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Least Tern Project info
2011
Season
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2010
Least Tern & Snowy Plover Project
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| July
19 - Got
Fish? Least Tern Mortality Information |
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Got
Fish?
For anyone like me who is nutty enough to wonder about the
foraging strategy of least terns, their successes and some
consequences, please read the following chatter from other
monitors about terns in the south coast area.
When are we done? We usually monitor the colony until we see
no more young of the year. Last year Ross Griswold observed
the last fledgling on the 29th of July. It looks like we will
end in the next couple weeks or sooner. Our monitors report
a few remaining nesting attempts, and we want to see if they
are successful. The end of the season will become obvious
and we will announce when to finish up. Thanks again for all
of your time and observations. I hope you’ve enjoyed
your natural resources at the beach.
—David
Pryor |
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Least
Tern Mortality in San Diego County |
Responses
from Dan Robinette, Robert Patton, Shauna Wolf,
Brian Collins, Brian Bonesteel, Tom Ryan, Pat Baird, |
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| from
Dan Robinette: |
| This
was an El Nino year, though the impacts seem to be varied
depending on species and location. The ENSO index was not
as positive as during major events like 82/83 and 97/98. Sea
Lions are definitely showing signs of food issues. And Least
Terns at Venice and Port of LA definitely had issues this
year as well. It sounds like San Diego terns were able to
find enough of a source to initiate nests, but not enough
to keep the older chicks fed. It's quite possible that food
conditions deteriorated as the season progressed. We've documented
within-season changes in diet that have led to changes in
breeding metrics at Vandenberg. We really need to start collecting
and analyzing diet samples at the major colonies throughout
California so we can stop guessing about this stuff. I have
info for some colonies in some years and a 10-year time-series
for Vandenberg, but we really need to develop good time-series
for multiple colonies. |
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| from
Robert Patton: |
The
highest numbers of dead chicks & fledglings that I've
heard reference to have been at Batiquitos Lagoon & the
Naval Base Coronado sites, which are also the largest colonies
in the area. I've heard little to nothing of mortality at
Camp Pendleton, the largest colony in the population, other
than scattered reports of dead fledglings that had been banded
as chicks elsewhere. There also seem to be relatively high
numbers of dead fledglings (including banded from elsewhere)
at Mariners Point in Mission Bay.
At Tijuana Estuary & sites on San Diego Bay:
Nest/brood attendance has appeared relatively depressed for
much of the season, but most notably early in the season &
then significantly so over the past two weeks. There have
also been relatively high nest abandonment rates, particularly
in the past two weeks. As Sandy suggested, assigning cause
simply to lack of access/availability of prey fish has been
complicated by presence of peregrines as well as other predators.
However, evidence of predation of adult CLTs at most of the
sites I'm at has been relatively low. Some mobbing by multiple
chicks/fledglings of adults flying in with fish has been observed.
There appears to have been relatively normal numbers of dropped
fish, although most appear to be surf perch tending to be
on the large side. Chick weights have been generally good
with a few exceptions & for the most part I've not seen
aberrant behavior, weak or sick chicks, BUT on the other hand
I've had extremely few recaptures in recent weeks so have
been unable to track any changes (however I understand that
dropping weights were recorded elsewhere in recent weeks).
Likewise, most of the young at the sites I'm at had reached
fledging age when this mortality episode began so were no
longer being recaptured & weighed. The recent dead I've
found have been fledglings & nearly fledged chicks, almost
all already desiccated by the time they were found. The couple
of fresh carcasses & one alive but emaciated that I found
were turned over to Project Wildlife for rehab or gross necropsy
but I've not heard results. The majority of carcasses that
I've seen had protruding sternal keels & lacked any vent
staining.
've heard suggestion that prey fish availability has decreased
as local water temperatures increased then fluctuated significantly
over the past two weeks, but I've not had a chance to check
sea surface temp data.
I've not seen mortality in the other nesting species coinciding
with the timing of the increased mortality of juvenile CLTs.
However, there are typically high mortality rates among Forster's
tern & skimmer chicks in South SD Bay (FWS pushing for
additional funding for toxicology testing). We've had above
average mortality of adult elegant terns throughout the season,
BUT we've also had record high numbers of breeding elegant
terns significantly higher than previous records & with
high productivity so far (fledgling dispersal just began in
past one to two weeks with many chicks/fledglings still on
site).
