The Loss of Avian Cavities by Injury Compartmentalization

Abstract

Cavity-nesting birds are dependent upon the availability of suitable substrates for nesting, foraging, and roosting. For nesting and roosting, substrates must be large enough in diameter to contain a cavity and soft enough for excavation to occur. Cavities are believed to be available to cavity-nesting birds and other species of wildlife until the cavity deteriorates, or until the tree or limb containing the cavity falls. Cavity deterioration may occur over a period of years, with the cavity entrance becoming too large, or the back, sides, or bottom of the cavity compartment decomposing to the point where the cavity becomes unsuitable. Cavities in fallen limbs or boles are typically no longer available to cavity-nesting birds; however, we have observed Blackcapped Chickadees (Parus atricapillus) using cavities in fallen, leaning limbs and House Wrens (Troglodytes aedon) using those in fallen boles along the South Platte River in northeastern Colorado (Sedgwick and Knopf, unpubl. data). Other species (e.g., Turkey Vultures [Cathartes aura]) may also use cavities in fallen limbs and boles (J. Tate, pers. comm.). Here, we provide evidence of another way in which cavities become unusable. Incidental to a larger study of habitat relationships of cavity-nesting birds along the South Platte River in northeastern Colorado (see Sedgwick and Knopf 1990 for a complete description of the study area), we monitored a pair of Black-capped Chickadees nesting in a live plains cottonwood (Populus sargentii) in the summer of 1985. The cavity was a knothole cavity (i.e., at the site of a previous limb break) and was in living substrate. Cavity height was 1.6 m and cavity entrance diameter was 3.0 cm, barely large enough for an adult to squeeze through. Adult chickadees were actively carrying food to the cavity and feeding young in June 1985. Upon revisiting the site two years later, we discovered that the cavity entrance had sealed shut (Fig. 1). New sapwood and bark had gradually grown over the cavity opening and sealed the cavity closed as the tree compartmentalized the wound. In 1990 we reexamined all cavities (n = 181: 157 in plains cottonwood, 10 in peachleafwillow [Salix amygdaloides], 14 in unknown species of [dead] trees) previously located in 1985-1986 and found an additional nine cavities that had become completely or partially resealed. A total of one House Wren, one Red'Received 7 January 1991. Final acceptance 7 February 1991. SHORT COMMUNICATIONS 781