RECURRENT INGESTION OF GRIT BY COMMON NIGHTHAWKS (CHORDEILES MINOR)

Abstract

Grit ingestion is widespread among birds, as an aid for grinding up food and as a dietary supplement of calcium, but is less common among insectivores, especially aerial feeders like nightjars (Caprimulgidae). I report recurrent visits by Common Nighthawks (Chordeiles minor) to a gravel driveway in British Columbia at dusk over a 4-y period. These ground visits were highly consistent (nighthawks landed during 93% of 72 dusk watches) and visits were highly concentrated both in time (95% of time on the ground between 10 and 57 min after sunset) and space (91% of the birds' time was spent in 2 areas comprising only 31% of the visible study site). Birds pecked at the ground in 97% of 198 focal-animal watches. In 43 visits the nighthawks picked up or swallowed some item; in 24 cases this was a small piece of gravel and no other items were identified. Surveys of ground arthropods in the study area over 3 y showed that potential prey were sparse (mean <2 insects m–2) and the most common insects were unsuitable prey (tiny 3-mm ants). I conclude that the only reason for the nighthawks to consistently visit this gravel area was to ingest grit. The gravel ingested was crushed limestone, which might support a calcium uptake hypothesis. Grit ingestion, occurring within a restricted dusk period, might not explain why nightjars often sit on gravel roads at night, which is a major source of mortality for several species. Information on ground behavior in Caprimulgidae is important for understanding their mortality risks and conservation.