Lunar Influence on Foraging and Nesting Activity of Common Poorwills (Phalaenoptilus nuttallii)

Abstract

-Whip-poor-wills (Caprimulgus vociferus) forage by from a perch, are active during periods of the night with moonlight, and synchronize hatching with the lunar cycle. In contrast, Common Nighthawks (Chordeiles minor) forage during continuous flight and do not increase activity during moonlit periods or synchronize their breeding activities with the lunar cycle. We used data on the foraging activity and nest timing of the Common Poorwill (Phalaenoptilus nuttallii) to test the hypothesis that moonlight influences the activity patterns and nest timing of this caprimulgid. If foraging strategy alone influences the response to the lunar cycle, activity by Common Poorwills should be correlated with lunar light levels and hatching should be synchronized with the lunar cycle. Our results support the general prediction that lunar light allows poorwills to forage during periods of the night. However, contrary to expectations, increased foraging activity was correlated only with increasing moon height and not the percent of the moon face illuminated. Furthermore, the nesting cycle was not synchronized with the lunar cycle. We conclude that knowledge of foraging style alone is not sufficient to predict the nature of lunarphilia by goatsuckers. We suggest that habitat choice, duration of twilight, the number of breeding attempts, and the physiological ability to enter torpor also may be important factors mediating lunar influence on foraging activity. Received 19 April 1991, accepted 13 January 1992. MOONLIGHT depresses nocturnal activity in many animals including crustaceans (e.g. Wolcott and Wolcott 1982), insects (e.g. Williams and Singh 1951), small mammals (e.g. Clarke 1983), bats (e.g. Fenton et al. 1977, Morrison 1978, Reith 1982), and birds (e.g. Watanuki 1986, Nelson 1989). An increased risk of predation often is cited as the reason for this response (Lockard and Owings 1974, Fenton et al. 1977, Morrison 1978, Watanuki 1986, Nelson 1989). In contrast, some birds increase activity during the bright phase of the lunar cycle (e.g. nocturnal singing by both nocturnal and diurnal birds; Cooper 1980, Elliot 1983, Barclay et al. 1985). To date, however, the general influence of moonlight on the behavior of visually orienting nocturnal predators is poorly understood. Intuitively, we would expect an increased level of activity with increased lunar light levels. The Whip-poor-will (Caprimulgus vociferus) is a nocturnal insectivore that exhibits increased locomotory, vocal, and nest-visiting activity during periods of bright moonlight (Mills 1 Present address: Department of Biology, University of Regina, Regina, Saskatchewan S4S OA2, Canada. 1986). Mills (1986) found that the nesting cycles of Caprimulgus species are synchronized with the lunar cycle, such that the first two weeks of the nestling period coincide with the period with the most moonlight. Observations of Caprimulgus species in North America, Africa, and Europe also indicate that moonlight positively influences singing behavior (Wynne-Edwards 1930, Brauner 1952, Cooper 1980, Jackson 1985), leading Mills (1986) to suggest that caprimulgids are lunarphilic, meaning that they are limited to foraging crepuscularly (dusk and dawn; Martin 1990) and during periods of the night with moonlight. In contrast to the lunarphilic response of Caprimulgus species, Common Nighthawks (Chordeiles minor) only forage crepuscularly, and not during the night, regardless of moonlight levels (Aldridge and Brigham 1991, Brigham and Fenton 1991). Furthermore, there is no evidence for synchrony of hatching dates in C. minor or C. acutipennis nests with the lunar cycle (Mills 1986). The different foraging styles of Chordeiles and Caprimulgus could explain the differences in nesting synchrony and activity periods. Whippoor-wills attack flying insects in upward-directed sallies of short duration from a perch or the ground (a sit-and-wait or sallying strategy; Mills 1986). Anecdotal evidence suggests