2002-03-13 CC Packet - Laserfiche WebLink

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\y.z z.....""~ a 169 (,T8Ug~~ero- l "," ,Howe~r. most of tkaspruce respect_ ~nvolved. ~e species is well (. At 5 sites used :hosen for nesting :oduced 118 nests ::ation to many other :5. camm.) that in )ny may have used (f a move by a he- : must, we have yet :: designations for }ecies in which they )ns and observations lave a growing body .nto smaller units :ted. Some of the . cases no reasons !s aCCount for ob_ :1 ve success from .imbing, in our .ctlcal, and per- le birds on a ~egu- :erving all nests .hers, once the leing fed. The in cottonwood and tOre above ground. or efficiency as of 3.1 per nest However, mean fled- variable, ranging r nest in 1979. a, mean fledging ervation (7 nests ompatible with blue herons are re- re breeding popula- nests produce up- t count and fled- f post-fledging and Europe that (Owen 1960, Henny avian predators, er than mortality. arassment and pre- are inconsistent. jeserted their re- because a pair of Table I. Great blue heron colonies studied between 1977 and 1979 showing the mean number of successful nests (nests from which young birds fledged) and the mean number of fledg- ings/nest. Where the period of observation is less than 3 years it is shown in brackets. Colony Mean No. of Mean No. Successful Nests Fl edged/Nes t Coquitlam 121 2.3 (2) Crescent Beach 42 2.9 . Edgewater 28 2.7 (2) Haney 10 Mcivor 7 2.3 Poi nt Roberts 231 2.7 Salweln 102 2.6 (1) Stanley Park 33 2.4 University of B. C. 101 2.9 Pe nde r Ha rbou r 29 (2) 2.6 (2) Seche It 32 (2) 2.7 (2) Powell River 6 (I) Total 742 Mean 2.6 great horned owls (Bubo virginianus) occupied 1 of the nests. The colony was reoccupied when the owls left and has been occupied since. By con- trast, a pair of great horned owls took up residence in a 102 nest colo- ny on the lower Fraser in 1979 with no apparent effect on nesting by the herons. In 1979, a pair of bald eagles (Haliaeetu8 Zeucocephalus) nested in the centre of the Coquitlam colony (Table 1) and apparently caused its complete abandonment even though some eggs were laid. Some of the herons successfully nested a few kilometers away. Again by contrast the Pender Harbour colony has an eagle nest within 15 m of its periphery. The nest was not occupied in 1979, but in 1978 the eagles appeared to be using the heronry as a free lunch counter. Several times during that summer eagles were seen to take young birds from nests, and occasional predation on a- dults was suspected. The heronry remained occupied, but occasional dis- turbance by the eagles caused the adult herons to leave the colony for much longer periods then would usually be the case. That in turn permit- ted predation by ravens (Corvus carax). Ravens took eggs and young birds. They would knock nestlings to the ground where they would eat the en- trails. Ravens and crows (Corvus brachyrhynchos) could make serious in- roads in a colony when large numbers of adults were frightened away, but they would also try individual unprotected nests. Bald eagles were seen to take young herons from nests at the Crescent colony on 2 occasions. In 1977 a 46-nest colony was abandoned after egg laying, perhaps because of .nearby nesting eagles, and the herons did not return in subsequent years. Crows apparently destroyed the abandoned eggs. It is believed ~hat the displaced herons joined a larger colony about 6 km away. In -71-