Going to the Birds – A Mapping Project in Review
GROWLS, the Islands Trust, and the Wildlife Tree Stewardship Program (WiTS) has asked Islanders to help catalogue favoured Bald Eagle perch trees on Gabriola. Although WiTS has been documenting eagle nest locations along southeast Vancouver Island for years, and started to map significant perch trees, this survey represents the first attempt to focus strictly on these sites.
We were delighted by the public’s response! Despite the relatively tight timing, many folks took a few moments to tally up their trees for the 3-day inventory process – a testament to the high value we place on raptors. In fact, time ran out before we could get to the 50+ trees reported; these sites we hope to visit in the near future. The following stats summarize the findings to date.
In total 49 sites were visited, including 46 individual trees, 2 small islands, and a stand of trees. Almost all of the trees were Douglas-fir with a diameter at breast height (or dbh) that ranged from 0.44 m for the younger trees right up to a few massive veterans that measured 1.5 m across. The latter are the classic old-growths that tower over the surrounding tree canopy. Their thick, heavily furrowed bark, often blackened from a previous fire, and open branch structure sets them apart from the younger trees. Vets such as these are also the preferred nest trees along the coast. Of the 15 nests in Douglas-firs on Gabriola where the trunk could be measured, the mean dbh was 1.3 m!
Not surprisingly, the majority of the perch reports (92%) were along the coast – the location, location, location quip certainly held true. Given that ocean habitat provides the bulk of the average eagle’s diet (fish and waterbirds), shore side perches contribute greatly to a nesting pair’s ability to keep food on the table. For instance, consider the findings of a Canadian Wildlife Service study near Crofton: for nests with 2 chicks, just over 7 prey deliveries were made each day to the nest and some pairs made 10+ trips.
Just like people, eagles also have their routines, and a few of the trees identified had very specific uses. It seems that some eagles have particular trees just for mating while other trees are the preferred location to bring the kids after they leave the nest.
Of some surprise was the number of heavily used trees that were more than 1 km from a known nest location. This doesn’t necessarily mean that these trees aren’t part of a breeding pair’s home range as eagles certainly do forage outside of the core nest area, especially when the nest isn’t active. Some of this use, however, is likely attributed to floaters, defined as resident but non-breeding adults. Although population dynamics are complex, biologists cite high numbers of floaters as an important component to eagle population stability. And then there’s the migratory eagles - young birds and adults that over winter here but breed elsewhere. One tree reported has held up to 11 eagles at a time!
Waterfront is at a premium for eagles and people. Seems we all prefer the view! The availability of high quality perches not only helps the local nesters, but also may increase the fitness of distant breeders that depend on the coastal food supply in winter while their nesting sites are in the deep freeze. Hopefully, this mapping project will help us make the most of what we have here on the coast. A huge thanks to all of the volunteers and landowners that made this project a success.
By Terri Martin
If you notice a perching tree in your area please notify the Gabriola eagle monitors.
Dar Mace, email@example.com or Iain Lawrence, firstname.lastname@example.org