. La Canadian field-naturaliste. 1971 WOLFORD ET BOAG : Bihoreau à couronne noire 15. T 1 â " ! 1 20 30 AVRIL 1020 MAI 30 Figure 4. La distribution de fréquence pour les oeufs récemment pondus de Bihoreaux gris en Alberta.* ont été bagués. Seule la petite colonie près de Rainier est demeuré relativement constant au cours des trois dernières années. Apparemment, le héron est arrivé à ces colonies à la mi-avril de chaque année et a commencé à jeter par la dernière semaine d'avril. Les nids ont été placés dans l'article les restes de l'ancienne année de croissance de la végétation émergente. Il est intéressant de noter que les tiges sèches de gumweed (G

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. The Canadian field-naturalist. 1971 WOLFORD AND BOAG: BLACK-CROWNED NIGHT HeRONS 15. T 1 â "! 1 20 30 APRIL 10 20 MAY 30 Figure 4. Frequency distribution for newly laid eggs of Black-crowned Night Herons in Alberta.* were banded. Only the small colony near Rainier remained relatively constant in numbers over the three years. The herons apparently arrived at these colonies in mid-April each year and began laying by the last week in April. The nests were placed in standing remnants of the previous year's growth of emergent vegetation. It is interesting to note that dry stems of gumweed (Grindelia squarrosa) from the surrounding prairie, and Russian thistle (Salsola kali) which had been blown into the marsh, were often used with cattail and bulrush for nest construction (Fig. 3). We found nests only in emergent vegetation; this seems to be the usual habitat for night herons in Alberta (Hampson, pers. comm.; Freeman, pers. comm.), though Ver- meer (1969) has recorded them using small trees. The nests were clumped in their distribution within the marsh, though within the clumps they tended to be uniformly spaced. The loca- tions within the marshes occupied by the colonies tended to be in stands of emergent vegetation that were farthest from shore and largely surrounded by open water. Such areas *On]y those eggs for which the laying dates were known, or were estimated from the hatching dates are included here. retained water througout the season and thus were the most secure from terrestrial predators. We investigated the extent to which the members of the colonies demonstrated syn- chrony in their reproductive cycles, and we chose the deposition of eggs as a manifestation of this phenomenon (Fig. 4). In 1964, when the numbers of breeding birds were somewhat greater than in 1965, the initiation of first clutches was eariier and the degree of syn- chrony was apparently greater. In 1964, when 199 eggs were laid in the initial clutches, 83 per cent of them were produ

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