Palo Alto Flood Control Basin (Spring/Summer):
A Noisy California Gull Colony
by Mary Ann Roberston and Eve Meier
If you are not sure if you are a larophobe or a larophile then come along the Adobe Creek Loop Trail between the Palo Alto Flood Control Basin and Charleston Slough in Mountain View and watch the antics of the California Gulls in their colony. We find these noisy birds so fun and hope you do too.
Trip Covers: April - August
Key Birds: Mallards with ducklings, American Coot, American Avocet, Marbled Godwit, Dowitchers, California Gull, Black Skimmer, American White Pelican, Barn Swallow, Cliff Swallow
How to Bird
Palo Alto Baylands Nature Preserve is a 1,940-acre tract of marshland and within it lies the 618-acre Palo Alto Flood Control Basin bounded on the east by Adobe Creek and on the west by Mayfield Slough and Byxbee Park. This basin of diked salt march (or muted tidal marsh) contains islands which are home to the California Gull colony and the American White Pelican. This guide covers the 1.3 mile one-way, flat walk along the Adobe Creek Loop Trail out to the California Gull colony.
Starting from the San Antonio trail entrance on Terminal Blvd in Mountain View, walk about 0.2 miles along the paved trail to the intersection of the Bay Trail and the Adobe Creek Loop Trail. (You will have passed Coast-Casey Forebay and the pump house on your right and we’ll talk about those on your return trip.) At the trail intersection, continue straight towards the bay for another 1.1 miles along the Adobe Creek Loop Trail. Along this section of the trip, Adobe Creek will be on your left and Charleston Slough will be on your right. Depending on how fast you walk this will take 40-50 minutes with a few brief stops.
As you get closer to the gull colony, you will see them in the distance - the large mass of white (Stop 1 on interactive map).
Figure 1 - Gull colony in the distance (Stop 1)
Continue along the Loop Trail to Stop 2 (37.448617, -122.091033) for an excellent view (with binoculars) of the colony. To get closer, leave the wide loop trail and walk down the dirt paths closer to the creek. Be aware that bicycles often use these dirt paths.
Figure 2 - Gull colony on islands (Stop 2 at 37.448617, -122.091033)
Figure 3 - Dirt path leading closer to the water (Stop 2 at 37.448617, -122.091033)
Figure 4 - Closer view of the gull colony
By now, you should be able to hear the colony. Thousands of California Gulls are vocalizing reminding us of cats meowing. They are busy building and sitting on nests. You may even see some gulls bringing in fresh nest materials. Amongst the thousands of gulls, you may also see some American White Pelicans which are much larger than the gulls.
Figure 5 - Video of noisy gulls
Figure 6: Video of California Gulls on their nests
California Gulls start arriving in March to establish territories. By April, they are building scrape nests. In 2019, San Francisco Bay Bird Observatory (SFBBO) counted 9,036 nests in the colony during their walkthrough survey. (There was no survey in 2020 because of COVID-19.) Clutches of 2 to 3 eggs are most common. With two parents per nest that was 18,072 adult California Gulls. Noisy, indeed!
Figure 7 - Gulls sitting on nests. A gull on the right has brought new material.
Egg laying is in early May with peak incubation occurring in mid-May. (That’s when SFBBO does its walkthrough survey to count nests.) In late May, the chicks hatch. They will remain in the nest for a few days but, by day four, will move short distances. Chicks fledge in June, 5.5 weeks after hatching.
Figure 8a - Gulls with young chicks (May 14, 2021)
Figure 8b: Adult and juvenile California Gulls and one fuzzy chick (June 24, 2021)
Figure 8c. All but two gulls are gone from the colony islands (August 4, 2021)
In July, the fledglings leave parental territory (age of 40–60 days), though may depart earlier if disturbed. According to Birds of the World website, after leaving the colony, juveniles congregate at the edge of the water and swim together. They often form groups in areas with high food concentrations (other than breeding grounds) before migrating.
Figure 9 - Juvenile California Gull in August (photo by Tom Grey)
After you’ve had your fill of the gulls, return the way you came watching for other birds. If the tide is out in the Charleston Slough many birds may be seen on the mudflats. You will probably see American Avocets, Black-necked Stilts, Western and Least Sandpipers, and more gulls. Black Skimmers may be flying over the slough.
Figure 10 - Shorebirds in the Slough (August 13, 2020)
As you continue your return trip, keep an eye on Adobe Creek (Stop 3). You will probably see pelicans and gulls in the creek. You may also notice that some of the gulls look different than the adults and the juveniles of the colony. These are most likely not different species of gulls but are California Gulls that haven’t reached maturity. (During this time of year, most of the gulls are California Gulls. This is not true in the winter months.) The California Gull takes four years to mature and the plumage is different during those years. See your field guide for more details.
