Interlocking
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Figure 4.8 The *grey-backed cameroptera (Cameroptera brevicaudata) attaches the inner nest lining to the outer envelope of growing leaves by driving through the leaf membrane dozens of plant-down pop-rivets. (Photograph by M. Hansell.)
by George de Mestral, a Swiss engineer, who in 1940 was inspired by the sight of plant burrs becoming entangled in the fur of his dog (Jacobs 1996). My own nest examinations show that some species from 25 of the 45 (56%) passerine families (Sibley & Monroe 1990) use silk as a structural material and, in the non-passerine family the hummingbirds (with about 300 species), silk use may be almost universal (Table 4.1). The silk samples in a survey of the nests of 110 genera from six families show that very few of these use silk as an adhesive. The only adhesive silk available to birds is the web silk of ecribellate spiders, which use sticky droplets to capture ying insects by adhesion. Many of these species produces orb webs, which we admire for the small amount of silk that can be used to produce such a large capture surface. For birds, such webs are not very valuable; instead, they collect thick sheets or blobs of silk from sheet webs, tubular retreats and spider egg cocoons, or from lepidopteran pupal cocoons. All these have dry threads and so can only be used in nest building as the looped material of a velcro fabric. Not all plant materials will provide the numerous projections