Vision and Foraging in Cormorants: More like Herons than Hawks?

Background Great cormorants (Phalacrocorax carbo L.) show the highest known foraging yield for a marine predator and they are often perceived to be in conflict with human economic interests. They are generally regarded as visually-guided, pursuit-dive foragers, so it would be expected that cormorants have excellent vision much like aerial predators, such as hawks which detect and pursue prey from a distance. Indeed cormorant eyes appear to show some specific adaptations to the amphibious life style. They are reported to have a highly pliable lens and powerful intraocular muscles which are thought to accommodate for the loss of corneal refractive power that accompanies immersion and ensures a well focussed image on the retina. However, nothing is known of the visual performance of these birds and how this might influence their prey capture technique. Methodology/Principal Findings We measured the aquatic visual acuity of great cormorants under a range of viewing conditions (illuminance, target contrast, viewing distance) and found it to be unexpectedly poor. Cormorant visual acuity under a range of viewing conditions is in fact comparable to unaided humans under water, and very inferior to that of aerial predators. We present a prey detectability model based upon the known acuity of cormorants at different illuminances, target contrasts and viewing distances. This shows that cormorants are able to detect individual prey only at close range (less than 1 m). Conclusions/Significance We conclude that cormorants are not the aquatic equivalent of hawks. Their efficient hunting involves the use of specialised foraging techniques which employ brief short-distance pursuit and/or rapid neck extension to capture prey that is visually detected or flushed only at short range. This technique appears to be driven proximately by the cormorant's limited visual capacities, and is analogous to the foraging techniques employed by herons

Prey ecology and behaviour affect foraging strategies in the Great Cormorant

The Wne link between a particular dive pattern and a speciWc prey item represents a challenging task in the analysis of marine predator\u2013prey relationships. There is growing evidence that prey type aVects diving seabirds\u2019 foraging strategies, dive shapes and underwater activity costs. This study investigates whether a generalist diver, the Great Cormorant Phalacrocorax carbo, modiWes the time budget allocated to prey-capture behaviour and breathing strategies (reactive vs. anticipatory) with respect to the prey type (pelagic vs. benthic). Video recordings of 91 Great Cormorants show how the ecology and behaviour of their main prey, Mullets (Mugilidae) and Flounders Platichthys Xesus, aVect dive/surface durations and the diving pattern. The demersal habit and the low mobility of Flounders leads to an easy access to prey with an anticipatory strategy. Moreover, the patchy distribution of this Wsh species increases prey-capture rates. Conversely, Mullets exploit the whole water column and are highly mobile, and this is reXected in the need of performing two sequential dives to capture a prey, both longer and likely more expensive, with a consequent switch of strategy from reactive in the searching phase to anticipatory breathing during prey-capture events. This study provides evidence that a generalist diver may switch between diVerent foraging strategies, and it shows how each of them may be optimal under particular ecological conditions. These constraints inXuence the dynamics that operate within the marine food chains and have relevant implications in managing lagoon areas, including Wsh ponds