in late April and early May, and average initial clutch size varied from to eggs. Some pairs laid replacement clutches which were consistently smaller but more successful than initial clutches. The average incubation period was days. Measure- ments of nestling growth are presented, and fledging required about six weeks. Production of young in both years (maximum of fledged per pair) was insufficient for maintenance of the population. The major factor responsible for low productivity was destruction of eggs and predation on nestlings by Ring-billed Gulls. Black-crowned Night Herons (Nycticorax nycti- corax) were recorded in Alberta for the first time in 1958, and their status in this province up to 1960 was reviewed by Salt (1961). Sub- sequently this species has been sighted, collect- ed, or found nesting at several additional localities throughout the province. During the summers of 1964 and 1965, we studied three colonies of these herons in the Eastern Irrigation District of southern Alberta. In this paper we document the presently known distribution of Black-crowned Night Herons in Alberta and discuss some population attributes of these birds in what is apparently a recently occupied part of their range. Distribution The locations at which Black-crowned Night Herons have been recorded in Alberta are shown in Figure 1. The northernmost record was of an immature bird (yearling), sighted by one of us (
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