Avian Conservation and Management

INTRODUCTION

Accurately assessing population distribution, numbers, and habitat associations is important for the management and recovery of threatened and endangered species (Joseph et al. 2006, Camaclang et al. 2015, Hughes 2015). Obtaining accurate and complete survey data may be especially challenging for rare or cryptic species, however, because of low or unknown detection probability (Gu and Swihart 2004, MacKenzie 2005, Martin et al. 2022), poor understanding of habitat preferences (Rosenfeld and Hatfield 2006), or temporal or spatial variation in occupancy (Wiens et al. 1987, Durso et al. 2011, Hayes and Monfils 2015). These factors all affect current understanding for the western distinct population segment (DPS) of the Yellow-billed Cuckoo (Coccyzus americanus; hereafter, “cuckoo”), a cryptic Neotropical migrant bird that was federally listed as threatened in 2014 (USFWS 2014).

Yellow-billed Cuckoos are widely distributed across sub-boreal North America (Hughes 2015). Although cuckoo numbers have decreased throughout their range, declines have been more significant across the western United States and extreme southwestern Canada (the “western DPS”; Gaines and Laymon 1984, Dettling et al. 2015, Hughes 2015), with most of the remaining population now in Arizona, New Mexico, southern California, and northern Sonora (Hughes 2015, USFWS 2021). Cuckoos are notably cryptic, exhibiting low call rates, large home ranges, a delayed breeding season, and rapid nesting cycle (Hamilton and Hamilton 1965, Halterman 2009, McNeil et al. 2013, Sechrist et al. 2013). They have also been hypothesized to have a nomadic period before nesting during which they may wander widely to assess prey availability (Sechrist et al. 2012, McNeil et al. 2015), making it difficult to determine whether detections in atypical vegetation types or early in the season are breeding or transient birds. These behaviors present challenges in assessing site occupancy, breeding status, population numbers, distributional patterns, and habitat preferences.

Cuckoos in the western DPS commonly breed in riparian areas in broad, low-mid elevation, low-gradient floodplains. Habitat is typically dominated by riparian trees including cottonwood (Populus spp.) and willow (Salix spp.), often with multi-story or early successional structure and adjacent trees such as mesquite (Prosopis spp.; Anderson and Laymon 1989, Ahlers et al. 2016, Johnson et al. 2017, McNeil et al. 2013, Wohner et al. 2021). This is referred to as “Rangewide Habitat” (hereafter “rangewide riparian habitat”) by the recent critical habitat rule issued by the U.S. Fish and Wildlife Service (USFWS 2021). These habitat conditions are supported by perennial surface or ground water (Snyder 2000), and the recruitment of riparian trees and resulting vegetation structure depend on seasonal flood regimes (Stromberg 1993, 2001, Lytle et al. 2017). In the southwestern United States, cuckoo survey efforts, ecological studies, and habitat modeling, have focused primarily on these cottonwood- and willow-dominated rangewide riparian habitats (Halterman 2009, Johnson et al. 2010, McNeil et al. 2013, Dettling et al. 2015, Wohner et al. 2021). Recent cuckoo surveys in mountain and foothill drainages in southeastern Arizona (hereafter “SE AZ”; Corman and Magill 2000, USFWS 2014, MacFarland and Horst 2015, 2017; C. Corson 2018, unpublished data), however, have documented cuckoo occurrence in mid- to upper-watershed drainages that are generally dry, with only intermittent or ephemeral surface water. The dominant trees along these drainage courses are typically a mix of velvet mesquite (Prosopis velutina), Arizona ash (Fraxinus velutina), Arizona walnut (Juglans major), netleaf hackberry (Celtis reticulata), and various oak species (Quercus spp.), depending on the elevation and aridity of the area. Tree density ranges from scattered individuals to small clumps to continuous bands along the edges of the drainage course, but the bands are typically narrow (one to a few trees in width; MacFarland and Horst 2016; C. Corson 2018, unpublished data). Cottonwood or willow are sometimes present but only in low numbers (MacFarland and Horst 2015, USFWS 2021). Adjacent uplands may comprise Madrean-evergreen woodland, semi-desert grassland, or desert scrub (Brown 1994, MacFarland and Horst 2015), again depending on aridity of the region. These areas are referred to as “Southwestern Habitat” in the critical habitat designation rule (USFWS 2021) and throughout this paper we refer to them as “SE AZ xeroriparian habitat” or simply “xeroriparian.” Although breeding behavior has been observed anecdotally at some of these sites (MacFarland and Horst 2015, 2017), the extent to which detections in these SE AZ xeroriparian habitats reflected breeding birds rather than transients or migrants was unknown.

