Challenging our understanding of western Yellow‐billed Cuckoo habitat needs and accepted management practices

Abstract

Riparian restoration in the southwestern United States frequently involves planting cottonwood (Populus spp.) and willow (Salix spp.). In the absence of flooding and gap-forming disturbance, planted forests often senesce without further young tree recruitment. This has largely been the case in California riparian systems that historically supported state-endangered western Yellow-billed Cuckoo (Coccyzus americanus; Cuckoo). A decline in Cuckoo population numbers in the past 30 years has been associated with forest maturation. Other riparian species of concern show a concomitant decline, indicating the problem is not specific to Cuckoos. Although varying hypotheses exist for recent decline, alternative management practices have not been sufficiently explored to rule out breeding ground habitat quality as a major contributing factor. Few intensive Cuckoo datasets exist to test hypotheses about breeding habitat quality due to extremely low populations in the remaining occupied sites. We used a historical (1986-1996) spot mapping dataset from the South Fork Kern River Valley, CA to identify vegetation characteristics related to Cuckoo and five other sensitive riparian bird territory densities. We found Cuckoo densities were positively associated with increased vertical vegetative structure 1-5 m above ground with a threshold for mean tree height. Sensitive species densities were also related to vertical structure and started to decline with stand height greater than 6-8 m. Naturally regenerated sites had higher densities of most sensitive bird species than planted sites. We provide ideas for restoring mature forest with little vertical structure.

Using Bioacoustics to Examine Vocal Phenology of Neotropical Migratory Birds on a Wild and Scenic River in Arizona

“…This Neotropical migratory bird breeds in areas with multi-layered riparian trees [32] and needs patches of habitat with trees of different heights and ages, which are often produced from episodic floods [33]. In the absence of flooding, native riparian forests have matured without replacing tree recruitment, further degrading habitat for the cuckoo [34]. The tanager and cuckoo are just two examples of birds experiencing declines from loss of habitat.…”

Early successional riparian vegetation is important for western Yellow‐billed Cuckoo nesting habitat

“…Although Cuckoos used restoration sites for foraging in KRP (Wohner et al 2020), only one active nest was found in the 10 restoration plots in 11 years. Restoration sites were ecologically different than natural areas (Wohner et al 2020) so we did not include restoration availability plots. We randomly removed some of the availability plots to take into account that the number of nests/site was unbalanced such that the ratio of use to availability plots was the maximum (Northrup et al 2013; ≤1:3) for each site.…”

“…We first used lowest scoring AIC c to determine which combination of nested random effects was the most informative to include as hierarchical random effects (Bolker et al 2009). These included three levels; the five sites (Patterson, South, Tanager, Riverbottom, and Turtle) or estimated territories via spotmapping (Wohner et al 2020) within sites ( n = 22), nested within forest type (natural regeneration vs. mixed‐age forest) or region (SFWA, including LIDDZ), KRP East, KRP West). Once a suitable random effects structure was identified, our model selection process was to determine the best fitting variables among similar variables in classification groups (Table S1) with the lowest AIC c score within ΔAIC c < 2.…”

“…Cuckoos can occasionally be found in marginal habitat while not occupying seemingly appropriate habitat (Wohner et al 2020), perhaps due to high site fidelity, making assessment for restoration success, allocation of future effort and management, and linking causal factors in population decline difficult. Another problem associated with Cuckoo recovery is their requirement for large, connected patches (>80 ha; Johnson et al 2017) and large estimated home range size (McNeil et al 2013), up to 91 ha in some parts of the range (Halterman 2009; Sechrist et al 2013).…”

“…We used historical KRV nest and vegetation data (1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996) because there are now so few western Cuckoos remaining that there are few current Cuckoo nest datasets to work with outside of restoration sites on the LCR. Possibly only one pair has nested in the KRV from 2015 to 2019 (Stanek 2017;Wohner et al 2020). We tested the hypotheses that Cuckoos select sites with smaller dbh and increased tree densities (reflecting a younger stand), areas with increased densities of willows and forbs, and sites with more complex three-dimensional vertical structure.…”