This lichen never fails to remind me of summer, though it is a snowy-looking subject. Parmelia sulcata, or hammered shield lichen, is a silvery foliose species in the Parmeliaceae with a dark, nearly black, underside possessing dense rhizines (hair-like growths from the body that anchor the lichen to the substrate). The photobiont of this symbiont is a green algae in the genus Trebouxia (one of the most common photosynthetic partners in lichen relationships). Parmelia sulcata is a very common lichen throughout Europe and North America, but it was particularly brought to my attention last summer when I discovered a hummingbird nest with this lichen species laced in. For an example of what I observed, visit the web site of Karen Crowe and browse her wildlife images gallery.
Hummingbirds are the only family of birds to have mastered flying backwards. North American species such as rufous or ruby-throated hummingbirds collect Parmelia sulcata in their beaks and drop it cleverly and carefully into a sticky web of spider silk. Once enough lichens are collected, the silk strands are laced about to form a cup, which is filled with fluffy seed down and moss to form a tiny nest maybe about the size of a toonie (~30mm or 1.25″ in diameter). The lichens do double duty as highly effective camouflage amongst conifer needles or hardwood foliage! Using lichens in this way is not uncommon and some birds will actually “shingle” their entire nests! Although a few different lichen species are used, Parmelia seems to be the favourite by far, although this could be because it is highly tolerant of pollution and therefore very common. You can find a full list of lichens’ faunal users at the Lichens of North America Wildlife page.
In the presence of K+ (potassium hydroxide, or caustic soda, available at hardware stores), the cortex or outermost layer of this lichen will flash orange-yellow (for examples, an orange result and a yellow result)! This is a chemical reaction that indicates the presence of a secondary metabolite called salazinic acid, which has demonstrated antibacterial and antifungal activity.
If you are interested in chemical identification of lichens, the British Lichen Society has an excellent resource page on chemical tests. More examples of spot tests are available from Sharnoff Photos: Lichen Spot Tests.