Breeding status of cuckoos in rangewide riparian habitat has typically been evaluated based on results of a standardized survey protocol using repeat-visit, call-playback methods along a survey transect (Halterman et al. 2015). To avoid unnecessary stress to cuckoos, nest searching is not a component of these protocol surveys, and few nests are incidentally detected because of the birds’ cryptic behavior and nest concealment. Instead, breeding status and site occupancy is inferred at the end of the survey season by reviewing the location and timing of cuckoo detections within the survey site. Although this approach has been shown to accurately reflect breeding status in rangewide riparian habitat (Halterman 2009, McNeil et al. 2013), its applicability has not been assessed in SE AZ xeroriparian habitat. Interpretation of cuckoo occurrence and breeding status in SE AZ xeroriparian habitats is further complicated by geolocator studies indicating cuckoos regularly use southeastern Arizona for migration (Sechrist et al. 2012, McNeil et al. 2015) and historical accounts of cuckoos in upland vegetation in California during migration (Shelton 1911). Therefore, the spatial extent of breeding in this understudied and potentially significant habitat in SE AZ cannot be reliably assessed because of limited surveys and lack of careful assessment of breeding status in SE AZ xeroriparian habitat.

To evaluate the breeding status and distribution of cuckoos in SE AZ xeroriparian habitat, we first tested whether site occupancy and breeding status as estimated using the standard USFWS-accepted survey protocol (Halterman et al. 2015) accurately reflected breeding status by conducting both protocol surveys and intensive nest searching at a subset of sites in the region. We also conducted opportunistic nest searching in additional sites being informally surveyed. We then reevaluated previously collected survey data and developed a map of known breeding distribution and occupancy in SE AZ xeroriparian habitat.

METHODS

Study area

Our study area included major mountain ranges and foothills between the San Pedro River and the Baboquivari Mountains, but additional historic data collected east of the San Pedro River were compiled and included for analyses of distribution (Fig. 1). The study area lies within the broader, international “Madrean Sky Island Archipelago” region (hereafter, Sky Islands), which consists of prominent mountain ranges separated by desert valleys that extend from the southern terminus of the Colorado Plateau in Arizona and New Mexico to the northern Sierra Madre in Sonora and Chihuahua, Mexico (Brown 1994).

Site selection

We sampled 83 sites within our study area between 2018 and 2020. Each site was located in a vegetated drainage, with a survey transect of 1.5–2.5 km in length following the drainage bottom. To maximize our ability to test whether current survey protocols accurately reflect breeding status, 61 of these sites were selected non-randomly by choosing sites where cuckoos had been previously detected. The remaining 22 sites were selected using a stratified random sampling approach, which was initiated in 2019. Our 2018 survey results, together with eBird (2021) detection data for SE AZ xeroriparian habitat, indicated that the major vegetation associations where cuckoos occurred in July and August (peak nesting period in Arizona; Hamilton and Hamilton 1965, Hughes 2015) were in Apacherian-Chihuahuan Mesquite Upland Scrub, Madrean Encinal, and North American Warm Desert Riparian Forest and Woodland (associations from LANDFIRE 2016). We then randomly selected grid cells having over 50% coverage of one or more of these LANDFIRE vegetation associations from a GIS coverage of the overall study area, with a 2 km x 2 km grid overlay. In each random grid cell, we selected a vegetated drainage bottom to survey. A total of 54 random sites were initially selected, and the first five or six drawn for each mountain range were used for survey sites, with the remaining selections serving as backups in the event we were unable to access the primary sites.

