(A) Toe pads slip and stick when dragged along a surface, as shown by the tangential force (friction) to normal force ratio, FT/FN, over time for a representative test. The violin plot illustrates the distribution of FT/FN values immediately before slip events (bw = bodyweight, ddrag = 5 mm) for Coast Live Oak (CLO), Floss Silk (FS), Sweet Olive (SO), birch, foam, and Teflon. Sandpaper, which yielded unnaturally high wear on claws and toe pads during pilot tests, was omitted from this testing. The number of peaks gives an estimate of the number of usable asperities, and the force before slip indicates how much friction those asperities provide. (B) Maximum friction values sustained at different normal forces demonstrate that toe pad friction is surface dependent and well described by a Coulomb friction model. Circles and error bars show mean ± SD for nnatural = 10 (trials for natural surfaces), nartificial = 5 (trials for artificial surfaces). (C) Claws also slip and stick, as shown for a representative quasi-static drag test (see Materials and methods), but reach higher FT/FN ratios with greater variance than toe pads (nnatural = 20, nartificial = 10). (D) Max FT/FN values, which approximate the friction a bird can expect from claw curling, increase with the angle between the claw and surface (α) on the roughest natural surface (SO), but remain relatively constant on the smoothest (FS). Diagrams in D show claws at the microscopic level to illustrate how α (defined at the macroscopic level shown in C) affects interactions with surface asperities. (E) To compensate for variability in max claw friction, birds may leverage load sharing over multiple claws. Standard deviation bars based on sampling more claws from the single claw distributions, and averaging the force they produce, show that (as expected) the spread of the average claw force gets smaller as more claws contribute to generating force. Gray error bars show SD for the single claw distributions. Dashed lines in (C-E) show the max FT/FN cutoff used to avoid claw damage during testing. (F) Birds can further improve available claw friction by increasing drag distance. The expected value of the maximum tangential to normal force ratio is denoted as E[FT,max/FN]. Dots and bolded segments of the curves denote estimated mean ± SD parrotlet claw drag distances, respectively, for each surface if the foot did not slip (see Materials and methods). The gray line indicates the estimated average drag length when landing, which was used for friction testing.