Sugar Thieves

Have you seen birds with feathers sticking up around their beaks as if they had a bad facial hair day? It happens often in late winter in our area to small birds with short and thin bills, such as Ruby-crowned Kinglet, and a number of warblers. These birds are insectivorous for the most part, but when there is nutrient-rich plant food around, they don’t hesitate to tuck into it.

Ruby-crowned Kinglet, near Jewel Lake. Feb. 8, 2016. By Minder Cheng.

When warblers pick bead-like berries, such as fruits of Pacific Wax Myrtle (Morella californica) and exotic palms, both favorites of Yellow-rumped Warblers, they swallow them in whole, and no feathers are messed up. The irresistible sugary tree sap, on the other hand, is a treat that leaves tell-tale sign on the guilty parties.

Yellow-rumped Warbler, Tilden Nature Area. Feb 1, 2016. By Minder Cheng.

Coming upon sap wells drilled by sapsuckers (and sometimes by other woodpeckers, see for example Kozma 2010), warblers and kinglets have to press their heads against the holes, and thrust their short beaks as deep as they can, in order to lap up the sweet liquid inside. That’s because the circumference and depth of the holes are made by bills much bigger than theirs, and if the sugary residue around the hole touches the base of a small bill, it can gum up the feathers surrounding the bill.

Orange-crowned Warbler, Valley-Foothill Section of Regional Parks Botanic Garden.
May 7, 2019. By Minder Cheng.

Here is a blog post that documents a Townsend’s Warbler visiting a Red-breasted Sapsucker’s handiwork in Fort Mason, San Francisco.

Hummingbirds, well-known for their love of sugary drink, actively pursue, and even defend, sap-producing holes (Sutherland et al 1982). I once saw an Anna’s Hummingbird dipping its bill into a sap well, then it got excited and zapped a Red-breasted Sapsucker working on other sap wells in the same tree. The feisty hummer chased the rightful owner away in no time.

But we don’t see hummingbirds with any matted facial feathers. They maintain their immaculate look because they have longer beaks, plus very long tongues adapted for nectar drinking (watch this video for the way their tongue works. They simply hover outside the holes, and let their bill and tongue do the job cleanly.

Tree sap is such an important part of hummingbird diet that it’s believed that the northern limit of Ruby-throated Hummingbird’s breeding range is affected by the availability of the sweet sap provided by Yellow-bellied Sapsuckers. In the west, the same relationship likely exists between Rufous Hummingbird and two sapsuckers, Red-naped and Red-breasted. (Miller and Nero 1983)

Rufous Hummingbird and Red-naped Sapsuckers. Painted by William Zimmerman.
Life Histories of North American Woodpeckers, 1992.

In fact, many birds enjoy swiping the sweet drink that the hard-working sapsuckers make available by drilling down to phloem or xylem tissue of trees. According to The Audubon Society Encyclopedia of North American Birds (1980) by John K. Terres, American Goldfinch, Pine Siskin, White-crowned Sparrow, Evening Grosbeak, Northern Cardinal, nuthatches, waxwings, and juncos are all sugar thieves. If we know there is free syrup out there in the woods, won’t we flock there, too?

Note:

Another source of matted facial feathers is the viscous nectar of Eucalyptus flowers. Let me quote David L. Suddjian (2004) “Birds and Eucalyptus on the Central California Coast: A Love–Hate Relationship”:

The great attraction of flowering eucalyptus for foraging birds may have its downside, though. As the birds spend time feeding amid the flowers, the feathers on their faces become matted with a black pitch-like residue (or gum) from the nectar. This affects their ability to maintain those feathers, and in some cases the gum may plug their nostrils or bills, and prevent breathing or feeding. Australian birds that regularly feed at Eucalyptus flowers in native settings have longer bills than North American species that feed at eucalyptus flowers, apparently permitting them to feed there without being affected by the gum.

Articles published in the Point Reyes Bird Observatory newsletter (Stallcup 1997) and in Audubon magazine (Williams 2001) have suggested that the effects of this black pitch cause substantial mortality among the North American species that feed at eucalyptus flowers. It seems to be a reasonable conclusion, and Stallcup (1997) cites some instances of mortality. But in my experience, and the experience of a number of other long time field ornithologists, we have seen very little evidence of such mortality. It has been argued that the bird carcasses do not last long on the ground before they are scavenged. However, when observers spend hundreds of hours under these trees over many years but find hardly any evidence of such mortality, then it seems fair to question whether the incidence of mortality is as high as has been suggested.