We selected 37 of the 83 sites to be used in our occupancy validation analysis, where we conducted protocol surveys in tandem with intensive nest searching between 2018 and 2020. Seven of these occupancy validation sites were from the randomly selected subset of sites while the rest were sites where cuckoos had been reported previously. In the remaining 46 sites (15 of which were from the randomly selected subset of sites), we either opportunistically documented breeding or conducted surveys without follow-up nest searching (see Mapping Distribution below), but these data were not used in the occupancy validation analysis.

Protocol surveys and nest searching

We surveyed for cuckoos using the currently established USFWS-accepted protocol (Halterman et al. 2015). Surveys consisted of call-playback at 100 m intervals along a pre-established transect that followed the main drainage at each site. To discourage detected cuckoos from following surveyors and inadvertently being double-counted, surveyors traveled a minimum of 300 m from detected birds before using playback again. All detection locations were recorded using handheld GPS, and the total number of individual cuckoos detected was estimated upon completion of each survey visit.

Following positive detections within a site, we conducted additional nest searching surveys to determine breeding status. Surveyors returned to the area of previous cuckoo detections with the goal of locating an active nest or observing other evidence of breeding including copulation, nest building, fledglings, or distraction displays. To avoid disturbance to potentially breeding birds, playback was used minimally, and we maintained a minimum distance of 10 m from birds while tracking individuals. Locations of nests or other breeding evidence were recorded using handheld GPS units. Breeding surveys were undertaken only after cuckoos were detected in a site during protocol surveys. If no cuckoos were detected in a site, no follow-up visits for breeding surveys were made.

The standard protocol (Halterman et al. 2015) calls for a minimum of four surveys per site, spaced 10–15 days apart, with the season lasting from 15 June to 15 August. We followed the protocol when conducting a survey on a single morning, referred to as a “survey visit.” However, at some sites we deviated from the standard protocol by shifting the surveys two weeks later, and at some sites we ceased survey efforts after two or three survey visits if both positive occupancy status and positive breeding status had been established (see Occupancy Validation, below). We refer to any of these survey methods as “protocol surveys.”

Occupancy validation

Upon completion of each survey season (2018–2020), data collected from sites were evaluated to estimate occupancy and breeding status according to an established protocol rubric (Halterman et al. 2015). “Occupancy” as used here refers to an evaluation of whether cuckoos were present at a site consistently during the breeding season; we did not perform a formal statistical occupancy analysis (cf. MacKenzie et al. 2002). We identified “occupied” territories as an area where cuckoos were detected during 2 or more survey periods, with survey visits separated by at least 10 days and with detection locations between surveys no greater than 500 m apart, and/or where evidence of breeding was observed incidentally during a survey visit or a follow-up to a survey visit. We therefore classified a site as “occupied” if one or more occupied territories were identified at that site. Sites were classified as “unoccupied,” and therefore not assigned a breeding estimate, under three scenarios: (1) if no cuckoos were detected during any of at least four survey visits, (2) if cuckoos were detected during only one of at least four survey visits, or (3) if cuckoos were detected in two or more of at least four survey visits but no two detections from separate survey visits were < 500 m apart. We categorized scenarios 2 and 3 as “unoccupied with detection.” These criteria allowed for sites to be included in occupancy validation if they were visited less than four times only if occupancy status had been established or breeding had been confirmed with fewer than four survey visits, as those results constituted a positive validation of the occupancy estimate without full effort. We did not include in our occupancy validation analysis either unoccupied sites with fewer than four survey visits or sites where cuckoos were detected but breeding was not confirmed because no additional follow-up nest searching was conducted.

