Butterfly Count in San Francisco – 2017

This year’s sunny butterfly count day – June 18th, 2017 – made up for last year’s dismal fog. Eighteen spotters in eight groups, led by butterfly maven Liam O’Brien, counted a record number of butterflies: 1435 or 1440, (we’re still trying to verify which, but realistically it doesn’t much matter). The count hasn’t exceeded a thousand before.

Illustration of a Mourning Cloak butterfly - public domain via Wikipedia

Illustration of a Mourning Cloak butterfly

They also counted 29 species, up from the more usual 24-26. One of them, the Mourning Cloak, hasn’t been counted before (at least since 2010). In all, 34  species have been recorded in San Francisco in the 7 years we’ve been following the data, but some don’t show up every year.


Each year, the North American Butterfly Association (NABA) sponsors the July 4th series of butterfly counts at locations all across the US. Volunteers go out up to one month before or after July 4th to count butterflies in specific locations.

We’ve followed the San Francisco butterfly count since 2010, with a gap in 2015 when we found no published data. (If data are made available, we’d be happy to publish it.)   The results for earlier years are here: 2010, 2011, 2012, 2013, 2014, and 2016.

The San Francisco count is tricky; San Francisco gets fog in summer and butterflies tend to lie low on foggy days. (The picture of Sutro Forest below shows typical summer weather in San Francisco.)

The 2016 count, on June 4th, had bad luck with the weather, with a persistent fog and only sporadic sunshine. The spotters were able to find 24 species, the same as in most years, but only 499 individual butterflies. In 2017, by contrast, spotters found 1435-1440 butterflies of 29 species.

Counts in recent years include Angel Island and Yerba Buena Island, both of which have slightly different species from San Francisco city. In 2016, three of the species were counted only on the islands: The Pipevine Swallowtail on Angel Island and Yerba Buena (though other, non-count reports say it has been seen on Mount Sutro!); the Common Wood Nymph and the Rural Skipper on Angel Island. In 2017, the islands yielded California Sisters, Rural Skippers, and the Common Wood Nymph. (The Pipevine Swallowtail is also on the “spotted” list, but this year’s notes don’t mention it as being confined to the islands.)


Two butterfly species accounted for half the sightings in 2017:  The Cabbage White, and the Echo Blue.  The top ten butterfly species accounted for 86% of the count numbers.

Cabbage White sitting on Oxalis

The Cabbage White butterfly topped the charts this year, as it has in five of the seven years for which we have data. There was a record sighting of 487 individuals, far exceeding the 378 in 2011. This butterfly especially likes brassicas, in the cabbage and mustard family like San Francisco’s wild mustard.

Source: Katja Schultz, Wikimedia Creative Commons

The second position this year went to the Echo Blue, a small blue butterfly. It’s shown up in third place twice before. Online information about this butterfly is sparse; its larval food plant seems to be ceanothus and a variety of others including, possibly, blackberry. If this proves accurate, it may explain why the species is seen so often in San Francisco – we have a lot of Himalayan Blackberry, which is an excellent habitat plant for a lot of wildlife of all sizes and species.

The third place went to the spectacular Anise Swallowtail, one of our prettiest butterflies. It breeds on fennel, a non-native plant introduced for its culinary and medicinal properties, but now hunted as a weed by San Francisco’s Natural Resources Department. Fortunately, fennel is still abundant in the city, and so are these butterflies.

Other highlights:

  • For Anise Swallowtails, this count bettered the National High count of 2011!
  • The Monarch butterfly showed up again, which is pleasantly unusual since these butterflies more usually overwinter at the coast and fly inland in summer. Maybe they’re adapting to year-round residence? Its main locations seem to be The Presidio and Treasure Island/ Yerba Buena.
  • The Mylitta’s Crescent, which feeds mainly on thistles, made a better showing this year.
  • The Western Pygmy Blue, common elsewhere but rare in San Francisco  showed up this year. It’s the smallest butterfly in the US, only a half-inch across and is copper-colored with only tiny bits of blue. It’s only been recorded once before, in 2012.
  • The Mourning Cloak butterfly – a handsome dark-brown butterfly with a yellow edge to its wings is rare locally though abundant elsewhere. It showed up in the count for the first time. This butterfly lives almost a year, and is a strong migrator so it shows up all round the world. Though the butterfly is gorgeous (its British name is “Camberwell Beauty”) its caterpillar – the Spiny Elm Caterpillar – chomps through tree leaves, sometimes destroying the trees. It prefers hardwood forests and cold winters, which may explain why we don’t see much of them in the city.


If, like us, you like to see the data in detail, here is our spreadsheet compiling the butterfly count numbers from 2010-2017 (except for 2015, the missing year). We’ve arranged them in alphabetical order for convenience.WHY NO MISSION BLUES?

What about the Mission Blue butterfly that SF Recreation and Parks Department has been trying to introduce on Twin Peaks?  Well, the count really doesn’t provide any information because those butterflies have an 8-10 week flight season in April and May. So unless the Count is very early and the flight season delayed, Mission Blues are unlikely to appear in this record. We’ll report on them separately when we have all the data.

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