Foraging energetics of arctic cormorants and the evolution of diving birds

Abstract

Efficient body insulation is assumed to have enabled birds and mammals to colonize polar aquatic ecosystems. We challenge this concept by comparing the bioenergetics of cormorants (Phalacrocorax carbo) living in temperate and arctic conditions. We show that although these birds have limited insulation, they maintain high body temperature (42.3 °C) when diving in cold water (1–10 °C). Their energy demand at these times is extremely high (up to 60 W kg−1). Free‐living cormorants wintering in Greenland (water temperature −1 °C) profoundly alter their foraging activity, thus minimizing time spent in water and the associated high thermoregulatory costs. They then meet their daily food demand within a single intense dive bout (lasting 9 min on average). Their substantial energy requirements are balanced by the highest predatory efficiency so far recorded for aquatic predators. We postulate that similar behavioural patterns allowed early diving birds (Cretaceous) to colonize cold coastal areas before they evolved efficient insulation.

Dive depth and plumage air in wettable birds: the extraordinary case of the imperial cormorant

“…120 min for P. albiventer [Kato et al 2000] and 194 min for P. nivalis [Green & Williams 1997], and up to a maximum of 390 min [our data]). This contrasts starkly with some other cormorants, notably P. carbo, which is known to contain very little plumage air (170 ml kg -1 ) (Gremillet et al 2005b) and is limited to a mean daily foraging time of just 9 min in water at -2°C (but will forage for a daily mean of about 45 min at 8°C and 150 min at 13°C) (Gremillet et al 2001, but see Gremillet et al 2005a). We note again that European shags (which do not belong to the blue-eyed complex) appear atypical, and may spend up to 7 h d -1 in northern European waters at temperatures around 9°C (Daunt et al 2006).…”

“…However, deep diving must be balanced by appropriate prey densities to make the increased transit time between the surface and bottom profitable. Future studies could address the rates of prey acquisition in cormorants in general (Gremillet et al 2001(Gremillet et al , 2004, and in blue-eyed cormorants in particular, in order to determine how prey density with depth equates with appropriate dive strategies and net energetic gain after taking into account the factors that modulate metabolic rate. Other than that, a global examination of the depths and temperatures exploited by various cormorant species in relation to plumage air volume (perhaps facilitated by the simple methods proposed by Wilson et al 1992) might go some way to help explain patterns of distribution in these remarkable birds.…”

“…They frequent both coastal and inland waters from polar to tropical regions, breeding in colonies numbering from just a few individuals to many thousands of birds (Johnsgard 1993). The obvious success of this group has been attributed to their efficiency as aquatic predators (Gremillet et al 2001(Gremillet et al , 2005a, this being related to their swimming and diving abilities (Gremillet 1997, Gremillet et al 2004) that are, in part, a result of their wettable plumage (Ribak et al 2005, Gremillet et al 2005b, which results in reduced buoyancy. Despite this, cormorants are often cited as diving shallowly, primarily exploiting the first 10 m of the water column in dives that rarely exceed 1 min (for review see Cooper 1986).…”

Flexible foraging behaviour of a sexually dimorphic seabird: large males do not always dive deep

“…Deeper water is colder, and, due to the increased pressure at depth, the plumage loses more of its insulating properties. Thus, in cold water, dive costs of up to 20 times the BMR have been estimated in birds (Grémillet et al 2001, Enstipp et al 2006. It has been noted recently (Shepard et al 2009(Shepard et al , 2010 that, due to reduced buoyancy, deep diving is proportionally less expensive metabolically than expected.…”

Fish are not safe from great cormorants in turbid water

“…They have a broad, flexible diet and show extremely high foraging efficiency (grams of fish caught per unit time underwater; Grémillet 1997), which allow them to exploit feeding habitats in a great variety of climate zones (Grémillet et al 2001), from coastal waters to alpine streams and across rivers, lake and ponds. The reasons for this foraging efficiency, which is higher than that of any other diving seabird (Grémillet 1997), are unclear.…”