The primary purpose of this study was to determine and document the occurrence and extent of nesting in these xeroriparian habitats, so we therefore assumed that occupancy of a site by a pair of cuckoos over the course of the nesting season was likely to be associated with attempted breeding. We further assumed that intensive nest-searching at a site where cuckoos were breeding would reliably confirm the breeding attempt. On this basis, each site was classified according to the standard survey protocol designation (Halterman et al. 2015) and its breeding status based on nest-searching efforts. This resulted in five possible classifications:

1. Occupied based on protocol surveys; breeding confirmed based on nest searching.
2. Occupied based on protocol surveys; breeding not confirmed based on nest searching.
3. Unoccupied based on protocol surveys; no breeding surveys due to unoccupied status.
4. Unoccupied with detection based on protocol surveys; breeding confirmed based on nest searching.
5. Unoccupied with detection based on protocol surveys; breeding not confirmed based on nest searching.

We then calculated the total number of each of these five classifications to determine overall proportion of sites in which occupancy status according to standard protocol survey designations accurately reflected attempted breeding, and proportions of sites where standard protocol survey designations of “occupied” or “unoccupied” did not agree with nest searching results (i.e., false positive or false negative). This evaluation, in turn, allowed us to evaluate whether site occupancy, as determined through protocol surveys, could reliably be used to assess extent and distribution of breeding by cuckoos in xeroriparian habitats. A total of 37 sites were included for our occupancy validation analysis, with three sites having two years of survey data, resulting in 40 survey records.

Mapping and evaluating distribution

Results from the occupancy validation were used to reevaluate occupancy estimation results from additional sites we surveyed between 2018 and 2020, as well as previous survey data collected between 2013 and 2020 (U.S. Fish and Wildlife Service, unpublished data) and contributed by other entities, including Tucson Audubon Society, Audubon Southwest, Saguaro National Park, Buenos Aires National Wildlife Refuge, Coronado National Forest, and the consulting firms Moors Biological Services, Archeological Consulting Services, and WestLand Resources Inc. Sites were classified according to their occupancy status based on standard protocol surveys. For sites with multiple years of data, the highest level of occupancy status was used to represent that site’s “occupancy potential” in our map. In some sites, only three survey visits were conducted rather than the four required by the protocol. In these cases, we excluded sites that contained detections on only one of three survey visits and were classified as unoccupied because results from a fourth survey visit could possibly yield detections and elevate the site’s status to occupied. However, we did retain sites classified as unoccupied with three negative survey visits, under the assumption that the fourth survey visit would not yield results sufficient for occupied status. Additionally, some surveyors shifted their survey window two weeks later to ensure their surveys extended into late August. We retained data for all of these sites given that this shifted schedule is intended to capture more of the breeding season in southeastern Arizona (Hamilton and Hamilton 1965). Sites with incidental detections not associated with protocol surveys were not included unless these detections confirmed breeding.

We mapped all sites that could be classified as occupied or unoccupied using the above criteria using ArcGIS (Environmental Systems Research Institute [ESRI] 2023. ArcGIS PRO. Release 3.2.2. Redlands, CA.), differentiating locations with confirmed breeding, occupied sites, and unoccupied sites. We also included polygons of designated critical habitat (USFWS 2021), representing known cuckoo habitat, which primarily consists of rangewide riparian habitat in perennial drainages and contains only a small subset of occupied xeroriparian habitat.

RESULTS

Occupancy validation

Twenty-four (60%) of 40 survey records resulted in an occupied status, with breeding evidence documented in 23 of them. Cuckoos were detected in 2 of 16 survey records classified as unoccupied, with no additional detections or breeding evidence documented in follow-up surveys. In the remaining 14 unoccupied survey records with no detections, unoccupied status was assumed to be valid, and no follow-up surveys or nest searching was conducted, so we therefore did not explicitly test for false negatives. Of the three sites with two years of survey data, one site was occupied in both years, one was unoccupied in both years, and one site was occupied in one year and unoccupied in the subsequent year.

Taken together, these results reflected only a single potential false-positive site (a site designated as occupied based on protocol surveys, but no evidence of breeding based on follow-up nest searching). Protocol surveys at this site (Brown Wash, Baboquivari Mountains) estimated three territories present based on detections in all six protocol surveys conducted between 26 June and 6 September, suggesting the lack of breeding confirmation at this site may have been failure of nest searchers to find the nests rather than absence of breeding. Regardless, these results demonstrated high confidence in the use of existing protocol survey criteria for estimating occupancy and breeding status.