To put in perspective the mortality (not including predation)
at the sites I'm monitoring:
SDIA - Lindbergh Field - 114 total nests, 38-47 fledglngs
estimated, 14 dead chicks in June (corresponded with raptor
predation), 13 dead chicks & 4 dead fledglings in 1st
2 weeks July
D Street Fill - 119 total nests, 26-30 fledglings,
9 dead chicks June, 8 dead chicks & 3 fledglings 1st 2
weeks July
Chula Vista Wildlife Reserve - 40 total nests, 2 fledglings
South SD Bay saltworks - 65 total nests, 6-7 fledglings,
1 dead adult
Tijuana Estuary - 219 total nests, 62-63 fledglings,
4 dead chicks June, 2 dead chicks & 10 fledglings &
2 adults 1st 2 weeks July (including dead fledgling from Batiquitos
& 2 from NBC)
Looking forward to hearing observations & perspectives
from other sites. |
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| from
Shauna Wolf: |
At
Batiquitos the only issue appears to be food related. The
CLT chick weights were very good until 6/26 when they suddenly
started dropping. We have few to no active CLT nests now
and the recently hatched chicks have all died. Indications
of food problems are that we had a fledgling CLT attempt
to eat a CLT chick. The chick had lost weight since the
attack and was only 4.4 grams so would have died anyway.
We have not seen the fledglings eating hair ties and fuzzy
string like have in some past years. There are still three
known chicks at Batiquitos that are medium size and all
three appear to be doing well. We have found several fledglings
from Camp Pendleton dead at BAT (all thin) and several of
the BAT have been found at other sites (SD river mouth,
NABO, Borderfield, Camp Pendleton, and Mariner's Point).
I know there has been a red tide most of the season, although
it didn't seem to be causing much of a problem. It
looked like it disappeared for a couple weeks near BAT,
but looked a little stronger again last Sunday when I did
a WSP survey at SCSB-SP. The black skimmer chicks at E3
appear to be doing well so far, but they mostly eat bigger
fish than the CLT.
We have had similar mortality rates in the past at the end
of the season at BAT, and in some years it has been much
worse. The symptoms are the same, chick weights drop, chicks
start accumulating feces stuck to their vent and then they
start dying. The biggest difference this year is that it
was a much more sudden shift. It usually takes about a week
of dropping chick weights before we see much death and this
year it was only a couple of days.
Adult predation
is not contributing to the mortality at BAT. We had some
predation at the beginning of the season, but that was before
there was much nesting. Recently we have had some fledgling
predation, but I suspect the weakness of the fledges is
making them an easier target.
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| from
Brian Collins: |
Is
anyone seeing evidence to suggest that this high mortality
rate is a result of something other than starvation?
No,
no gross evidence of wildlife disease has been observed or
reported so far.
Does anyone have any information about food sources/ocean
conditions that suggest unfavorable conditions for least terns?
We
just had a big weather shift here in the last week or so.
Oceanic conditions have likely changed as we have been seeing
large black jellies in the nearshore waters, are having many
more stingray problems with people at the beach (although
the warm weather is an obvious link with more people in the
water), so it may be likely that the food source for small
terns has shifted off shore, also a number of peregrines are
very active in the area, so adding those conditions together
may be pushing the fledges over the edge.
Are dead or rehabilitated
chicks being examined (I know at least one is)?
It's
hard to get much evidence from them. Is someone wants, I can
do a bulk shipment to our FWS lab in Madison, WI but who knows
how useful that would be. If we want to archive chicks for
forensic examination, please do not freeze them, refrigerate
them only. If we want to send them to Madison, please let
me know ASAP and I will arrange for shipment early next week
after checking with the lab to make sure they have staff who
can respond in a timely manner. With the BP disaster ongoing,
they may be swamped with other work. Our local SD County lab
no longer takes in birds for necropsy unfortunately.
Are there any results from these examinations?
Are other species exhibiting similar mortality rates?
No,
we are not really seeing anything unusual at least in the
SD Bay area. Our larger terns seem to be doing fine with fledgling
survivability looking very good for now. We are still seeing
many adults with large sardine looking fishes flying to and
fro over the Strand and beaches and the bay heading for salt
works to feed their young. To date, I have neither witnessed
or heard about any other mortality clusters that raised any
red flags.
We have had problems with Black Skimmer chick mortality that
has been exacerbated by a coyote getting into one subcolony
and tearing it up pretty badly, but relatively high chick
mortality was obvious prior to that occurring. This early
age class mortality is a regular yearly observation in the
skimmer colony at salt works. we have plans for further contaminants
analyses of skimmer eggs in future seasons, hopefully next
year if we get the funding. But this too may be food item
related, we don't know. Skimmers tend to take fishes closer
in size to least tern prey items so there may be a parallel.
