Southwest Biological Science Center
Yellow-billed Cuckoo ©Bob Steele
This 2006 annual report details the first season of a 2-year study documenting western yellow-billed cuckoo (Coccyzus americanus occidentalis) distribution, abundance, and habitat use throughout the Lower Colorado River Multi-Species Conservation Plan boundary area. We conducted cuckoo surveys at 55 sites within 17 areas, between 11 June and 13 September. The 243 visits across all sites yielded 180 yellow-billed cuckoo detections. Cuckoos were detected at 27 of the 55 sites, primarily at the Bill Williams River National Wildlife Refuge AZ sites (n = 117 detections) and the Grand Canyon National Park–Lake Mead National Recreation Area AZ delta sites (n = 29 detections). There were also cuckoos at the Gila River–Colorado River Confluence, AZ (n = 9), Overton Wildlife Management, NV area (n = 7), and Limitrophe Division North, AZ (n = 6); however, at these sites the numbers were much lower and very few of these birds were considered to be paired or breeding. The greatest number of detections (n = 79) occurred during the second survey period (3–23 July). In 2006, we confirmed five breeding events, including one nesting observation and sightings of four juveniles; all confirmed breeding was at the Bill Williams River NWR and Grand Canyon NP–Lake Mead NRA delta sites. The breeding status of most of our detections were unknown, however, we observed 17 adult cuckoos carrying nest material or food and 40 cuckoo detections were detected while counter-calling occurred in same area during repeated surveys.
We used playback recordings to survey for western yellow-billed cuckoos. Compared to simple point counts or surveys, this method increases the number of detections of this secretive, elusive species. It has long been suspected that cuckoos have a fairly low response rate, and that the standard survey method of using playback recordings may fail to detect all birds present in an area. In 2006, we found that the majority (72%) of cuckoo detections were solicited through playback at all study sites. The number of solicited detections peaked during the first half of July and then declined as the breeding season progressed, while the number of unsolicited detections (cuckoos heard calling before playback was initiated) remained fairly constant. The majority (64%) of cuckoo detections, solicited or unsolicited, were aural; 27 percent were both heard and seen and nine percent were visual detections only. Cuckoos in areas with the largest populations had the highest rate of vocalizations before playback or after the first broadcast. In contrast, more than half the responses at sites with fewer cuckoos (with < 10 detections per site) first occurred after three or more playback recordings. This type of baseline information will be used to help refine the survey protocol for 2007, and to create hypotheses that can serve as the foundation for a full-scale evaluation and optimization of this survey technique.
Our preliminary analysis of vegetation data from occupied and unoccupied sites in 2006 focused on general patterns in the distribution and abundance of woody species. The density and composition of woody riparian vegetation varied considerably among the study areas. Much of the variation in tree density was due to the patterns of abundance of trees in the smallest size class (< 8 cm dbh). The dominant tree species at the cuckoo survey sites were cottonwood, willow, and tamarisk. Tamarisk was the most common tree, due to the abundance of small (< 8 cm dbh) individuals. When occupied and unoccupied sites were compared, occupied sites tended to have higher average canopy cover, attributable to higher average cover of the mid and low canopy. The dominant canopy at occupied sites most often consisted of cottonwood or willow trees. In addition, occupied sites in most areas had lower than average total tree density whereas unoccupied sites were denser than average. When densities of trees in different size classes were compared between occupied and unoccupied sites within areas, it appeared that cuckoos did not use areas with the highest density of small trees (< 8 cm dbh), mostly tamarisk.
We also measured microclimate variables (temperature, relative humidity, soil moisture) at occupied and unoccupied sites. Microclimate sampling in 2006 was delayed due to equipment procurement difficulties, so our preliminary conclusions are based on late-year data only; conclusions and patterns may change as new data (especially from the early season) are collected in 2007. Microclimate measurements at Grand Canyon NP–Lake Mead NRA and Bill Williams River NWR showed that locations occupied by yellow- billed cuckoos were generally slightly cooler and more humid than unoccupied sites. This was not true at Cibola NWR, where only mean nocturnal temperature was lower at occupied sites. On average, soil moisture was slightly higher at occupied cuckoo locations. Although microclimate conditions may play a significant role in cuckoo habitat selection or breeding ecology, the factors underlying the microclimate conditions in riparian patches are not currently known.
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