Breeding surveys

In addition to confirmed breeding at the 23 sites used in the occupancy validation analysis, we opportunistically documented breeding at an additional 22 sites. Multiple breeding territories were documented at several of these sites, resulting in a total of 55 known breeding territories (Appendix 1). We documented 24, 26, and 5 breeding territories in 2018, 2019, and 2020, respectively (survey efforts were reduced in 2020). Evidence of breeding included 39 active nests, 11 observations of fledglings/juveniles, 3 observations of copulation, and 4 observations of distraction displays. Nests were placed in several tree species, including oak (Quercus spp.; 14), hackberry (Celtis reticulata; 12), mesquite (Prosopis spp.; 4), juniper (Juniperus spp.; 3), acacia (Senegalia greggii; 2), ash (Fraxinus velutina; 2), yew-leaf willow (Salix taxifolia; 1), and cottonwood (Populus fremontii; 1). Nest monitoring was not a specific goal of our study, but we documented confirmed nesting success in 15 locations (based on observations of fledglings), nest failure in 2 locations, and were unable to determine nest fate in 38 locations where nests were not revisited. The earliest and latest dates of nesting activity were 3 July (active nest) and 11 September (nest with nestling), respectively. The earliest fledgling observation was 29 July, and the latest observation of copulation was 15 August.

Random sites

We surveyed 22 random sites across 7 mountain ranges in our study area. Of these, 68% (15) were occupied, including 6 of 7 random sites used for our occupancy validation. We were unable to survey sufficient random sites per mountain range to evaluate spatial trends in occupancy. However, our observed overall random site occupancy rate of 68% was consistent with our 61 non-random sites, where 70% of sites were occupied (Table 1). When we removed non-random sites from mountain ranges with no random sites, the occupancy rate of non-random sites was 73%. These results indicate targeted survey efforts tracked closely with results from randomly selected sites.

Mapping distribution

We combined data we collected between 2018 and 2020 with available survey and nest searching data collected by other entities between 2013 and 2020 in SE AZ xeroriparian habitat, resulting in data for 163 sites in our southeastern Arizona study area (Fig. 2, Appendix 2). We documented breeding at 46 sites, either based on nest-searching or anecdotal evidence (serendipitous sighting of fledglings, copulation, etc.), and 54 sites were considered occupied based on surveys but were not revisited for nest-searching and had no anecdotal evidence of breeding, totaling 100 sites with confirmed breeding or occupancy. Eighteen sites were considered unoccupied sites with detections, and 45 were considered unoccupied sites with no detections (Fig. 2).

As summarized in Table 1, occupied sites were numerous in the western portion of the Sky Islands, particularly in the region extending from the Baboquivari Mountains in the west, east through the Altar Valley, Atascosa Highlands, Santa Rita Mountains, Patagonia Mountains, Canelo Hills, San Rafael Valley, and finally the western side of the Huachuca Mountains. Some occupied sites were documented in the Whetstone Mountains, Rincon Mountains, and Santa Catalina Mountains, but we observed a higher proportion of unoccupied sites and surveyed fewer overall sites in these ranges. East of the San Pedro River, data contributed from other surveyors resulted in two occupied sites in the Dragoon Mountains and no occupied sites in the Chiricahua Mountains.

DISCUSSION

Previous studies of cuckoo habitat in the western United States stressed the importance of native riparian vegetation along major rivers and streams (Gaines 1974, Girvetz and Greco 2009, Johnson et al. 2017), while information on cuckoo breeding in SE AZ xeroriparian habitat has remained largely anecdotal. Our results expand the known range of breeding cuckoos and identify additional habitat for protection. We also demonstrated strong support for the use of the established survey protocol (Halterman et al. 2015) to determine breeding status in SE AZ xeroriparian habitat by verifying that breeding occurred in 97% (23 of 24) of sites designated as “occupied” using standard survey protocol estimates at our intensively monitored occupancy validation sites. Moreover, the percentage of randomly selected sites that were occupied was similar to that of non-random sites, suggesting our estimates of site occupancy were not biased by a priori site selection. This allowed us to extrapolate the results from the occupancy validation set of sites to other sites and to surveys carried out in other years, providing a larger temporal and spatial scale for assessing cuckoo presence across the Sky Islands region.

