[Tweeters] Seabirds down? -- a decline in Glaucous-winged Gulls

Wayne Weber contopus at telus.net
Wed Mar 18 15:01:09 PDT 2015



Tweeters,


Following up on Dennis Paulson's question about whether seabirds are
declining in the Puget Sound/Salish Sea area, I am attaching the following
article, which was copied from a free e-mail newsletter distributed by The
Wildlife Society (the professional society of wildlife biologists). It is
based on an article published in the February 2015 issue of "The Condor" by
Louise Blight et al. Among other things, the article reports that there has
been about a 50% decline in breeding populations of Glaucous-winged Gulls in
southwestern BC since the 1980s.


This is indeed a significant decline in one of the most numerous seabirds in
the area. However-- because the Glaucous-winged Gull is still one of the
most abundant birds in the Fraser Delta/Boundary Bay area, with a wintering
population in the tens of thousands-- this decline is not at all obvious to
birders, even those who keep track of numbers like I do. It helps to
underline what I said, that the only way to really ascertain whether
populations are increasing or decreasing, and by how much, is to make
systematic counts, rather than relying on general impressions or on our
memories, which may be unreliable.


Wayne C. Weber


Delta, BC


contopus at telus.net





Assessing the Big Picture on Declining Gull Populations


By Joshua Rapp Learn


Gull populations off Canada's southwest coast have dropped by half since the
1980s, likely due to a decline in quality food, according to a new study
<http://www.aoucospubs.org/doi/full/10.1650/CONDOR-14-113.1> .

But at the risk of crying wolf, researchers at the University of British
Columbia looked at long-term data compiled about the Glaucous-winged Gulls
(Larus glaucescens) over the past 150 years to make sure the 30-year trend
isn't a passing phase.

"The larger implications in part are to point out that using long-term
information on population trends, diet and demography gives us a much richer
picture of the factors potentially driving bird population trends, said
Peter Arcese, the Forest Renewal British Columbia chair of conservation
biology at UBC and coauthor of the study.

"The [gull] population increased into the 1980s, but that's really only when
people started monitoring birds systematically there."

Arcese said the team focused their study on Mandarte Island - a small rocky
outcropping in the Salish Sea near Vancouver Island. On that island, the
population of birds had increased from 200 breeding pairs around the
beginning of the 20th century to about 2,500 breeding pairs in the 1980s.
But he said this spike could be representative of the effects of the
Migratory Bird Treaty Act of 1918. Prior to that, humans harvested gull eggs
or hunted the birds and historical factors like the gold rushes of the 1800s
could have affected populations.

The study found that even though gull populations started to recover
throughout most of the 20th century, egg clutch sizes tended to be small,
and diet analysis showed a decline in food quality.

"Eventually the decline in diet reduced clutch size and began to impair the
survival of young to the point that that increase could no longer be
sustained," Arcese said.

He also added that other factors like the recovery of bald eagles
(Haliaeetus leucocephalus) that prey on gulls could have had an effect on
the latter's numbers. And the lack of food for gulls may only make it easier
for the eagles to hunt them. "When food is short, predators are often more
effective."

A Gauge for Ecosystem Change

While Arcese said that many people often think about gulls as "garbage
birds," the birds didn't always feed on human trash and leftovers. Much of
their diet was based on so-called forage fish.

"This bird 150 years ago was a bird that fed almost exclusively on herring
and other marine organisms," Arcese said. But these fish stocks have
declined drastically over the past 150 years due to human development and
fishing.

"We think there needs to be focused attention on the recovery of forage fish
stocks that sustain a whole host of marine organism and marine birds," he
said.

And it isn't just gulls. The birds were chosen because their wide-ranging
diet means their population dynamics are a good gauge for health of the
ecosystem in general. "Because they're generalists, and omnivores, they also
tell us something about the foods available over the past 150 years," Arcese
said. "We think the gulls are riding on top of that long-term change."

Arcese said the gull study is part of a larger work on the history of bird
populations on the Salish Sea that sits roughly between Vancouver Island and
the mainland of B.C. and Washington state. They have done other studies on
Mandarte island looking at marbled murrelets (Brachyramphus marmoratus) and
western grebes (Aechmophorus occidentalis), which have declined by 90
percent since the 1950s and 1970s, respectively. But while the historical
information on the numbers of these birds is often fragmented, they have
much better information on gulls. This is necessary, he stresses, because in
order to create working conservation and management plans, it's necessary
for governments and conservationists to know what the situation was like in
the past.

"It's that long term perspective that we feel is essential if someone is
going to be able to create a reliable and defensible long term conservation
plan," he said. "When we talk about restoring ecosystems, we need to know
where we come from. You can't know what's going on by looking at the past 20
years."





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