Swainson’s Hawk Survey in Los Angeles County

[Posted by Chuck Almdale]

The following message is from Lance Benner lbenner@charter.net, birder and near-earth astroid-finder extraordinaire. The group Los Angeles Birders wants help spotting Swainson’s Hawks who fly over L.A. County in their northward migration from the grasslands of Argentina.

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Swainson’s Light Morph adult – eBird, Steven Mlodinow, 31 May 2017, Colorado

Hi Everyone,

Los Angeles Birders are organizing a survey this spring to monitor the movement of Swainson’s Hawks through Los Angeles County.  Following up on the email posted a few days earlier, we want to provide more information and to request your help. 

Based on results from previous years, there appear to be two peaks in Swainson’s hawk numbers locally with large flocks totaling 50+ birds moving through between about March 11-20 and a smaller, but still substantial, peak roughly two to three weeks later.  The date of the first peak fluctuates by +- a few days. The full migration extends from roughly mid February through early May.

Several reports with kettles of 10+ birds have already appeared in eBird and on the L.A. County listserve within the last few days.

There’s a lot we don’t know about Swainson’s Hawk movement through the county in the spring, so Los Angeles Birders are organizing efforts to track them more thoroughly and systematically in 2022.  In previous years most of the Swainson’s hawks seen in LA County have been along the foothills between Claremont on the east and Big Tujunga Wash on the west, but where the birds go beyond Tujunga isn’t well known.  There has also been significant movement through Griffith Park and the eastern San Fernando Valley and occasionally along the north slope of the San Gabriel mountains.  

Swainson’s Dark Morph adult – eBird, Brian Sullivan, 25 Apr 2009, Fresno

The table below lists all Los Angeles County eBird reports of at least 50 Swainson’s Hawks from 2017-2021:

No.     Date      Time  Location  

80    15-Mar-17 18:00 East of Lancaster (G-8 at 100th E). desert
100   24-Feb-18 17:14 Pitzer College, Claremont. coastal slope
100    3-Mar-18  7:40 Peck Rd. Water Conservation Park. coastal slope
250   11-Mar-18  9:14 Hahamongna. coastal slope. mostly cloudy, rain previous night
200   11-Mar-18 16:00 Los Feliz/Atwater Village. coastal slope
145   11-Mar-18 16:00 Scholl Canyon, Glendale. coastal slope, partly cloudy
75    11-Mar-18 17:00 Myrtle/Greystone, Monrovia. coastal slope
100   27-Mar-18  9:00 Chilao, San Gabriel Mtns. mountains
55    23-Mar-19  7:00 Hansen Dam. coastal slope overcast
70    23-Mar-19  9:12 Altadena. coastal slope, had roosted nearby
120    8-Apr-19  9:05 Piute Ponds. desert, sunny, calm
200   15-Mar-20  6:36 Hahamongna. coastal slope,partly clear (cloudy, rainy previous days)
516   15-Mar-20 10:00 Vina Vieja Park, Pasadena. coastal slope, cloudy
75    17-Mar-20 10:42 Highland Place, Monrovia, coastal slope
80     5-Mar-21  8:45 Horsethief Canyon Park, San Dimas. coastal slope
400   13-Mar-21  9:40 Hahamongna, coastal slope
400   13-Mar-21  9:30 East Alta Loma Dr., Altadena, coastal slope, sunny, calm
100   13-Mar-21 12:12 Bygrove St., Covina. coastal slope
125   14-Mar-21  7:47 Bernard Field Station, Claremont. coastal slope, mostly cloudy
150   15-Mar-21 12:30 East Loma Alta Dr., Altadena. coastal slope, overcast, light rain
150   15-Mar-21 15:05 Horsethief Canyon Park, San Dimas. coastal slope, cloudy, drizzle
375   15-Mar-21 17:10 Altadena. coastal slope
75    15-Mar-21 17:56 Bowring Dr., Altadena. coastal slope
70    20-Mar-21 13:01 Claremont Wilderness Park. coastal slope
141   20-Mar-21 13:45 Juniper Hills. desert foothills, 10% cloud cover

Migrant flocks through the desert (Antelope Valley, and north of us from eastern Kern and the Owens Valley) tend to be later in the season (mainly late March through April) than flocks on the coastal slope (which can begin in mid-February and peak by early April). Of course we emphasize that more data are desirable.

Large flocks of Swainson’s Hawks in LA County have been reported throughout the day but not usually before about 9 am.

