Overnight foraging trips by chick‐rearing Nazca Boobies Sula granti and the risk of attack by predatory fish

Abstract

Most tropical booby species complete breeding foraging trips within daylight hours, thus avoiding nights at sea. Nazca Boobies Sula granti are unusual in this respect, frequently spending one or more nights away from the nest. We used GPS dataloggers, time‐depth recorders, and changes in body weight to characterize foraging trips and to evaluate potential influences on the decisions of 64 adult Nazca Boobies to spend a night at sea, or to return to their chicks on Isla Española, Galápagos, in daylight hours. The tagged birds foraged east of Isla Española, undertaking both single‐day (2–15 h, 67% of trips) and overnight trips (28 h–7.2 days, 33%), and executing 1–19 foraging plunge‐dives per single‐day trip. Birds might forage longer if they are in nutritional stress when they depart, but body weight at departure was not correlated with trip length. Birds might be expected to return from longer trips with more prey for young, but they returned from single‐day and overnight trips with similar body weights, consistent with previous indications that Nazca Boobies forage until accumulating a target value of prey weight. Birds with a lower dive frequency during the first 5 h of a trip were more likely to spend the night at sea, suggesting that they might choose to spend the night at sea if prey capture success was low. At night, birds almost never dived and spent most of their time resting on the water’s surface (11.8–12.1 h, > 99% of the time between civil sunset and civil dawn). Thus, the night is an unproductive time spent among subsurface predators under low illumination. The birds’ webbed feet provided evidence of this risk: 24% of birds were missing > 25% of their foot tissue, probably due to attacks by predatory fish, and the amount of foot tissue lost increased with age, consistent with a cumulative risk across the lifespan. In contrast, other tropical boobies (Blue‐footed Sula nebouxii and Brown Boobies Sula leucogaster), which do not spend the night on the water, showed no such damage. These results suggest that chick‐rearing Nazca Boobies accept nocturnal predation risk on occasions of low prey encounter during a foraging trip’s first day.

Pacific Decadal and El Niño oscillations shape survival of a seabird

“…ENSO warm events are associated with lower primary productivity and some studies already documented how it affects survival of juvenile seabirds (i.e., Blue-footed Boobies S. nebouxii; Oro et al 2010). Nazca Booby adults forage at fish concentrations in association with other large predators, including some tunas and sharks (Au andPitman 1986, Anderson andRicklefs 1987) that could maim or kill Nazca Boobies (Zavalaga et al 2012). While a negative influence of warm events on fish biomass is well documented (Bertrand et al 2004), a positive effect of cool events is less evident, and high fish catches appear not to be related directly to higher primary productivity (Carr 2001).…”

“…We obtained monthly values , smoothing them with a 12-month filter from the onset of the Nazca Booby breeding season (October; data available online). For each month, we averaged nine values in the 2°9 2°block between 0.5°S and 2.5°S and 87.5°W and 89.5°W, matching known foraging areas of chick-rearing Nazca Boobies from Isla Española (Zavalaga et al 2012). Annual values reflect the duration of ENSO warm or cool events.…”

“…The Nazca Booby Sula granti is relevant as a biological model because it is endemic to the Eastern Tropical Pacific ecosystem (Friesen et al 2002) and feeds offshore on pelagic fish whose stocks are sensitive to climate oscillations (Anderson and Ricklefs 1987, Anderson 1989, Zavalaga et al 2012. The Nazca Booby Sula granti is relevant as a biological model because it is endemic to the Eastern Tropical Pacific ecosystem (Friesen et al 2002) and feeds offshore on pelagic fish whose stocks are sensitive to climate oscillations (Anderson and Ricklefs 1987, Anderson 1989, Zavalaga et al 2012.…”

Demographic effects of artificial nighttime lighting on animal populations

“…The light cycle may pose marked constraints on feeding time. Thus, when predominantly diurnal organisms forage at night, or predominantly nocturnal organisms forage during the day, they can experience increased predation pressure (Fenn and MacDonald 1995;Zavalaga et al 2012); predation pressure often shapes activity patterns of night feeding species in response to changes in the lunar cycle (e.g., Daly et al 1992;Skutelsky 1996;Kramer and Birney 2001); and when predation pressure is alleviated some organisms expand or change their temporal niches (e.g., Fraser et al 2004;McCauley et al 2012).…”

Geographical variation in the foraging behaviour of the pantropical red-footed booby

“…Sharks may rely on vision to detect seabirds on the surface, implying that resting at night may not be a high-risk behaviour. Birds could then afford nocturnal predation risk, for example, in cases of low prey encounter during the previous day (Zavalaga et al 2012). …”