Restoration of priority habitats for Neotropical migratory birds in the Madrean Sky Islands region, northwest Mexico II

Understanding how ecological restoration projects influence target vegetation and wildlife is fundamental for evaluating efforts and guiding future applications.  In 2011, we began a multifaceted ecological restoration, outreach, and education project in northern Sonora, Mexico to enhance riparian vegetation dominated by cottonwood and willows, augment and improve habitat for populations of breeding and migratory birds, and provide training and examples of successful restoration to local landowners and managers.  With help from numerous current and former collaborators including Borderlands Restoration and Sky Island Alliance, we treated 569 ha of riparian areas with restorative fencing (e.g., cattle exclosures) and locally with erosion control structures and pole plantings in 2012, developed management agreements focused on cool-season cattle grazing with landowners, and subsequently repaired structures and negotiated new agreements in 2017-‘18.  To foster long-term monitoring and assess the influence of treatments, I use of a before-after/control-impact design and gathered baseline data on vegetation structure and bird communities within a large set of treatment and control areas in 2012, and subsequently gathered follow-up measurements in 2019 to foster evaluations.  This report documents results linked to these monitoring activities and provides guidance on future efforts.

Understory vegetation volume increased in restoration treatments relative to controls across time (P = 0.09, for time period by treatment interaction), especially when data from one transect with the lowest pre-treatment grazing impacts were censored (P = 0.01), indicating that restoration treatments had important positive impacts.  Magnitudes of change, however, varied widely across space among the three project sites.  Understory vegetation volume increased in treatments relative controls across time at San Lazaro and Milpillas, but larger overall increases within treatments at Cocóspera were similar within controls.  Canopy cover increased markedly at all sites across time, but in contrast to patterns in the understory, there was little evidence of differential changes in cover in treatments relative to controls (P ≥ 0.23), suggesting factors other than restoration treatments drove these patterns.  

Observed changes in the densities of various bird populations across time largely matched those for temporal changes in vegetation structure.  Densities of 16 focal bird species and one species group (understory species), which totaled 71% of species (or groups) considered by analyses, showed at least some evidence of increases across time in treatments, controls, or both.  Many of these bird species that showed changes across time are dependent on mid- or high-canopy resources, tree trunks, or understory vegetation.  Most importantly, and often as predicted, densities of the White-winged Dove, Sinaloa Wren, Lucy’s Warbler, Yellow-breasted Chat, and Summer Tanager all increased in treatments relative to controls (P ≤ 0.05).  Moreover, point estimates of densities of understory obligates such as Common Yellowthroat and Song Sparrow increased as much as 2-8 fold in many treatment reaches relative to controls, especially at Cocóspera, but precision of estimates was low due to small local sample sizes.  Such results were consistent with hypothesized changes whereby reduced grazing in the understory driven by restoration treatments results in rapid vegetation recovery and concomitant increases in associated bird populations, and largely validates the overall success of this effort.  In contrast, over the relatively short time period considered (~7 years), increases in canopy cover and associated bird populations were likely driven by natural maturation and growth of riparian trees, many of which germinated following major floods of 1992 and are thus maturing rapidly.  Continued and enhanced improvements of vegetation, habitats, and bird populations as a result of restoration treatments can be fostered by future outreach and collaboration with landowners, and additional work such as localized pole plantings, erosion control structures, and follow-up repairs to existing infrastructure in conjunction with future monitoring.