Eagle attacks on Beacon Hill Park’s Great Blue Heron colony are dramatic wildlife spectacles. As the predator swoops low over the nests to grab a chick, thunderous cries of Heron parents can be heard. A mass flight of up to 100 screaming, circling Herons darkens the sky. If an Eagle succeeds in grabbing a chick in its talons, it lands in a nearby Park tree to dismember and consume it. Heron feathers falling like snow lead observers to the Eagle’s perch.
The Bald Eagle (Haliaeetus leucocephalus) is a huge, powerful bird. Eagles weigh an astonishing 9.5 pounds (4.3 kg.) and their wingspan is six and a half feet (2 metres). Adults have distinctive white heads and tails; their yellow legs are thick; their talons razor sharp. Park visitors frequently witness Eagles catching smaller prey such as Mallards, gulls and heron chicks, but they are capable of capturing and carrying away prey equal to half their own weight, about five pounds.
Though impressive to Park observers, Heron cries and mass flights are completely ignored by Eagles. Herons have no effective way to defend their nests; they must take off to protect themselves. Researchers report Eagles killing and eating adult Great Blue Herons (Ardea herodias). Herons--four feet tall, including long legs and necks, with a six-foot (182.8 cm.) wingspan and weighing 5.3 pounds (2.4 kg.)--must be a difficult prey target. It is likely Heron juveniles are attacked more frequently than more experienced adults.
Eagles did not nest in the Park in three of the last four years--2001, 2003 and 2004--but the mated pair returned to their territory every year to feast on chicks in the Heron colony nests. Though cause and effect are difficult to prove, it seems likely Eagles kill more Heron chicks in the years they nest a few feet from the colony.
In 2002, the Eagles reared two young in the nest site by the old Aviary near Goodacre Lake. There were 90 active Heron nests that year, but only 30 chicks successfully fledged. The colony success rate was greater in the three years when the Eagles did not nest in the Park. In 2001, there were 65 active Heron nests and 65 chicks fledged; in 2003, 99 active nests counted and 75 chicks fledged; in 2004, 96 nests and 60 chicks fledged.
The effect of Eagle predation on the productivity of Heron colonies has been the focus of research by wildlife biologist Ross Vennesland. When asked what he predicted for the Park colony in 2001, he said: “We’re quite worried about the future. The Beacon Hill colony is, generally, getting attacked pretty heavily by Eagles, so nest productivity is pretty low. The number of chicks is low because of eagle attacks and that pretty much goes for all Vancouver Island colonies.” Vennesland reported the abandonment of other Heron colonies in his study area when Eagles nesting nearby constantly hunted at the colonies.
Vennesland gathered data at thirty-five Heron colonies--including Beacon Hill Park--over the two year period 1998-1999. In 2004, Vennesland and Robert Butler published an article comparing that data with data gathered by Butler ten years earlier. They concluded breeding failures were more frequent in 1998 and 1999 because of the combined effects of human disturbance from land development and an increased frequency of Eagle predation.
Vennesland said the increase in Eagle predation on Heron nesting colonies might be caused by the ten-fold increase in Eagle populations in B. C. since the 1960's. Eagle populations were previously at an artificially low number because of human poisons. Since pesticides like DDT were banned and hunters were required to use steel shot instead of toxic lead shot, Eagle populations could be back up to normal levels. It is also possible Eagles are targeting Herons more often because of decreased salmon and other fish stocks. Biologist Ann Eissinger believes the collapse of in-shore fish stocks is affecting Eagle behaviour, especially that of juveniles.
Park visitors tend to identify with the prey more than with the predator, but Vennesland pointed out both birds have existed since the ice age. “We should not put Herons higher than Eagles in value just because they are the ones getting eaten.”
Eagle attacks might cause panicked Heron chicks to fall out of their nests. In 2001, Park gardener Al Cunningham reported picking up twelve dead Heron chicks one morning under the nest trees. He thought the body count was the result of chicks trying to escape Eagles. He said they “plain run off into the air and drop down 80 feet and bounce off the ground. It’s a natural death but it’s still pretty brutal.”
There are many other reasons chicks fall. Flimsy nests in constantly moving tall treetops are dangerous places to live; nests become increasingly crowded as chicks grow. Chicks perch unsteadily on wobbling branches outside the nests, as in this June, 2004 photo by Brian Hepburn. Gusts of wind can catch flapping young birds by surprise. They rush to meet returning parents, jostling siblings in the process. Though an accurate count is not possible because Park visitors remove carcasses, there appears to be more dead chicks after high winds. Park gardener Margaret Marsden reported collecting a total of 30 dead Heron chicks from under nest trees in 2002.
Visitors to the Park anticipate seeing Herons gather to begin a new nesting season each February, but it is never certain they will return. Neither can we predict the Eagles will nest again; it is possible the Park Eagles are getting too old.