Red‑throated
Loon Gavia stellata
The
Red-throated is the smallest and scarcest of the three loons known from
San
Diego County, an
uncommon winter visitor both on the ocean close to shore and in San
Diego Bay.
There are only a few records inland, suggesting the Red-throated Loon
uses San
Diego County far
less as an overland migration route than does the Common Loon.
Winter: The
Red-throated Loon occurs rather uniformly in low density all along San
Diego County’s
coastline. There may be some concentration off Torrey Pines State
Reserve (N7), site of several of our higher counts 1997–2002, including
the highest, of 31 on 23 December 2001 (S.
Walens). On San Diego Bay, the
Red-throated, like the other loons, is more numerous in the north bay.
During the atlas period our maximum count there was 15 near North Island
(S8) 18 December 1999, while
weekly surveys through 1993 returned a January–March average of 10 and
a maximum of 18 on 2 February (Mock et al. 1994). We found only
a single individual in Mission Bay, at
the southeast corner (R8) 8
February 2001 (P. Unitt).
The one found inland during the atlas period, at Sweetwater Reservoir
(S12) 15 December 1998 (P. Famolaro), may have been a late fall migrant,
though a bird evidently wintering remained at Santee Lakes (P12) 27 January–19
February 1978 (AB 32:393, 1978).
Christmas bird counts suggest considerable variation from year to year
in the number of Red-throated Loons in San
Diego County.
Since 1970 the San Diego count
has varied from a low of two in 1974 and 1993 to a high of 117 in 1983;
that year yielded high numbers of Red-throated Loons on all three of the
county’s coastal Christmas bird counts.
Migration: The
Red-throated Loon may arrive in October (one at Point Loma, S7, 25 October 1976, J. L. Dunn; two on
central or south San Diego Bay in
October 1994; Manning 1995). But the species’ earliest date on the
weekly surveys of north San Diego Bay (Mock
et al. 1994) was 7 December. Spring departure is largely or entirely
completed in April. During the atlas period our latest date was
2 April (2000, two at Torrey Pines State Reserve, D. K. Adams).
The various systematic surveys of San
Diego Bay recorded
the species up to 14 April, except for one on the north bay 18
May and 18 June 1993 (Mock
et al. 1994), one of the few records of a summer straggler.
Inland, the Red-throated Loon has been found in fall twice at Lake Henshaw
(G17), one on 12 November 1978 (AB 33:213, 1979), two from 12 to 17 November
1985 (AB 40:157, 1986), and once at Borrego Air Ranch (H26), one picked
up 21 November 1996 and released on a pond in Borrego Springs the next
day (R. Thériault, NASFN 51:119, 1997). Inland spring records number
six, from 8 March (1964; one at Lower Otay Lake, U13/U14, G. McCaskie)
to 26 April (1983, one on Lake Henshaw,
R. Higson, AB 37:912, 1983). One of the birds at Lake Henshaw in
spring 1983 stayed into summer, noted on 8 August (R. Higson, AB 37:1026,
1983).
Conservation: Groves
et al. (1996) reported that the number of Red-throated Loons breeding
in western Alaska fell
by 53% from 1977 to 1993, but this change is not paralleled in the number
wintering in San Diego
County,
as suggested by Christmas bird counts.
Pacific
Loon Gavia pacifica
The
Pacific is the most abundant but also the most oceanic of San
Diego County’s
three loons. As a winter visitor it is often common on the ocean
1 to 10 miles offshore but uncommon to rare in the bays and lagoons.
Because of the concave shape of the southern California coast, most of
the hundreds of thousands of Pacific Loons that migrate past California
take a short cut closer to the Channel Islands and so miss San Diego County.
But strong northwest winds sometimes drive fall migrants close to shore
even here.