We documented 100 occupied sites between 2013 and 2020, including many sites with multiple nests or territories. Given that xeroriparian habitat was not adequately accounted for in previous studies, our results confirm the cumulative abundance of cuckoos using SE AZ xeroriparian habitat represents a significant addition to the known cuckoo population in Arizona. Direct comparisons to other cuckoo studies are difficult, however, because most previous studies occurred prior to the adoption of current survey protocols (Halterman et al. 2015), and used different survey and reporting methods. For example, Corman and Magill (2000) conducted statewide surveys in riparian habitat with detections in 84 of 145 sites, but estimating occupancy was not possible because they only surveyed each site once or twice per season. However, we documented cuckoos in a greater number of sites in the Sky Islands region than Corman and Magill (2000) documented statewide. Moreover, aside from relatively large populations in Arizona and New Mexico, cuckoos are uncommon elsewhere in the western DPS (USFWS 2021). In California, cuckoos have declined precipitously on the Sacramento River, with only 8 and 10 individual detections in 2012 and 2013, respectively, and no estimated territories (Dettling et al. 2015), while relatively small populations have fluctuated on the Kern River (Stanek 2013, 2017). Cuckoos are currently very uncommon in other western DPS states such as Colorado (Beason 2012), Utah (Howe and Hanberg 2000), and Idaho (Coates and Carlisle 2022). Given these notably small populations outside of Arizona and New Mexico, our results underscore the regional importance of SE AZ xeroriparian habitat.

Cuckoo habitat selection in riparian areas has been linked to canopy composition and cover, age class, and vegetation phenology (Gaines 1974, Wallace et al. 2013, Johnson et al. 2017, Stanek et al. 2021). Our results indicate cuckoos in SE AZ use a broader range of vegetative communities, with sites representing diverse physiographies from broad alluvial flats to narrow canyons, and a continuum of riparian conditions, with local vegetation ranging from isolated or narrow patches of riparian trees (cottonwood, willow, ash, walnut, etc.) to xeric ephemeral drainages with only oak, mesquite, and hackberry. The contrast in the xeroriparian vegetative communities occupied by cuckoos as documented here, compared to the cottonwood-willow riparian areas used by cuckoos, is also reflected in our list of nest tree species, with 78% (31 of 40) of nests found in oak, hackberry, or mesquite. In most cases, adjacent uplands contained either Madrean encinal woodland, semi-desert grassland (typically with interspersed shrubs and trees), or desert scrub (Brown 1994), all of which were used for foraging. These results expand the range of geophysical conditions and vegetation communities traditionally considered habitat for cuckoos. Drivers of occupancy in xeroriparian habitat, however, remain poorly understood and merit further investigation.

Geographically, our combined results indicate lower cuckoo occupancy in the northern and eastern portions of the Sky Islands region, with most occupied sites and breeding locations in the southern and western portions of the region. In particular, cuckoos were detected in most drainages surveyed in the Patagonia Mountains, Santa Rita Mountains, Canelo Hills, Atascosa Highlands, and Altar Valley. Many of these sites were located in the upper watersheds of rivers or creeks that support rangewide riparian habitat known to contain high numbers of cuckoos (e.g., San Pedro River, Santa Cruz River, Sonoita Creek). Whether and how often cuckoos move between these riparian and xeroriparian habitats, both within and between years, remains an important question. Importantly, our mapped distribution (Fig. 2) depicts known breeding locations and occupied survey sites and is not an estimate of actual distributional limits. Large geographic gaps in available data include the Baboquivari Mountains (west side), Galiuro Mountains, Winchester Mountains, Santa Teresa Mountains, Mule Mountains, Pinaleño Mountains, and Peloncillo Mountains, lower-elevation drainages in the Chiricahua Mountains, and in sub-ranges, foothills, and valleys between these major mountain ranges. Many of the gaps in survey coverage are either on private property or are remote and difficult to access. Although survey data from Mexico are limited (Macías-Duarte et al. 2015, 2023) and not included in our analyses, they are consistent with our findings in southeastern Arizona, suggesting use of xeroriparian vegetation in ephemeral drainages may extend into the Sierra Madre of Mexico. Therefore, additional survey efforts in under-surveyed areas of the Sky Islands region in both the United States and Mexico will help to further refine distribution and population estimates.

