Energetic consequences of plunge diving in gannets

Abstract

Seabirds that forage by plunge diving dive less frequently than those that dive from the water surface, and spend less time in flight than more generalist foragers. We hypothesised that this is due to foraging by plunge diving entailing a high energetic cost, which in turn is due to high energetic costs of take-off and flight. Using heart rate as a proxy for metabolic rate, we evaluated the energetic costs of foraging by plunge diving in the Australasian gannet Morus serrator. As expected, flight entailed a high energetic cost, and energy expenditure during foraging was equivalent to that during flight and significantly higher than that when animals were resting during foraging trips or were inactive on land. These values represent the highest costs of foraging yet recorded in a seabird, and the low frequency of plunge diving can be attributed to these high costs. On average, Australasian gannets perform 2.6 dives h -1 when foraging, with a mean dive duration of 3.4 s. As a result, they spend < 0.25% of the duration of each foraging trip submerged. We combined this information with previously obtained data on diet to calculate an estimated rate of prey capture of ~287 g (min submerged) -1 . This rate is at least 7 times greater than rates recorded in other diving birds. For plunge divers, therefore, the high costs of foraging are offset by high rates of energy gain.

Heart rate and estimated energy expenditure of flapping and gliding in black-browed albatrosses

“…The high heart rate during and shortly after landing might have been due to feeding activity. Therefore, it seems reasonable that landing and takeoff, although energetically expensive, are essential parts of foraging (Green et al, 2009). …”

“…Heart rate can reflect the rate of oxygen consumption when both the stroke volume of the heart and the extraction of oxygen by the body tissues change in a systematic fashion (Fick, 1870). Although these values may change and thus the relationship may not be linear, heart rate is correlated with the rate of oxygen consumption under most conditions (Butler et al, 2004;Green et al, 2009;Green, 2011). Thus, once the relationship between heart rate and the rate of oxygen consumption is established, properly calibrated and validated, heart rate can serve as a good indicator of energy expenditure.…”

An increase in minimum metabolic rate and not activity explains field metabolic rate changes in a breeding seabird

“…This was particularly important in the present study for two reasons. Firstly, it meant that it was not necessary to define an unambiguous continuous timebudget for the gannets as this has previously been shown to be impossible with this data set (see Green et al, 2009c). Secondly, it allowed us to account for the fact that calibration data could not be obtained from flying or swimming birds.…”

Diet composition and provisioning rates of nestlings determine reproductive success in a subtropical seabird

“…Österblom et al (2008) suggest that the negative influence of lower-energy food items is particularly pronounced in certain species of seabirds, especially species specialized to carry single prey items or small masses of prey, species with energetically expensive foraging strategies, and species with low digestive efficiency. Although plunge-diving is energetically demanding (Green et al 2009), pelicans are able to capture and carry large volumes of prey, which may allow them to buffer the effects of reduced prey quality by increasing prey quantity with minimal increases in foraging effort. It is interesting to note the apparently opposing relationship between meal delivery rates and meal mass, the 2 primary drivers of nest productivity.…”