Nesting of the California Least Tern and Western Snowy Plover at Oceano Dunes State Vehicular Recreation Area, San Luis Obispo County, California 2024 Season

1
SUMMARY
Staff of Oceano Dunes State Vehicular Recreation Area (Oceano Dunes SVRA, ODSVRA) and Point Blue
Conservation Science (Point Blue) monitored breeding California least terns (Sternula antillarum browni)
(least tern, tern) and western snowy plovers (Charadrius nivosus nivosus1) (snowy plover, plover) at
ODSVRA, San Luis Obispo County, California, in 2024. This is the 23rd consecutive year of both intensive
banding of snowy plover chicks and predator management that includes options for selective relocation or
removal of predators. Banding of least tern chicks began in 2003.
Least tern
There were an estimated 40 least tern breeding pairs in 2024, slightly fewer than the 41-42 breeding pairs
in 2023, and similar to the average of 40-43 pairs (range=23-60) in the 18-year period 2005-22. During the
2024 season a minimum of 14 banded birds with known origins were documented breeding at the park,
with 13 banded as chicks and fledged from ODSVRA, and one banded as a chick and fledged from
Vandenberg Space Force Base (VSFB).
There were 40 known nesting attempts, including three with unknown nest location. Of these, three were
abandoned. The hatching rate for known location and fate nests was 91.9% (34/37), compared to the average
of 85.0% (663/780) during the previous 19-year period 2005-23.
Sixty-four chicks hatched and of these 60 were color-banded to individual. Forty-eight chicks (45 banded,
three unbanded) are known to have fledged (seen when 21 days old or older), for a fledging rate of 75.0%
and an estimated 1.20 chicks fledged per pair. This is above the average for the previous 18-year period
2006-23 (banding chicks to individual began in 2006) of 45 juveniles produced per year, a 70.2% chick
fledging rate, and 1.07-1.14 chicks fledged per pair.
Snowy plover
There was a minimum of 281 breeding snowy plovers (152 males and 129 females) in 2024, an increase of
21.1% from the previous year. One hundred and six banded birds with known origins were documented as
breeding with 92.5% (98/106) banded as chicks and fledged from ODSVRA.
There were 284 known nesting attempts, including 35 identified only by detection of brood (unknown nest
location). Of the 249 nests from known locations, 150 (60.2%) were in the Southern Exclosure, 41 (16.5%)
in Oso Flaco, 55 (22.1%) in the Foredune closure, one (0.4%) in the open riding area, one (0.4%) in the
closed buffer area, and one (0.4%) in Eucalyptus Tree revegetation area east of 7 exclosure. Of the 243
nests with known location and fate, 175 hatched for a nest hatching rate of 72.0%. This is similar to the
average of 72.9% for the previous 22-year period 2002-23. Sixty-eight nests failed, attributed to the
following causes: abandoned pre-term (14); abandoned post-term (2), abandoned unknown pre- or postterm
(1); abandoned, partial loss of clutch (5); vandalism (1); overwashed by tide (5); cause unknown (2);
eggs removed by staff (3); unidentified predator (5); unidentified avian predator (2); raven (16); event,
raven (7); corvid (2); opossum (2) and coyote (1).
Of the 529 hatching chicks, 334 were color-banded to brood with 44.6% (149/334) fledging, and the fate
of the 195 unbanded chicks is believed known with 49.7% (97/195) fledging. A total of 246 chicks fledged
(seen when 28 days old or older) for a fledging rate of 46.5% and is above the average fledge rate of 39.1%
(range=7.4-66.0%) for the 22-year period 2002-23. One chick fledged per breeding male is the estimated
number needed to prevent the population of snowy plovers from declining and productivity of 1.2 chicks
fledged per male should provide for moderate population growth (assuming approximately 75% annual
1 Scientific name of western snowy plover was changed to Anarhynchus nivosus nivosus in 2024 by the American
Ornithological Society.
2
adult survival and 50% juvenile survival) (U.S. Fish and Wildlife Service 2007). In 2024, an estimated 1.62
chicks fledged per breeding male at ODSVRA. For the previous 22-year period, average productivity was
1.45 chicks fledged per breeding male.