American Goldfinch (Spinus tristis)
   
 
Narrative and Science Lens
 
   Audubon shows these birds foraging on thistles. He also shows them feeding upside down and the female (the one without the yellow back, from plant materials in its dietand black cap, clinging to seed heads, which is typical of these birds. Thistles offer not only food but down used to line its water-tight nest (which is so compact nestlings can drown if parents fail to cover it during storms), and feed its young. So reliant on these seeds, the birds breed late in the season, timed to the availabilty of these seeds. Thistle feeders are popular and attract these birds.

Bull Thistle (Cirsium vulgare), shown here, is invasive. It was first reported in the San Francisco Bay Area in 1925 as "rather infrequent”, but is now common. Plants can produce about 100 to 300 or more seeds per flowerhead , and anywhere from 1 to over 400 flowerheads per plant. Does foraging on the seeds of these thistle reduce their spread, enhance it, or play an insignifcant role? One clue may be found in their natural dispersal potential: The down readily detaches from the seed at maturity, so more than 90 percent of seeds fall close to the parent plant. If the seeds pass through the bird unscathed (unlikely) the birds may be assisting their spead. More likely the seeds are carried in their feathers. But they are also carried by water, transport vehicles and farm machines, in mud on the fur of animals, and in animal manure, but mostly they are carried in baled or loose hay.
 
  Campus Locations
Fairly common migrant and winter resident in various habitats virtually throughout campus. Often forages in flocks (occasionally at feeders) during the nonbreeding season.
  Campus sustainability
  Yellow Starthistle (Centaurea solstitalis) apparently arrived in California much earlier than Bull Thistle, about the mid-1800s, and is one of California's most noxious pests. Each plant produces 700 to 10,000 seeds, which can remain dormant for a decade, enabling the plant to have spread across roughly 8 million acres.
.      What you can do
       1. See if Bull Thistle or Yellow Starthistle are on campus. Their spread can be controlled by cutting them shortly before plants flower;this may need to be done repeatedly until the first frost. Cut stems may still contain viable seeds, and should be removed.
       2. If these thistles are on campus and in flower, see if the goldfinches are using them.
       3. See if goldfinch-loving native plants can be added to the Native Species garden adjacent to the Keck building
  Science
       Essays from The Birder’s Handbook:
            Bird Biology and the Arts; Site Tenacity; Mixed-Species Flocking; Incubation Time; Incubation: Heating Eggs; Brood Reduction; Cowbirds
       References: Balph, 1979; Ketterson and Nolan, 1982; Smith and Andersen, 1982.
       Videos:
  Art
       Photos:
            Rohan Kamath: Brown-headed Cowbird (male and female)
       Drawings and Paintings:
            DW drawing after Audubon
 
  To add reports or images please submit them via the Art at Exits home page