To understand movement of Swainson’s Hawks through this area, we request that everyone please report them to eBird and, if you see flocks of more than several birds, please post them to the L.A. County listserve promptly.

Swainson’s hawks appear to fly ~100-200 km per day (~60-120 miles), so it could take a couple of days for them to get from staging areas near Borrego Springs to, say, Pasadena.  Large numbers lifting off from Borrego Springs might provide an early alert for big flights farther west-northwest on the next day or two.

In more detail, we ask your help with monitoring the hawks as follows:

1. Conduct systematic surveys on specific dates from sites that have a good chance for seeing the hawks.  For these surveys, we ask observers to watch for at least one hour between 9 am and sunset. 

Target dates:
March 12 Saturday
March 13 Sunday
March 17 Thursday
March 18 Friday
March 19 Saturday
March 20 Sunday

We chose March 17 and 18, even though they’re weekdays, because experienced observers will be watching from Bear divide on those mornings.

2. Please report *all* Swainson’s hawks seen as part of other, regular birding in your eBird lists.  

3. If you see a flock of several dozen or more birds, PLEASE post the sighting to the L.A. County listserve  as soon as possible so we can try to marshal observers to track its movement. 

We are hopeful that migration monitoring at Bear Divide (which officially starts on March 17) and Gorman (see the recent listserve post by Richard Crossley) will help us understand movement through those areas. 

Negative reports with zero hawks will help us understand their movements and are also welcome.

Where should one look?  Previous experience suggests that the hawks preferentially move along the foothills on the coastal slope, but there are signs of substantial movement along the north slope as well (Juniper Hills, along Pine Canyon Road west of Lake Hughes, near Quail Lake, and Gorman).  

Here’s a list of sites where significant numbers have been reported in LA County previously or where we think chances are good for seeing the birds, listed in order of southeast to northwest:

Wheeler Park, Claremont
California Botanic Garden, Claremont
Blaisdell Ranch Preserve, north Claremont
Claremont Wilderness Park
Horsethief Canyon Park, San Dimas
South Hills Park, Glendora (along the ridge)
Bonelli Park
Santa Fe Dam
Encanto Park
Grand Avenue Park, Monrovia (Zone-tailed hawk area)
Peck Road Water Conservation Area 
Los Angeles County Arboretum
Bailey Canyon Park, Sierra Madre
Eaton Canyon, including the road up to Henninger Flat
Vina Vieja Park, Pasadena
Cobb Estate, Altadena
Loma Alta Park, Altadena
Chaney Trail Road/Sunset Ridge/Millard Canyon
Hahamongna Watershed Park
Griffith Park
Verdugo Mountains and adjacent parks on the north and south sides
Hansen Dam
Big Tujunga Wash
Veteran’s Park, Sylmar
Bear Divide
Placerita Canyon State Park
Castaic Lagoon
Pyramid Lake
Pine Canyon Road west of Lake Hughes
Quail Lake
Gorman area where the rough-legged hawk turned up in 2021

The above list of sites is by no means complete and it wouldn’t be surprising if large flocks turn up in other areas near the foothills.  In general, though, there aren’t any records of large flocks moving along the coast.

Thus far in 2022, reports of 10 or more birds have occurred on the following dates:

#   Date     Location & observers
12  March 3  north Monrovia. Ron Cyger
30  March 4  Gorman. Richard Crossley et al. 
11  March 5  Bonelli Park.  Keith Condon
28  March 5  Pacific Crest Trail, Three Points.  Alan Brelsford
11  March 5  Pacific Crest Trail, Three Points.  Alan Brelsford

There have been recent reports of modest numbers (< 10) in Borrego Springs, a traditional staging area in previous years, and numerous reports of more than 10 birds/flock in the Imperial Valley in fields near Calipatria.  So far in 2022, it appears that Swainson’s hawks are staging more near Calipatria than at Borrego Springs. There was also an intriguing report of about 50 birds settling to roost in Morongo on March 4, which raises the obvious question of where they went on March 5 (north slope..?).

Please report sightings to eBird, and in your comments, please note the weather conditions.   Anecdotally, there are indications that the hawks are easier to see on days with low overcasts, which appear to keep the birds closer to the ground where they are more visible.  

Kimball points out that it’s also important to track migration by turkey vultures, which have already started to move through in relatively large flocks, with kettles exceeding 20 birds seen going west along the foothills as recently as March 6.  Please report those in eBird and (especially large groups) on the listserve.

Thank you,

Lance Benner
Altadena, CA
Community Science Chair,
Los Angeles Birders
lbenner@charter.net