Introduction

A noisy, chattering, social species, the Pygmy Nuthatch lives in long-needled pine forests—in the United States, principally ponderosa pines (Pinus ponderosa). Frenetic movements—head first, up tree, down tree, along branches, right-side up and upside down—accompanied by constant chatter, convey the busyness and nervous activity of this gregarious bird as it searches for food.

Because it uses cavities for roosting and for breeding, the Pygmy Nuthatch reaches its highest densities in mature pine forests little affected by logging, firewood collection, and snag removal. It feeds mainly on weevils and leaf and bark beetles (but also eats pine seeds), and is 1 of the few cooperatively breeding passerines in North America and 1 of the world’s 2 cooperatively breeding nuthatches. A third of the breeding pairs have 1–3 male helpers, usually progeny or other relatives. Helpers feed incubating females, nestlings, and fledglings and participate in defense of the nest site. Breeding units and their offspring form the nucleus of winter flocks. During the nonbreeding season, family groups flock with other Pygmy Nuthatch families that range over a foraging territory. They also join with other flocking birds of the conifer forests. Pygmy Nuthatches generally dominate these species in the social hierarchy and in volubility.

The Pygmy Nuthatch roosts in cavities, which it selects depending on season and weather. It picks roosts based on insulation and ventilation provided by roost cavities and relies on hypothermia to survive cold winter nights. Although a few other North American birds are capable of controlled hypothermia, only the Pygmy Nuthatch links hypothermia with protected roost sites and communalism. Pairs roost together; juveniles roost with parents, and collectives of several flocks roost together. Stacks of 6, 8, and 10 birds, and more, roost together in formations of squares, oblongs, triangles, diamonds, wedges, and tiers. Two articles report more than 150 individuals roosting in 1 tree (Knorr 1957, Sydeman and Güntert 1983).

Robert A. Norris (1958) conducted the most extensive research on Pygmy Nuthatches. His thorough monograph covers all aspects of the biology of this species and compares it to its sister species, the Brown-headed Nuthatch (Sitta pusilla). Russ Balda and associates have addressed many aspects of Pygmy Nuthatch biology, such as cooperative breeding, roosting behavior, physiology, flocking behavior and hierarchy, feeding strategy, and habitat use (Balda 1967, 1969, 1975; Szaro and Balda 1979, 1982; Balda et al. 1983; Hay 1983; Brawn 1985, 1987, 1988; Brawn and Balda 1988a, 1988b; Güntert et al. 1988; Löhrl 1988; Sydeman et al. 1988; Sydeman 1989, 1991; Szaro et al. 1990; Ghalambor 1998; Ghalambor and Martin 1998; Martin and Ghalambor 1999).