Winter: The
bight between Point Loma and Imperial Beach is the Pacific Loon’s prime
habitat in San Diego County, used by as many as 679 on 18 December 1976
and 421 on 20 December 1997 (D. W. Povey). The numbers in this area
vary greatly, however: in spite of Povey’s consistent coverage of it on
San Diego Christmas bird counts 1975–2002, the figures for the Pacific
Loon on the count have been as low as 8 in 1975 and 18 in 1994, averaging
141. Along the coast of northern San Diego County Pacific Loons
are usually fewer. On 22 counts 1980–2003 the Rancho Santa Fe circle
has averaged 65, with a maximum of 583. On 28 counts 1976–2002 the
Oceanside circle
has averaged 38, with a maximum of 214. Presumably the birds shift up
and down the coast with schools of fish. Variation in Pacific Loon
numbers on San Diego County’s three coastal Christmas bird counts is only
slightly if at all coordinated. For example, 1995 and 1997 generated
the highest totals since 1976 on the San
Diego count but totals well below average on
the Oceanside count.
Inside San Diego Bay the
Pacific Loon is uncommon in the north
bay and becomes scarcer farther
south. In weekly surveys of the north bay through 1993, just once
did Mock et al. (1994) count more than 10 (20 on 5 January). Weekly
surveys of the central bay in 1994 yielded no more than six (Preston and
Mock 1995). Covering all of San
Diego Bay south
of the bridge weekly from April 1993 to April 1994, Manning (1995) found
no more than five per day. In Mission Bay the
Pacific Loon is inconsistent; from 1997 to 2002 our maximum number there
was four on 20 January 1998 (B.
C. Moore). The only north county lagoon inside which we found the
Pacific Loon was Batiquitos (J6/J7), undoubtedly as a result of the deepening
of the lagoon carried out 1994–96. Four sightings at this site included
a maximum of four individuals 27
December 1997 (F. Hall).
Migration: Most
of the Pacific Loon’s population migrates along the California coast.
In central California the
birds hug the shoreline, but between Point Conception and the Mexican
border they take a more direct route over the ocean, out of sight from
the mainland (Russell and Lehman 1994). As a result the huge numbers
seen at places like Point Piedras Blancas and Pigeon Point are rarely
seen at San
Diego. Russell and Lehman (1994) found
that northbound migrants halted when headwinds
were too strong, but southbound migrants may take advantage of northwesterly
tailwinds (rare in fall), even if they have to fight their way around
bends in the coast. On 6
December 1998, during strong northwest wind, the loons were streaming
past La Jolla (P7) in flocks of up to 100, and thousands passed by over
the course of the day (G. McCaskie).
Fall arrival has been recorded as early as 8 October (1973, two at Point
Loma, S7, J. L. Dunn), but the weekly surveys of San
Diego Bay did
not detect the species until 9 November in 1993. Spring migration
lasts from late March to early June, as attested by 25 off San Diego 3
June 1972 (G. McCaskie), three at Torrey Pines State Reserve (N7) 3 June
2001 (P. A. Ginsburg), and one on north San Diego Bay 6 June 1995 (Preston
and Mock 1995). Stragglers remaining to summer are rare; the only
one during the atlas period was at the San Diego River mouth (R7) 20 July–11
August 2000 (C. G. Edwards).
Pacific Loons wintering in the Gulf of California evidently
make their overland crossing south of the international border, in Baja
California (Huey 1927). As a result,
the species is only casual inland in San
Diego County.
In spring there are only two records, listed by Unitt (1984). In
fall there are three records from Lake Henshaw, each
of two birds: 19
October 1983, 12–15 November 1985 (R. Higson, AB 38:245, 1984; 40:157,
1986), and 7–8 December 1996 (C. G. Edwards, NASFN 51:801, 1996).
Conservation: In
San
Diego County any
trend in the Pacific Loon’s numbers is obscured by the great variability
from year to year. Daily counts of migrants in central California
through the entire spring yielded about 1,000,000 individuals in 1979
but only about 450,000 in 1996; the difference could be due to a population
decline, a northward shift of the winter range, or both (Russell 2002).
Because of the Pacific Loon’s funneling in migration through narrow bottlenecks,
it is especially vulnerable to oil spills at those points.
Common
Loon Gavia immer
The
Common Loon is at best a fairly common winter visitor to San
Diego County, widespread
along the coast both on the ocean near shore and in tidal bays and estuaries.
But the number of individuals wintering rarely if ever exceeds 150.
The Common is the only loon likely to be seen inland, a few wintering,
exceptionally summering, on the larger lakes. Migrants presumably
headed to and from the Gulf of California cross
San Diego County regularly
but seldom stop unless compelled by storms.
