Nest height of Black-billed Magpies: is it determined by human disturbance or habitat type?

Abstract

The impact of human disturbance on the choice of trees in which to nest and the heights at which to place the nests by Black-billed Magpies (Pica pica) was studied by comparing nest placement in three areas: A, urban with frequent human disturbance at the nest; B, urban with occasional disturbance; and C, suburban with minimal disturbance. Magpies nested in conifers more frequently than expected, based on availability, in all three areas. Neither absolute nor relative nest height (nest height/tree height) changed over the 2 years of this study, despite frequent human disturbance. The relative nest height did not differ from that 12 years before the current study was initiated. Moreover, nest height (absolute and relative) did not change when 10 pairs renested at new sites in 1987 in area A (after their original nests failed because of a spring snowstorm) even though the original nest sites had been periodically climbed to examine nest contents. Magpies used taller trees in area B than in area A but maintained the same relative nest height, which suggests that they either nested at an optimum height in the trees for ecological reasons or placed their nests as high as they could in the urban environment. Birds in suburban area C nested in shorter trees and at relatively lower levels than those in the urban areas, probably in response to the habitat type. The relative height of magpie nests was significantly greater in urban than in suburban habitat for conifers but not for deciduous trees, this variable being relatively constant within each habitat. Thus, habitat type seemed to have the greatest influence on nest height of magpies, but human disturbance did cause magpies to change trees for renesting in the same season.