The yellow-billed cuckoo is a long-distance migrant that breeds in low, dense scrub near streams, marshes, and wetlands within otherwise open woodlands (Hughes 1999). Yellow-billed cuckoos are among the most common birds recorded in floodplain habitats along the Mississippi River and are found in both young cottonwood-willow stands and mature silver maple forests (Knutson and others 2005). The species exhibits some area-sensitivity. Conner and others (2004) found yellow-billed cuckoos most abundant in riparian strips >70 m wide, and Aquilani and Brewer (2004) recorded greater abundances of yellow-billed cuckoos in forest tracts >55 ha.
Breeding success is correlated with insect outbreaks, particularly those of hairy caterpillars, and population densities vary greatly with food supply. Nests are located in dense, broad-leaved, deciduous shrubs or trees within 10 m of the ground. Twedt and others (2001) did not find a difference in nest success between bottomland hardwoods and cottonwood plantations, nor did Wilson (1999) find a difference in stands subject to alternative thinning rates in Arkansas. Based on anticipated harvest scenarios, Klaus and others (2005) predicted yellow-billed cuckoo numbers would decline ~37 percent on the Cherokee National Forest over the next 60 years.
The habitat suitability model for the yellow-billed cuckoo contains seven parameters:
- successional age class
- edge occurrence
- midstory (11–25 cm d.b.h.) tree density
- percent forest in the landscape (10-km radius)
- forest patch size
The first suitability function combines landform, landcover, and successional age class into a single matrix (SI1) that defines unique combinations of these classes
. We then directly assigned habitat suitability index scores to these combinations based on yellow-billed cuckoo habitat associations outlined in Hamel (1992). We increased suitability index scores within floodplain/valley and terrace-mesic landforms to account for the higher abundance of yellow-billed cuckoos on these sites in the CH and WGCP.
Yellow-billed cuckoos are more abundant within edge (SI2) habitats than forest interiors (Kroodsma 1984). We used a 9 × 9 moving window to identify habitat edges and assumed these locations represented optimal habitat (suitability index score = 1.000). Nevertheless, non-edge habitats are also used by yellow-billed cuckoos so we assigned these sites only a slightly lower suitability index score (0.667;
Yellow-billed cuckoos breed in forest stands containing well-developed midstories (SI3). We fit a quadratic function
to data from Annand and Thompson (1997) on the relative densities of yellow-billed cuckoos in stands with different midstory tree densities
to predict how suitability index scores responded to changes in this habitat variable.
Although a forest-breeding species, yellow-billed cuckoos are associated with fragmented landscapes (Robbins and others 1989, Hughes 1999). We assumed 70–80 percent forest in a 10-km landscape (SI4) was characteristic of ideal habitat (suitability index score = 1.000;
and fit a function that reduced suitability index scores symmetrically as forest compositions departed from these ideal proportions
. Nevertheless, cuckoos exhibit area-sensitivity and may be absent or at low densities in small fragments (Robbins and others 1989, Bancroft and others 1995, Hughes 1999). Therefore, we used data from these sources to derive a logistic function
that quantified the relationship between yellow-billed cuckoo habitat suitability and forest patch size (SI5;
To calculate the overall suitability index score, we determined the geometric mean of SI scores for forest structure (SI1 and SI3) and landscape composition (SI2, SI4, and SI5) separately and then the geometric mean of these means together.
Overall SI = ((SI1 * SI3)0.500 * (SI2 * SI4 * SI5)0.333)0.500