Does Human Intrusion Alter the Seasonal Timing of Avian Song during Breeding Periods?

Abstract

ABsTRAcr.-The seasonal timing of male song during breeding affects the timing of territory establishment, mate attraction, pair formation, egg laying, and transmission of information about breeding songs to young. Thus, alteration of the seasonal timing of song may influence reproductive success and survivorship. Human intrusion has the potential to influence avian singing behavior. Accordingly, we determined whether repeated intrusion affected the seasonal timing of male song for three passerines (Ruby-crowned Kinglet [Regulus calendula], Yellow-rumped Warbler [Dendroica coronata], Dark-eyed Junco Uunco hyemalis]) in Wyoming forests. Intrusions involved one person walking through habitats for 1-2 h (Snowy Mountains, 1989-1993) or for 5 h (Pole Mountain, 1991-1993) each week for 10 consecutive weeks of each breeding season. For most comparisons, we did not detect a significant difference in the seasonal timing of singing activity between control and intruded sites. Two exceptions were: (1) the mean singing date for Ruby-crowned Kinglets was 11 days earlier on intruded sites than on control sites at Pole Mountain; and (2) the proportion of intruded sites with singing by Yellow-rumped Warblers (0.40) was smaller than that for control sites (1.00) during midJune at Pole Mountain. Moderate and large differences in the timing of singing activity were detectable with a probability of 0.80, but small differences were not reliably detectable. Thus, our results are a conservative estimate of the effects of intrusion on the seasonal timing of song. Curtailment of singing on intruded sites may have reduced breeding activity and, consequently, the quality of those sites for producing young. Because intrusion-induced effects often are context-specific and can vary among individuals and species, and because our results are conservative, avian ecologists should continue to assess whether intrusion influences seasonal patterns of singing activity. Received 13 May 1996, accepted 12 September 1996.