Diurnal and nocturnal feeding rate in Kentish plovers Charadrius alexandrinus on an intertidal flat as recorded by telescopic video systems
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To elucidate the effects of light, site, temperature, time after emersion, and wind speed on foraging attempt rate (FAR) (attempts/unit time) and feeding success (FS) (captures/attempts) in the obligate visual foraging shorebird, Kentish plovers Charadrius alexandrinus, field observations were performed at a sandflat in Tokyo Bay, using a visible-light telescope camcorder during the day and a thermal infrared telescope camcorder at night. The re-analysis capability and frame-step replay of highly magnified video-images can contribute to accurate measurement of feeding behaviour over conventional focal observation techniques. Kentish plovers increased both FAR and FS in areas of high prey (polychaetes and crabs) density, resulting in a synergistic increase in feeding rate (captures/unit time). In areas of high prey abundance, FAR was higher at lower wind speed. FS increased with increasing time after emersion. Increasing temperatures induced a positive effect on FAR but a negative effect on FS. The effect of light on FAR was not observed; however, time-to-defecation occurrence was 2.2-fold shorter at night than during the day, indicating that feeding rate and FS are higher at night. These results are attributable to an increase in availability of cues due to higher nocturnal activity in polychaetes. Since available foraging time (emersion time) at night was 1.7-fold longer than during the day in the present study period, Kentish plovers could capture 3.7-fold (2.2 × 1.7) more prey at night than during the day. Results from this study indicate that nocturnal feeding in overwintering plovers is not a compensation but a major foraging activity to meet their energy requirements.
KeywordsPolychaete Prey Density Intertidal Flat Feeding Success Defecation Rate
I thank E. Miyoshi for help during fieldwork, A. Kuwae and N. Oura for assistance on video image analysis, S. Harada and O. Masuko for video system information, and H. Shimura and K. Ozaki for shorebird information at the site. I also thank R. W. Elner, R. C. Ydenberg, K. J. Mathot, A. C. Pomeroy, and R. W. Butler for valuable comments. The manuscript was greatly improved by the valuable comments of three anonymous reviewers. This research was supported by grants to T.K. from the Port and Airport Research Institute Exploratory Research and the Japan Society for the Promotion of Science (JSPS) Postdoctoral Fellowships for Research Abroad (FY2004).
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