Nocturnal courtship, copulation, and egg-laying in the western grebe (Aechmophorus occidentalis) and Clark’s grebe (Aechmophorus clarkii)

Abstract

Grebes (Podicipedidae) are primarily diurnal although migration occurs at night and some species forage opportunistically at night. We used motion-activated cameras to study nocturnal reproductive activities of the western grebe, Aechmophorus occidentalis, and Clark’s grebe, Aechmophorus clarkii, at Clear Lake, California, USA, during 2014–2016. During 9283 h of monitoring, 19.4% of rushing displays (n = 75) and 8.4% of copulations (n = 435), plus two weed dancing displays and four instances of egg-laying, occurred at night. Nocturnal copulation and egg-laying often occurred on empty, recently abandoned nests, but some incubating pairs of grebes also copulated at night. Nocturnal reproductive activities occurred more frequently at nests attached to submergent vegetation in open water far from shore when water levels were low than on nests attached to emergent vegetation close to shore when water levels were high, probably due to a reduced risk of predation by nocturnal mammals farther from shore. Nocturnal reproductive activities were not in response to elevated light levels at night, daylight disturbances, diurnal predation, high temperatures on hot days, or sexual interference during daylight. Instead, grebes attempted to increase mating investment and minimize parental investment by exploiting recently abandoned nests at night, and perhaps by undertaking extrapair copulations under the cover of darkness, but some pairs of grebes may simply extended routine reproductive activities into the night. Reproductive activities are probably limited at night by the need to feed during daylight and sleep at night, and by the increased risk of predation by mammals at night. Given the high costs of construction and maintenance of floating nests and the obvious benefit of quickly exploiting abandoned nests, nocturnal reproductive activities may occur more frequently in grebes and other waterbirds using floating nests than in other groups of birds whose reproductive activities occur mostly during daylight.