This report summarizes information on the status, richness, abundance, and habitat use of birds on and around the Northern Jaguar Reserve (NJR) in east-central Sonora, Mexico based on surveys spanning four seasons over 13 years (2007-2019). In 2018 and early 2019, we surveyed birds along up to 18 transects in each of four seasons for the first time since 2012, quantified variation in vegetation community cover, vegetation structure, and habitat resources along survey transects, and evaluated data gathered since 2007 at a much finer level of detail than during past efforts. This effort focused on assessing patterns of seasonal abundances of birds, inter-annual variation in abundance within each season based on three years of repeated counts, habitat relationships, and implications for management and conservation of birds and their habitats. Since initial surveys in spring 2007 by the late Dr. Peter Warshall, we have observed 220 species of birds on the NJR or ~41% of species known to occur in Sonora and ~21% of all species known in Mexico, indicating the remarkable biodiversity of this region. Recent efforts revealed five new bird species on the NJR including the northernmost records in the world of Lesser Roadrunner and Rufous-bellied Chachalaca. We also documented an increasingly large number of new winter resident species of birds found at the northern ends of their winter ranges, changes that may have been driven by marked increases in temperatures linked to climate change.
In regards to seasonal variation in abundances, bird communities varied widely among seasons with patterns characterized by an influx of migratory species dominated by flycatchers, vireos, and warblers in spring and again in late summer and fall some of which likely remain in winter but differed widely from communities of summer residents. Based on more reliable data from 2007 and 2011, overall bird densities were roughly 50% higher during summer than in other seasons when a relatively small number of summer resident species are found at remarkably high densities. In regards to inter-annual variation in seasonal relative abundances over three different years, variation in estimates for all individuals combined and many species were lowest in winter, especially for permanent residents, suggesting fairly high levels of population stability. In spring and fall, however, inter-annual variation in abundances was greater with significant differences observed for as many as 14 of 20 species in spring and patterns often suggesting increases in 2018 relative to past years. In the important summer breeding season, however, most species showed much greater evidence of inter-annual variation, but major inconsistencies in walking speeds by two observers that completed summer 2018 work likely biased estimates. Walking speeds during summer 2018 surveys were ~2-times faster than in past summers, resulted in ~2-times fewer bird encounters, and observers ended surveys roughly 1.5-2 hours earlier than that in past summers. In regards to bird-habitat relationships, palms and broadleaf deciduous vegetation were important substrates for birds with use rates that far exceeded availability; use of palms by orioles and woodpeckers, for example, was ~6-10 times greater than availability. Relative abundances of all bird species combined increased markedly with cover of mesic riparian vegetation, and species richness also increased with cover of this community and with cover of oak woodland and tropical deciduous forest, but decreased with cover of montane scrub. At finer scales, both relative abundances and species richness in spring and species richness in winter increased with presence of surface water, which is often rarest in these seasons at sites in the interior. In general, abundances and/or richness increased with structure (e.g., cover and height) of riparian shrub vegetation in winter, both riparian shrub and mid-story structure in spring and to some extent in fall, whereas in summer both short- and especially tall-tree structure in riparian areas and overall upland structure were the most important drivers of abundances.
In regards to management and conservation implications, preserving, enhancing, and restoring riparian areas dominated by broadleaf deciduous vegetation can help promote overall bird abundance and richness across seasons, but areas with high degrees of upland structure are also important targets especially for breeding birds. To promote bird species richness, acquiring and protecting areas that also support oak woodland and tropical deciduous forest, which are rarer in the region and largely limited to higher elevations and the best developed stands of foothills thornscrub on the NJR, respectively, can also help promote bird, landscape, and biodiversity preservation. At finer scales, conserving and augmenting key habitat and vegetation resources such as palms and other substrates that support both cavities and other safe sites for nesting, and areas with surface water are good targets for managers. These goals can be achieved by planting and protecting young palms, especially in areas where they are rare, and by repairing, securing, and constructing gabions, represos, and other water features and retention structures, which may also help enhance habitats for other wildlife.
Future work should explore more detailed population- and guild-specific analyses of habitat use by both common and less common species, which is possible with data already gathered. I also recommend surveying transects in summer 2021 to address bias and uncertainty issues present in 2018 data and motivate future monitoring. Once more than three years of data are available within each season, explicit assessment of population trends will be feasible and can be estimated with more detailed methods. Information on population trends and impacts of local management and conservation efforts on wildlife are fundamental questions for NJP and many other conservation organizations that manage conservation lands. Birds are excellent foci for such monitoring efforts because they are reliable surrogates of specific habitat resources and conditions and linked to broad biodiversity values and key ecological functions. Hence, continued and ongoing monitoring of birds can help evaluate the degree to which conservation and management efforts by NJP are working, and help fill major current gaps of knowledge.