Winter: The
Common Loon occurs all along San
Diego County’s
coast. On the ocean, the birder walking the beach or scanning from
a lookout seldom sees as many as 10 in a day; 20 at Oceanside (H5) 27 December 1997 (D.
Rorick) is a maximum count. The birds may prefer the calm water
within the bays. Our numbers in Mission Bay ranged
up to eight in the northeast quadrant (Q8) 7 December 1998 (J. C. Worley).
Various studies of San
Diego Bay found
that the Common Loon, unlike many other water birds, is more numerous
in the deeper water of the north
bay and becomes less numerous
farther south. In weekly surveys of the north bay (bridge to mouth)
through 1993 Mock et al. (1994) found an average of 10.4 from December
through March and a maximum of 24 on 26 January. In monthly surveys
of the central bay through 1993 and weekly surveys there through 1994
Preston and
Mock (1995) found an average through the same months of 8.5 and a maximum
of 18 on 22 November 1994.
In weekly surveys of central and south bay April 1993–April 1994 Manning
(1995) found no more than 10.
The Common Loon also winters occasionally on inland lakes, at a rate of
less than one per year. Our only such records from 1997 to 2002
were of one at Lake Murray (Q11) 7
January 1998 (N. Osborn), one found dead at Sweetwater Reservoir (S12)
19 December 1998 (P.
Famolaro), and one at Lake Morena (T21) 27 December 1998 (R.
and S. L. Breisch). Over 18 Christmas bird counts from 1986 to 2002,
the Escondido count
recorded single Common Loons twice; over 22 counts from 1981 to 2002,
the Lake Henshaw count
recorded only one, on 31
December 1990.
Migration: A
Common Loon heading south has been seen in San Diego County as early as
26 September (1976, one over Point Loma, S7, G. McCaskie), but the regular
surveys of San Diego Bay did not detect arrival before 13 October and
found that numbers were still building into November. Spring departure
is largely in late March and early April, but some remain until May and
occasional birds remain through the summer. From 1997 to 2001 only
one was found summering along the coast, at Los Peñasquitos Lagoon (N7)
2 June 2001 (K. Estey).
The Common Loon migrates inland over San Diego County, primarily using the same
route along Highway S2 and over Lake Henshaw (G17) as the Brant and Surf
Scoter. It is seen less frequently inland in fall than in spring,
but there are several fall records extending from 1 October 1983 (14 at
Lake Henshaw, R. Higson, AB 38:245, 1984) to 24 November (one stranded
alive in the north fork of Fish Creek Wash, L28/L29, ABDSP database).
The only large flock reported in fall was of 211 on Lake Henshaw 12 November
1985 (R. Higson, AB 40:157, 1986). In spring, migrants have been
found at Lake Henshaw from 21 March (1983, R.
Higson, AB 37:912, 1983) to at least 12 May (2000, G. Grantham), but occasional
individuals coming down inland remain through the summer. During
the atlas period we found four on Lake Henshaw 17 July 1998 (C. G. Edwards)
and up to three on Lake Cuyamaca (M20/M21) 28 May–3 September 1998 (A.
P. and T. E. Keenan, P. D. Jorgensen)—note that these followed El Niño
with its repeated spring storms that can interrupt the loons’ overland
journey. Another year of El Niño, 1983, yielded the largest flock
of Common Loons ever recorded in San Diego County, 317 at Lake Henshaw 18
April (R. Higson, AB 37:912, 1983).
Conservation:
Because of their habit of diving and diet of fish, the Common Loon is susceptible
to oil spills, other types of water pollution, and contamination of its
food. But Christmas bird counts suggest no long-term trend in the
number wintering in San Diego County through the late 20th century.
Stephens’ (1919a) and Sams and Stott’s (1959) assessment of the species
as “common” suggest it may have been more numerous in the past.
Overland migration is a hazard for a bird that can take flight only from
water and is doomed if it crashes on land. On 11 April 1989 an estimated 200 died
when they landed along Highway S2 (R. Thériault). Casualties of
that fallout are preserved from 3 miles east of Borrego Springs (F25),
1 mile southeast of Scissors Crossing (J23), and near Canebrake (N27;
SDNHM 45700–2).