The active space of red-winged blackbird song

Summary

Active space, that distance from the source over which signal amplitude remains above the detection threshold of potential receivers, was determined for Red-winged Blackbird (Agelaius phoeniceus) song in an upland pasture near Ithaca, New York. Song amplitude, the rate of signal attenuation, the amplitude of ambient noise, and the sensitivity of redwings to song masked by noise determine active space and were measured in the field:

• 1. Maximum root-mean-square song amplitude at 1 m from a singing bird ranged from 88.5–93.5 dB SPL ($\bar X$± SE = 90.8±0.21 dB SPL).

• 2. Close to the source, song attenuates at a rate which closely matches that predicted by spherical spreading (6 dB/doubling of distance) alone. At distances beyond about 30 m, however, excess attenuation becomes important (Fig. 4). The rate of attenuation varies with relative wind direction (Table 1).

• 3. Ambient noise level is relatively low in early morning, rises in late morning and afternoon as air turbulence increases, and then drops again in the evening as turbulence decreases (Table 2 and Appendix). Measured on a 4 kHz octave filter scale (center frequency = 4 kHz, octave pass-band = 2.83– 5.66 kHz), noise during one day ranges from 15 dB SPL (at 06∶00 h) to 36 dB SPL (at 11∶00 h), more than a 10-fold variation in ambient sound pressure.

• 4. A 3 dB signal to noise ratio is sufficient for detection of song masked by noise in the field (Figs. 5 and 6). Also, redwings are able to detect differences in this ratio as small as 3 dB.

• 5. In the absence of wind, the maximum active space of redwing song is about 189 m for a signal SPL of 93.5 dB at 1 m. This distance varies with changes in ambient noise level during the day, and with relative wind direction.

Maximum measurements of active space correspond closely to the maximum distances across 2 individual upland redwing territories. This suggests that redwing song is adapted in its structure and amplitude at the source to communicate a male's presence throughout the extent of his neighbors' territories.