In 1993, a minimum of 2,400 pairs of the endangered California least tern (Sterna antillarum browni) nested at 35 sites along the coast of California, and produced approximately 1848-2009 fledglings. This 14% increase over 1992 breeding population size continues the trend since 1987 of continued growth of the population, and is directly attributable to the efforts of people working on behalf of recovery of the species. The statewide total of 2,400 pairs is the highest number recorded since systematic monitoring began in 1973, and represents a four-fold increase over the estimated 600 pairs of that year.
Predation on tern eggs, chicks, fledglings, and adults, and abandonment of eggs and chicks as a function of predation pressure, were the major causes of breeding failure in 1993. Monitors at 15 of 22 sites with low fledgling production (<0.9 fledglings/pair) attributed the lack of success to predation. The adverse effects of predation were manifested at all stages of breeding. Many types of human-related disturbance also constrained fledgling production in 1993. Breeding success and failure were strikingly localized; successful (>0.9 fledglings/pair) and unsuccessful sites were distributed throughout the State. Seven sites were particularly successful at fledging high numbers of tern chicks: NAS Alameda, Venice Beach, Seal Beach, Mission Bay/Mariner's Point, and Delta Beach North combined produced approximately 82% of the total fledglings produced statewide.
In 1995, a minimum of approximately 2,585-2,611 pairs of the endangered California least tern (Sterna antillarum browni) nested at 37 sites along the coast of California. This 7% decrease in breeding population size from 1994 brings to an end the trend since 1987 of continued growth of the population, and is likely attributable, at least in part, to the poor fledgling production experienced statewide in 1992. In addition to the drop in pair numbers, heavy predation pressure at many sites, an apparent shortage of food at two large sites, and a heavy storm in mid-June across the State, combined with a variety of human-related constraints on tern reproductive success, resulted in the lowest statewide fledgling-to-pair ratio recorded since fledgling production estimates were incorporated into monitoring protocol (1978). A minimum of approximately 963-1,174 fledglings was produced, 41% fewer than in 1994, resulting in a statewide fledgling per pair ratio of 0.37-0.45.
As usual, successful and unsuccessful sites were distributed rather evenly throughout the State. Terns themselves were more unevenly distributed: 50% of the statewide population bred at only five sites (Venice Beach, Santa Margarita River/North Beach, Mission Bay/Mariner's Point and FAA Island, and Tijuana River/South); inclusion of an additional four sites (NAS Alameda, Bolsa Chica, Huntington Beach, and Delta Beach/North) accounted for 73% of all breeding pairs, and the inclusion of two more (Ormond Beach/Edison and Seal Beach) accounted for 81%. The fledglings produced at Santa Margarita River/North Beach, Mission Bay/Mariner's Point, and Delta Beach/North constituted 33% of the State total; the balance were distributed relatively evenly among sites.
In 1996, 3330-3392 pairs of the endangered California least tern (Sterna antillarum nested at 35 sites along the coast of California. This 29% increase in breeding population size over 1995 resumes the predominant pattern since 1987 of exponential growth of the statewide population. Fledgling-to-pair ratios at most sites increased relative to those in 1995, and terns nesting at several sites in 1996 were very successful. However, heavy predation at many San Diego County sites (including a majority of those with high pair numbers) and a local food shortage at Venice Beach compromised statewide fledgling production somewhat. Approximately 1981-2120 young terns fledged across the State, a record high and almost twice the number fledged in 1995; statewide F/P = 0.58-0.64.
Most sites experienced increases over 1995 in the number of breeding pairs present, and several sites attracted more than 200 pairs. In 1996, 58% of the statewide papulation bred at only seven sites (NAS Alameda, Venice Beach, Huntington Beach, Santa Margarita River/North Beach, Mission Bay/FAA Island and Mariner's Point, Delta Beach/North), with the balance being distributed pretty evenly throughout the rest of the State. Almost half of the fledglings produced in the State came from only four sites; 45% of all potential new breeders produced this year fledged from NAS Alameda, Huntington Beach, Santa Margarita River/North Beach, and Delta Beach/North.
