Unveiling factors that determine abundance and distribution of endangered wildlife species has relevant implications in their conservation across international boundaries. For instance, the Western Distinct Population (as defined by the U.S. Fish and Wildlife Service) of the yellow-billed cuckoo Coccyzus americanus (Linnaeus, 1758) has disappeared in most of the species’ range across the western United States and southwestern Canada, but little is known about the conservation status at the southern edge of its breeding distribution in Mexico. To fill this information gap, we estimated abundance and occupancy rates of yellow-billed cuckoos using a standard broadcast call survey protocol. We used Bayesian spatial count models to estimate cuckoo population density at survey sites. We used Bayesian hierarchical models to estimate the effects of geography, climate, and vegetation on occupancy rates while accounting for imperfect detection. Mean cuckoo count per transect for all sites was C = 9.00 ± 0.45 cuckoos. Overall cuckoo density was D  = 13.18 cuckoos/km² (SD(D ) = 5.61 cuckoos/km²). Overall cuckoo occupancy in Sonora was φ = 0.538 (95%CrI(φ) = 0.544–0.600) but showed strong geographic variation. Relatively high occupancy levels suggest yellow-billed cuckoo populations in Sonora may be robust, but they are largely reliant on declining high-tree cover.

We surveyed for cuckoos following the standard broadcast survey protocol for the species (Halterman et al. 2015) in the summers of 2015–2019 (5 years), but we reduced the number of surveys per sampling unit. We surveyed each site twice per year, with one survey occurring in each of the second and third standard survey periods. We reduced the number of visits per site to increase the number of sites surveyed. We conducted our first survey period from site from late June to late July and our second survey period from late July to no later than 16 August. For one upland site, we were denied access to conduct the second survey, and we instead surveyed the closest alternate upland site. We started the first survey period at the onset of the monsoon rains, which meant that at some sites, the rain effects of a greening vegetation were not always evident during the survey. 

At each site, we surveyed for cuckoos using point counts placed on transects along the riparian gallery forest or desert arroyo vegetation, keeping as close to the river channel as possible. For upland sites, we used dirt roads, cattle tracks, or dry drainages. We began surveys just before sunrise and continued until 11:00, until temperatures reached 40°C, or until the surveyors identified potential human health risks, such as heatstroke or dehydration. The broadcast consisted of 5 contact/kowlp calls (70 db), each spaced one minute apart. We identified cuckoos by sight using size, form and plumage characteristics listed by Hughes (2020). We identified cuckoos by ear when we heard the contact/kowlp or coo call (Halterman et al. 2015), usually as a response of broadcasted contact/kowlp calls. Most cuckoo detections were from birds heard but never seen. We arrived at the broadcast-point and waited at least one minute to detect cuckoos. When we detected no cuckoos during the initial listening period, we began the first broadcast. When we detected no cuckoos at the broadcast-point after five broadcast calls, we continued 100 m along the transect and started a new broadcast as described above. After we detected an individual or a pair, we moved 300 m further along the transect before resuming the survey. When we encountered cuckoos between broadcast points (i.e., an unsolicited detection while traveling between broadcast points), we stopped and recorded all information in the same manner as if the detection had been made during a broadcast.

Literature cited

• Halterman, M., Johnson, M. J., Holmes, J.A., and Laymon, S.A. 2015. A Natural History Summary and Survey Protocol for the Western Distinct Population Segment of the yellow-billed cuckoo. U.S. Fish and Wildlife Techniques and Methods, 45 p.
• Hughes, J. M. 2020. Yellow-billed cuckoo (Coccyzus americanus), version 1.0. In Birds of the World (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, New York.

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