2002-03-13 CC Packet - Laserfiche WebLink

Home Browse Search

l-; a mixture of red alder and broadleaf maple (Acer macrophyllum), and a 169 nest colony used a mix of Sitka spruce, western hemlock (Tsuga hetero- phylla), Douglas Fir and lodgepole pine (Pinus contorta). However, most. of the birds in those colonies used broadleaf maple and Sitka spruce respect- ively almost to the exclusion of the other tree species involved. The fidelity of nesting herons for a particular tree species is well illustrated by the University of British Columbia colony. At 5 sites used by part, or all, of the colony only red alder has been chosen for nesting even though, during a move of the entire colony which produced 118 nests in 1979, the trees chosen were inferior in size and location to many other possible choices. There is a suggestion (E. Taylor, pers. cornm_) that in the past some components of the large Point Roberts colony may have used broadleaf maple and Douglas Fir as .well as red alder.. If a move by a he- ron colony from I tree speCies to another occurs, and it must, we have yet to see it. It would be premature to suggest subspecific designations for local populations of Ardea herodias based on the tree species in which they nestl Time does not permit a dissertation on our deductions and observations regarding the mobility of heron colonies. However, we have a growing body of evidence that they move, join with others, break up into smaller units and are, in general, far more dynamic than we had suspected. Some of the observed or historic movements are explicable. In other cases no reasons are apparent. Interchange between colonies may sometimes account for ob- served variability in numbers of nests from year to year. Renroduction. In 1977, we attempted to follow reproductive success from the egg to fledging by repeated visits, and SOme tree climbing, in our study colonies. It was soon concluded that it was impractical, and per- haps damaging, to visit, climb nest trees and disturb the birds on a regu- lar basis. Subsequently we followed reproduction by observing all nests in some colonies, and randomly chosen sample nests in others, once the young are hatched and large enough to be visible while being fed. The nesting trees are very large. Nest height, particularly in cottonwood and Douglas fir but sometimes even in alder may be 30 rn or more above ground. We found it prudent to hire professional tree climbers for efficiency as well as for our safety. Fledging success'varied from a low of 1.9 to a high of 3.1 per nest per colony per year during our 3 ye~rs of observation. However,mean fled- ging SUCcess for all colonies in each year was much less variable. ranging from 2.5 young per nest in 1977 and 1978 to 2.9 young per nest in 1979_ Among 5 colonies from which we have 3 years reliable data, mean fledging success ranged from 2.3 in the smallest colony under observation (7 nests only), to a hign of 2.9 (Table 1). Our figures appear compatible with those determined elsewhere in North America where great blue herons are re- producing satisfactorily. Our study colonies likely represent almost the entire breeding popula- tion of the southwestern British Columbia mainland. The nests prQduce up- ward of 1,930 young birds per year, judging from our nest count and fleq- ging succes's (Table 1). At the moment we have no idea of post-fledging survival but it has been suggested in both North America and Europe that 71% of young birds do not survive the first year of life (Owen 1960, Henny 1972) . Predation. .In discussing interaction between herons and avian predators, most observers (e.g. Bayer 1979) mention harassment rather than mortality. We have gathered many incidental observations of avian harassment and pre- dation on herons. Some are thought provoking, and some are inconsistent. A colony of about 36 heron nests at Crescent Beach deserted their re- gular nesting site for 2 years in 1972-1973, apparently because a pair of -70- Table t. Colony Coquitla' Crescent E dgewa te I' Haney Mcivor Pol nt Ro: I Sa lwein Stanley II Universi Pender H; Seche 1 t Powell R great horned owl~, was reoccupied wtl trast, a pair of ny on the lower F herons. In 1979, a I the centre of thE complete abandonn successfully nest Harbour colony h~ was not occupied heronry as a freE were seen to takE dults was suspect turbance by the ~ much longer perie ted predation by They would knock trails. Ravens c roads in a colon) they would also t to take young her In 1977 a 46-nest of nearby nestin~ years. Crows apt that the displaCE