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

Ganzen: geliefd, maar met mate

In het boerenland zijn ganzen een alledaagse verschijning geworden, niet alleen in de winter, maar ook in de zomerperiode zijn er grote aantallen. Vanuit het natuurbeleid wordt de toename toegejuicht, maar er zijn ook negatieve aspecten : de schade aan landbouwgewassen. Meer recent zijn er berichten dat de royale aanwezigheid van ganzen ook schade aan bepaalde vormen van natuur veroorzaakt. Daarmee is het tweeslachtige beeld van de ganzen in onze maatschappij gekenschetst : ze zijn welkom, maar met mate. Een overzicht van het aantal ontwikkelingen, de belangrijkste beleidsmaatregelen en de nieuwste bevindingen uit recent onderzoek

Le problème de la dynamique des ions dans l’élargissement Stark

L’influence de la dynamique des ions sur le profil d’une raie spectrale émise par un plasma est analysée en termes de trois effets principaux : élargissement par modulation de fréquence, par modulation d’amplitude et par effets non-adiabatiques. A chacun de ces mécanismes d’élargissement sont associés différents taux de relaxation du dipole atomique. Pour juger de l’importance de la dynamique des ions, ces taux de relaxation sont à comparer à ceux correspondant à l’élargissement par ions quasi-statiques et par électrons. Une comparaison détaillée des différents taux est faite pour la série Lyman. Selon les caractéristiques du plasma’ et de la raie considérée, certains taux de relaxation correspondant à la dynamique des ions peuvent être négligeables par rapport aux autres ; de tels cas sont favorables à un traitement approximatif de la dynamique des ions

Unified Model for Stark Broadening

Abstract Stark broadening of spectral lines is considered as a semi-classical many-body problem. Starting from a Liouville equation for a distribution function depending on the atomic Hilbert space vector and the coordinates and velocities of the classical plasma particles, BBGKY hierarchy techniques are used to derive a complete line profile for the electron contribution. The line shape formula is expressed in terms of the atomic time evolution operator for a collision with a single plasma electron. This operator is approximated by a strictly unitary exponential expression, yielding more accuracy than second order perturbation theory and being valid also in the quasi-static limit. The resulting line shape expression covers the whole frequency domain from the impact regime to the quasi-static regime. The results of the impact and quasi-static approximations are recovered as special cases for small and large distances from the line center. A numerical application to Lyman - α shows very good agreement with an experiment of Boldt and Cooper.</jats:p