Lesser Woodcreepers (Xiphorhynchus fuscus) excavate nest cavities in trees

To understand the evolution, life-history trade-offs, and population ecology of cavity nesters, it is critical to identify the avian lineages and circumstances in which birds excavate tree cavities. Woodcreepers (Furnariidae: Dendrocolaptinae; 56 species) are considered non-excavators dependent on existing cavities. We overturn this assumption by providing definitive evidence that the Lesser Woodcreeper (Xiphorhynchus fuscus, 23 g) is a facultative tree-cavity excavator. From 2007 to 2022 in the Atlantic forest of Misiones, Argentina, they nested in pre-existing tree crevices (4 nests), or excavated in trunks of large-diameter trees or stumps in advanced stages of decay (mean: 58 cm diameter; range: 22–121 cm; 22 nests). Nest entrances were vertically elongated and chambers were usually pocket-like, excavated in the exterior of the trees (sapwood), with floors that curved along the trees' circumference. Excavating woodcreepers pulled out elongated, fibrous pieces of decayed wood with a spongy texture, tapping only when inside cavities. Published and online photographs of nests of Xiphorhynchus species suggest that excavation may be widespread in the genus. Our observations that woodcreepers tore out elongated pieces of spongy wood (rather than hammering) are consistent with the idea that their long, thin bills are more resistant to torsion and less resistant to impact compared to the stouter bills of other excavators in Passeriformes and Piciformes. Research has tended to focus on birds with chisel-shaped bills, perforating harder sapwood to create nesting chambers in the center of heartrot-infected trees (resulting in typical woodpecker cavities, with circular floors). We hypothesize that Lesser Woodcreepers have adopted an alternative strategy, selecting large trunks with soft outer wood (sapwood), stopping their excavation radially if they reach harder wood, and then expanding the nest chamber laterally. Furnariidae may offer a useful model family for understanding ecological and evolutionary factors that influence cavity excavation.

Methods
We studied Lesser Woodcreeper at Cruce Caballero Provincial Park (San Pedro department, Misiones, Argentina; 26°31'S 54°00'W; 550–600 m asl). Carlos Ferreyra and Milka Gomez found trees with woodcreeper excavation activity. They made videos of excavating woodcreepers using a camera in hand, camera mounted on tripod, or iPhone (original) coupled to a Celestron telescope. Videos were trimmed for length.