Document: ROB-3-51-30

Expansion of breeding Bewick's Wrens into riparian forests of the middle Rio Grande: The role of forest succession.

TAYLOR, R.V.*

Utah State University, Logan, UT 84322 ¹

Abstract:
Bewick's Wren (Thryomanes bewickii ) was only recently observed to breed in the riparian forests of the middle Rio Grande in central New Mexico. Currently, it is one of the most abundant breeding birds there. I used territory mapping, nest location characteristics, abundance data, and a nest-box addition experiment at 12 sites near Albuquerque, NM in an attempt to understand the environmental factors that may have driven this change in the wrens' breeding distribution. Both the invasion of exotic Russian olive and salt cedar shrubs and changes in the age and condition of cottonwoood populations were hypothesized to be important. Wrens used only native species for nesting (n = 22) and cottonwoods selected for nesting (mean DBH SD = 57.9 39.1) were larger than random (40.2 21.6; p < 0.01). Multiple logistic regression of the floristic composition of wren territories vs. areas not used by wrens showed that territories had greater coverage of cottonwood and lesser coverage of grass and forbs (n = 42, p < 0.0001). The abundance of wrens appeared to be limited by the availability of nest cavities. From 1997 to 1998, wren abundance increased more in areas where nest boxes were provided (mean = +1.6) compared to control areas (mean = +0.4; n = 20; p < 0.05). . These results suggest that exotic plant species actually played little role in the expansion of wren distribution. Rather, it appears that wrens inhabit patches where mature cottonwoods dominate. These cottonwoods, established in the 1940s as a result of flood-control structures, provide suitable nest sites for breeding wrens. Reduction of overbank flooding has virtually eliminated cottonwood recruitment in the riparian forests of the Middle Rio Grande and it is projected that cottonwood populations will decline. This casts doubt on whether these newly established wren populations will persist.

Keywords: Bewick's wren, Thryomanes bewickii, Rio Grande, Populus deltoides, cottonwood, range expansion

This abstract is being presented at: 10:30 AM in session:
AVIAN ECOLOGY