Approximately 4,017 pairs of California least terns (Sterna antillarum browni) nested at 38 sites along the coast of California in 1997, as reported by least tern monitors. This represents a 19%increase from 1996 pair estimates, and a 55% increase from 1995 pair estimates, more than compensating for the 7% decrease between 1994 and 1995. Recruitment cannot entirely account for this increase in least tern pairs, as reported fledgling production for both 1994 and 1995 was low. Immigration from other least tern populations has been suggested, and improved survival on wintering grounds may be a factor, but supporting data for these hypotheses are lacking. Overestimates of pair numbers and/or underestimates of fledglings by monitors may also partially explain this apparent increase. Reported pair and fledgling values are always imprecise estimates that are not scientifically derived; moreover, consistent methods for obtaining them, as recommended in annual monitoring packets, are not being used at all nesting sites.
Reproductive success and adult survival in 1997 was affected by a number of predators, particularly at San Diego County sites. However, fledgling estimates (3,140 to 3,322) for 1997 were 55% to 64% higher than 1996 and 200% higher than 1995 estimates. The statewide fledglings-per-pair value (0.78 to 0.83) was also higher than for the previous three years. San Diego County supported over 57% of 1997 statewide pairs and produced over 58% of statewide fledglings at 20 nesting sites; the Santa Margarita River nesting sites alone supported over 18% of statewide pairs. More than 50%of the statewide breeding population was concentrated in six nesting sites (NAS Alameda, Venice Beach, Huntington Beach, Santa Margarita River North Beach, Mariner's Point, and Delta Beach North); these sites also produced over 64% of the State's fledglings. Santa Margarita River North Beach and NAS Alameda had the highest (over 1.2) fledglings-per-pair values. The Tijuana River nesting sites reported the lowest fledglings-per-pair value (0.01 - 0.02) in the State due to unprecedented burrowing owl predation on adults, a host of other predators in the site vicinity, and human disturbance and intrusion into the nesting site. No evidence of local prey shortages was reported by monitors for any nesting site in 1997.
An estimated 4,141 to 4,182 pairs of California least terns nested at 39 nesting sites in 1998 and produced an estimated 2,686 to 2,810 fledglings. Statewide pair estimates increased 3.9%, but fledgling estimates decreased by 14.6% from 1997 estimates, likely due to high chick mortality at many sites. Seven sites (NAS Alameda, NAWS Point Mugu, Venice Beach, Huntington Beach, Santa Margarita River North Beach, Mariner's Point, and Delta Beach North) supported a combined total of 65% of statewide pairs and produced 66% of the state's fledglings in 1998. Fledglings per pair were 0.64 to 0.68, lower than 1997 (0.80).
One of the more interesting findings of 1998 was a report of a nesting pair on evaporation pond dikes near Kettleman City in the San Joaquin Valley. Both eggs hatched and one chick apparently fledged.
It is likely that monitors continue to underestimate renesting, as reported pair estimates are only 378 lower than statewide nest numbers of 4,541, despite 64 eggs lost to flooding, 900 observed dead chicks and minimum losses to predators of 147 eggs and 165 chicks. Another method of estimating pairs was requested and attempted by some monitors in 1998, based upon the number of renesters that a given site may generate, rather than the number of renesting pairs at that site. This estimate was 3,483 pairs, or 84% of estimates derived by the traditional method. Statewide mean clutch size was 1.66 eggs per nest, lower than for the previous three years, suggesting limitations in prey availability, as reported by several monitors. However, statewide mean hatching success was 0.80, similar to the previous two years.
After a 54% increase in pairs and a 200% increase in fledglings between 1995 and 1997, pair numbers increased only 3.8% and fledgling numbers decreased by 14% from 1997. This is likely related to limitations in prey availability during 1998, as evidenced by high chick mortality, poor nest attendance, abnormal chick feeding and kleptoparasitism.
An estimated 3,451 to 3,674 pairs of California least terns nested at 36 nesting sites in 1999
and produced an estimated 671 to 711 fledglings. These estimates result in 0.18 to 0.21
fledglings per pair, the lowest productivity recorded since statewide censuses were initiated in
1976. Statewide pair estimates decreased 11% from 1998 values, but fledgling estimates
declined by 74.9% due to exceedingly high predation and chick mortality at many sites. Over
30% of the nesting population was concentrated at two sites (Mission Bay Mariner's Point and
Santa Margarita North Beach); ten sites supported a combined total of 76.6% of statewide pairs.