Our results support using existing protocols for estimating occupancy and breeding status based on survey results in xeroriparian habitat. However, we did not explicitly account for imperfect detection probability or test for false negative occupancy estimates, and it is therefore possible that sites classified as unoccupied were actually occupied. Furthermore, our data also suggests that multiple years of survey data may be necessary to reliably estimate occupancy at any given site. For example, in Upper Box Canyon in the Santa Rita Mountains, we discovered three nests in 2019, although no cuckoos were detected during any protocol surveys in 2020. Cuckoo occupancy in riparian habitat has also been shown to fluctuate between survey years (McNeil et al. 2019, Wohner et al. 2021). Therefore, given documented interannual variation in site occupancy and that individual detection probability with surveys is estimated to be 80% (Halterman et al. 2015), caution should be taken when designating sites as unoccupied, especially when only one year of data are available.

Our nesting data indicate cuckoos may often breed through August and as late as September in some sites (Appendix 2). This is consistent with Hamilton and Hamilton’s (1965) findings of cuckoos in southern Arizona nesting later than cuckoos in southern California. Under current protocols, the fourth and final survey could hypothetically be completed on 1 August, potentially resulting in a nesting cycle occurring after surveys have ended. We suggest that for xeroriparian sites in southeastern Arizona, a more accurate breeding window may be captured with an additional late-season survey between 15 and 31 August or shifting the four required surveys approximately two weeks later (1 July–31 August).

Cuckoo populations are believed to have declined in rangewide riparian habitat primarily because of loss and degradation of bottomland riparian vegetation (USFWS 2014). Southwestern riparian ecosystems are projected to continue declining in response to climate change and increased anthropogenic change (Stromberg et al. 2013, Giermakowski et al. 2015, Eastoe 2020). Xeroriparian habitat, often occurring at higher elevation than rangewide habitat, may be less likely to be affected by some of these stressors such as dams and conversion to agriculture and therefore potentially serve as important refugia for cuckoos amid continued declines to bottomland riparian habitat. These xeroriparian drainages have also undergone varied and often significant anthropogenic changes, however, many of which present threats to habitat quality and resilience. For example, although livestock grazed some of our sites, and cuckoos have previously been documented utilizing actively grazed riparian areas (Hamilton and Hamilton 1965), livestock grazing may result in degradation to riparian and xeroriparian habitat (Stromberg 1993, Fleischner 1994, Brock and Green 2003, Goodrich et al. 2018). Alternative grazing practices and exclusion of cattle from riparian and xeroriparian drainages in southern Arizona have resulted in improvements to hydrological and ecological function (Krueper 2003, Beard 2004), and may benefit cuckoo habitat. Likewise, although cuckoos in our study area occupied sites that had experienced historical mining activity, modern industrial mining often occurs on a much larger scale with greater potential impacts to watershed hydrology, geochemistry, and habitat quality through direct habitat destruction, dewatering of aquifers, or otherwise redirecting or altering flows in drainages (Lewis and Burraychak 1979, Brock and Green 2003). Finally, climate change may act to increase aridification of southwestern uplands and exacerbate the risk of drought and fire, posing a threat to the resilience of SE AZ xeroriparian habitat (Bock and Bock 2014, Friggens et al. 2014). In spite of these potential threats, the number of breeding cuckoos we documented in these varied xeroriparian habitats, and the fact that many of those birds bred successfully, indicates these areas represent an important addition to known habitat for western Yellow-billed Cuckoo conservation and management.

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