Summary
Black-capped Chickadees (Parus atricapillus) sing a two-note song termed fee bee. Frequency descends within the fee note and between the fee and bee notes. Within-song frequency ratios are highly invariant, while among songs, absolute frequency is variable. We conducted sound-transmission studies to analyze how Black-capped Chickadee songs degrade in different native habitats. We then subjected colour-banded territorial males to playback of undegraded and degraded songs to see if they could estimate the distance to a conspecific singer by means of degradation cues. In transmission studies conducted in an open field and a dense deciduous forest, significant effects of reverberation and amplitude modulation were measured for songs at three different frequencies. A new technique, cross-correlation of song spectrograms, allowed for a measure of holistic song degradation, which varied significantly over distance in open versus forested habitat. Polynomial regressions for the effects of reverberation, amplitude modulation, and cross-correlation measures of general degradation revealed different trends in open versus forested habitat, regardless of absolute frequency. Male Black-capped Chickadees did not differ in their response to playback of undegraded and degraded songs when the two types were broadcast from the same position in the territory at the same amplitude. We suggest that heterogeneity of Black-capped Chickadee native habitat may preclude the use of song degradation as a reliable distance cue.
Methodology
We conducted song-transmission studies on April 27-28, 1992, from 12:00 to 18:00 EST in an open field and a dense deciduous forest near the Queen's University Biological Station, Chaffey's Lock, Ontario, Canada (45"N,76"W). In the open habitat, low-lying grasses and staghorn sumac (Rhus typhina) were the dominant vegetation, with occasional dead trees. Over three randomly sampled 64-m2 quadrats there was an average of 100 sumac trees, 62% with diameter at breast height (dbh) >3 cm and 38% with dbh < 3 cm, in addition to grasses (height < 10 cm). All sumac was less than 3 m in height. In the forested habitat, the dominant canopy trees were red oak (Quercus rubra), largetooth aspen (Populus grandidentata), sugar maple (Acer saccharurn), American white birch (Betula papyrifera) , and beech (Fagus g.rand&olia) . In three randomly sampled quadrats, there was an average of 31 trees per 64 m2 with dbh > 8 cm, in addition to a dense understory of herbaceous plants (height <20 cm). Of the dominant canopy trees sampled, 41 % were red oak, 22% largetooth aspen, 19 % sugar maple, 12 % American white birch, and 6% beech (at this point in the season, the leaves had not yet come out). The terrain was flat in both the open and forested areas. Both transmission sites were typical of Black-capped Chickadee habitat in southeastern Ontario; in both habitats, several Black-capped Chickadee males defended territories in the early spring and bred successfully in natural cavities (personal observation). Transmission studies were carried out during calm and clear weather conditions.