Figure 11 - Pelicans and Young California Gulls in Adobe Creek (July 2, 2020): From left to right: adult breeding; 2nd winter; juvenile; and 2nd winter (Stop 3)
Figure 12 - First cycle California Gull in winter by Tom Grey
Figure 13- California Gull in 1st winter plumage by Brooke Miller
Figure 14 - California Gull in third-cycle winter plumage, similar to adult plumage, but without the red on the bill by Tom Grey
After you return to the intersection with the Bay Trail, turn right down a slight slope dirt trail to an overlook at Adobe Creek (Stop 4). You may see Black-crowned Night-Herons in the reeds across the creek and if you are lucky a mama Mallard and ducklings may paddle by.
Figure 15 - Mama Mallard and ducklings (June 16, 2020) (Stop 4)
Now go back up the slope and cross the trail to the Pump House (Stop 5). Beginning in April, both Cliff and Barn Swallows build nests in this area and will be flying over the Forebay and Adobe Creek looking for insects. The Cliff Swallows use the mud along the edges of the forebay to make their nests.
Figure 16 - Swallows on the roof of the pump house (July 22, 2019) (Stop 5)
Figure 17 - Swallows at the pump house (June 26, 2020) (Stop 5)
As you pass the forebay on the way to your car, look to see what ducks and shorebirds are gathered there. This is also a good location to study the various plumage stages of California Gulls.
Ready for More?
For more spring and summer birding in Palo Alto Baylands, check out our spring guide covering the visitor center area and our summer guide on the Emily Renzel Wetlands. Come back to Charleston Slough and Coast-Casey Forebay area in winter to see thousands of wintering shorebirds and ducks!
History of California Gulls in the bay area and how you can help.
SFBBO has been monitoring the growth of the breeding population of California Gulls since 1980 when there were only 24 gulls. The Gulls may have relocated to the Bay Area from Mono Lake (at the eastern edge of the Sierra Nevada in California) when its water level was reduced in the late 1970s (KALW 2019). Alviso Pond A6 formerly held an average of 76% of the breeding population in the Bay. When A6 was restored to tidal action in December 2010 the Gulls redistributed to nearby colonies, particularly A9/10/11/14 and the Palo Alto Flood Control Basin. (SFBBO 2019 report, p. 6, 15-16) The latter is now the largest colony.
The SFBBO volunteer program has many opportunities for SFBBO members to interact with birds as citizen scientists. You can don a hard hat and count nests as you walk through the Gull colony. Check out these links:
Interactive Map
Directions: From US-101 in Mountain View, exit San Antonio Rd. Continue north to the end of San Antonio Rd then turn right onto Terminal Blvd. Park along Terminal Blvd. Latitude/Longitude: 37.433605, -122.099551
Parking: Street parking, 2 ADA spots
Fees: none
Public Transportation: Not available
Park and/or Trail Hours: Palo Alto Baylands: 8 am to Sunset, Shoreline At Mountain View: Dawn to dusk
Facilities: Wheelchair accessible bathroom and drinking water available at the trailhead.
Trip Mileage: About 1.3 miles one way, 2.6 miles round-trip
Trail Conditions: Paved from the trailhead to the intersection of Bay Trail and Adobe Creek Loop Trail. The Adobe Creek Loop Trail is wide, packed dirt with some gravel. Paths nearest the colony are narrow packed dirt sometimes used by bicycles. There is no elevation change. No shade; several benches along the way.
Accessibility: On Terminal Boulevard there are two ADA parking spots and a curb cut and path leading to the trailhead, at the San Antonio Gate. The trail leading to the Bay Trail is wide and paved. The Adobe Creek Loop Trail between the Creek and the Slough is packed dirt with some gravel. Paths nearest the colony are narrow packed dirt sometimes used by bicycles, not recommended for wheelchairs. Pedestrians, dog walkers and bicycles use it.
Bikes: This is a great trip to do by bike
Dogs: Dogs are allowed on 6-ft leash
Helpful Hint: To print or download this guide before you go, select the text you want (don’t include the banner photo), then print to PDF, or use a free service such as printfriendly.com, which lets you shrink or remove photos.
More Information
California Gulls
Donate or Volunteer: SFBBO California Gull Colony Nest Surveys
2019 Gull Nest Count (see Table 1 on page 13)
YouTube video at SFBBO's 2019 California Gull Research shows how volunteers count the nests
Crosscurrents Podcast A Chain Reaction Brought Gulls To The Bay... Now, They Are Eating Other Birds
Location Information
Palo Alto Baylands Map. Note that this map calls Pond A1 “Mountain View Slough”
Valley Water’s Palo Alto Flood Basin Tide Gate Structure Replacement Project webpage and report
City of Palo Alto’s report Palo Alto Baylands Existing Conditions (Dec 2017)
Charleston Slough Tides
eBird Hotspots
eBird Hotspots: Charleston Slough/Coast Casey Forebay, Adobe Creek North of US 101, Palo Alto Flood Control Basin
More Resources
For more trips like this one, visit Self-Guided Field Trips.
Visit the SCVAS Birding Resources page for more information on where to bird, our birding community, bird identification resources and more.
Read “What to Look for Now” by SCVAS Executive Director Matthew Dodder.
Photo Credits: All photos and videos by Mary Ann Robertson unless otherwise noted
Last Updated: 5/14/2022
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