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| from
Brian Bonesteel: |
| Two
key things that I have observed on the navy sites in San Diego
bay are #1 the weather has been colder than average with heavy
mists that have been falling for many of the days when dead
fledglings are found and #2 I have seen a great increase in
the stealing of fish being brought in by least terns by Gull-billed
Terns over the last several weeks. |
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| from
Tom Ryan: |
At
the Venice Colony our birds appear particularly susceptible
to poor foraging conditions in the nearby ocean, especially
when it comes to anchovies. I am going to try to get out a
quick report later today or over the weekend with some data
I just received from Nate Mudry tracking anchovy populations
using CalCofi data and it mirrors the success and failure
of the Venice Colony very well.
What we observe on the ground during these low anchovy years
is that there are 1) fewer nests being established on any
given day, 2) fewer adults roosting/loafing within the colony,
3) fewer adults in attendance at the colony, 4) attendance
declines from mid-morning to the evening. All of these combine
to allow the local predator, the American Crow to enter the
colony, nearly unchallenged and locate and consume the eggs
of all active nests each day.
Regarding 2010, Wally Ross has been in contact with the local
bait fishermen who indicated that they were not catching any
anchovies during the early part of the season (April-May).
One other observation that taken on it's own may not mean
much, but given the current situation may be a piece of the
puzzle, we did find a dead adult elegant tern at Malibu Lagoon
on March 30, 2010 (specimen at LACMNH). It appeared emaciated
with no other signs of injury. |
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| from
Pat Baird: |
This
kind of discussion on causation of chick deaths has occurred
many times before, as far back as the late '80's. It is all
well and good to say that the chicks were starving and that
this was an El Nino etc. That is good background. However,
unless someone has actually looked at foraging intensely this
summer, you cannot state that food was a problem. Chicks dying
co-occur with an El Nino--that is correct-- and they "appear"
to be starving-- you just can't say this. That is not the
scientific method, assuming something without testing.
I did a 4-year foraging study in the '90's in San Diego Bay
and one last year. If my study on foraging in San Diego Bay
in 2009 had continued this year, we would have a definitive
idea for starters, whether or not food was an issue. In both
studies, I looked at the following.
geographical areas where terns foraged
frequency of return of birds with prey to the nests
type of fish consumed by chicks
type of fish refused by chicks or dropped by parents
temperature of seawater at foraging sites
In my study in the '90's, I could draw inferences on causation
of low fledgling success, because there were differences in
prey/foraging areas in the years of low productivity. Unfortunately,
I really cannot compare data from last year completely with
data from the 1990's because there has been a regime shift
in the prey all along the Pacific. But if I had done a study
this year, at least we would have had a time-true comparison.
Too bad. I just did not get funded for this year.
As you know, you cannot look at dropped fish as an indication
of what the birds are eating or feeding to their chicks. More
often than not, these are the fish that are of the wrong shape
or size to be fed to chicks. All of my data show that. I also
have evidence from a mini-study in Mission Bay in 1997 and
1998 on dropped fish, in one year where chicks starved.
So, unless you have had someone looking specifically at a)
foraging areas b) frequency of return of birds with prey to
nests and c) type/size of prey fed to chicks, you cannot assume
that starvation/lack of food were what made the chicks die.
You also need a concurrent fish study to show what is available.
However, you also must be careful in this, for the depth to
which least terns can plunge dive is very shallow. If someone
has fish sampling data at the top half a meter for this year,
then that would suffice, for the least terns cannot plunge-dive
much deeper than that.
I've also shown, for instance in Alaska, that the preferred
prey can be available, but that the surface feeders (in this
case, arctic and Aleutian terns, glaucous-winged gullls, and
black-legged kittiwakes) could not reach them because the
prey were stratified at depth, under a low-saline lens of
water due to a record rainfall for a month and a half.
However, another thought on fish sampling is: many nets are
so fine, that the topsmelt etc. at the surface can swim out
of them, for they generate so much drag. On the other hand,
if the mesh is such that the nets move through the water quickly
enough to trap these small fish, they can often swim out of
them through the larger mesh holes. This has always been a
problem, sampling for prey for least terns.
Moreover, just because something is available in the water
column does not mean that the terns will take it or eat it.
It all depends on so many variables. You need to look at what
they are bringing back to the colony. |
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