One site (Los Angeles Harbor) contributed nearly 24% of the state's fledglings in 1999; Los
Angeles Harbor and three other sites (NAS Alameda, Ormond Beach and Mission Bay Mariner's
Point) produced over 50% of 1999 statewide fledglings.
Four sites that supported least tern pairs in 1998 reported no nesting in 1999 (Batiquitos
Lagoon W-1 and E-2; Mission Bay Fiesta Island, Mission Bay South Shores); four additional
sites (Venice Beach, Seal Beach, Bolsa Chica, Chula Vista Wildlife Refuge) supported nesting
but had no productivity in 1999.
1999 pair estimates were 18% lower than corresponding statewide nest numbers; in 1998 they
differed by only 9%. Renesting may have occurred far less frequently in 1999 than in 1998 due
to far higher predation (16.5% of all eggs and 7.7% of chicks hatched) and other factors
contributing to chick mortality (26.5% of all hatched eggs) in 1999. The greatest egg losses in
1999 were attributed to coyotes, crows and ravens; highest chick/fledgling losses were to
American kestrels, coyotes and peregrine falcons. Chick mortality due to factors other than
predation was 26.5%, higher than 1997 and 1998 and is believed to be related to prey
deficiencies and unknown factors.
Monitoring of nesting sites in 2000 resulted in an estimate of 4521 to 4790 breeding pairs of California least terns establishing 5301 nests at 37 locations. This represents a 31 percent increase in the minimum estimated number of breeding pairs from 1999. An estimated 3710 to 4013 fledglings were produced, or 0.77 to 0.89 fledglings per pair. This represents a 453 percent increase over productivity of the 1999 season, and 38 percent over that of 1998.
Depredation was the primary limiting factor to reproductive success. Other reported causes of mortality included a heat wave in the San Francisco Bay area, nest abandonment, and human activity, including loss of chicks to vehicles.
The nesting colony at Camp Pendleton continues to be the largest in the state, accounting for 22.8 percent of breeding pairs and producing 27.6 percent of this season's fledglings. Other colonies numbering over 200 nests included Alameda Point, Point Mugu, Venice Beach, L.A. Harbor, Huntington State Beach, Mariner's Point, Naval Amphibious Base Coronado North Delta Beach and ocean beach, and Tijuana Estuary. The nesting site first documented in 1998 at Tulare Lake bed evaporation ponds outside of Kettleman City in Kings County was used again in 2000, and newly created Caltrans mitigation islands in Albany in Alameda County were colonized and produced at least one fledgling.
Monitoring to document breeding success of California least terns (Sternula antillarum browni)
continued in 2015, with observers at 41 nesting sites providing data. An estimated 4202-5295
California least tern breeding pairs established 5504 nests and produced 1514-1887 fledglings at
49 documented locations across California. The fledgling to breeding pair ratio was 0.29 to 0.45.
Statewide, 9654 eggs were reported, with a Statewide clutch size of 1.7 eggs (St Dev = 0.26) for
Type 1 sites where monitors walk within the colony.
Numbers of nesting least terns were not uniformly distributed across all sites. Camp Pendleton,
Naval Base Coronado, Huntington, Point Mugu, and Alameda Point each had over 300 minimum
breeding pairs, which represented 64% of the state total, and produced the most fledglings,
contributing 60% of the state’s production (Table 1). Sites with greater than 35 fledglings each
(the five aforementioned sites plus Mariner's Point, Hayward, Batiquitos, Bolsa Chica, and
Oceano Dunes) contributed 86% of the state’s production.
The 2015 statewide non-predation chick mortality rate was 18%, similar to that in 2014 (20%).
With the exceptions of Camp Pendleton and Naval Base Coronado, the larger nesting colonies
experienced non-predation chick mortality rates less than the average, similar to that documented
in 2014. The predators known to be responsible for the greatest number of depredated least terns
in 2015 were coyotes (Canis latrans), common ravens (Corvus corax), corvid species, raptor
species, peregrine falcons (Falco peregrinus), and American crows (Corvus brachyrhynchos).
The monitoring effort of 2015 is scheduled to continue in 2016.
