Silvano Benvenuti

Sex-specific foraging behaviour in a monomorphic seabird

Sexual differences in the foraging behaviour of parents have been observed in a number of sexually sizedimorphic birds, particularly seabirds, and the usual inference has been that these sex–specific differences are mediated primarily by differences in body size. To test this explanation, we compared the foraging behaviour of parents in a monomorphic seabird species, the northern gannet Morus bassanus. Using specially designed instruments and radio telemetry we found that individuals of both sexes were consistent in the directions and durations of their foraging trips. However, there were significant differences in the foraging behaviour of males and females. Female gannets were not only more selective than males in the areas where they foraged, but they also made longer, deeper dives and spent more time on the sea surface than males. As the sexes are morphologically similar in this species, then these differences are unlikely to have been mediated by body size. Our work highlights the need to investigate sexual differences in the foraging behaviour of seabirds and other species more closely, in order to test alternative theories that do not rely on differences in body size.

FORAGING FLIGHTS OF BREEDING THICK-BILLED MURRES (URIA LOMVIA) AS REVEALED BY BIRD-BORNE DIRECTION RECORDERS

A bird-borne data logger, which stores flight directions at regular intervals, was used to reveal the foraging patterns of Thick-billed Murres (Uria lomvia) breeding at Latrabjarg in northwestern Iceland. Tracked birds traveled 10 to 168 km one-way to foraging sites in a narrow sector to the northwest of the breeding colony. The pattern of foraging activity was rather constant. Outbound birds stopped briefly several times, possibly testing prey availability. The longest stops occurred at sites close by the turning point of the out- bound journey. During the inbound flight, stops were less frequent and shorter than on the outbound flight, suggesting that the sequence of stops was not the expression of a distur-

Central-place Foraging in an Arctic Seabird Provides Evidence for Storer-Ashmole's Halo

ABSTRACT. Breeding seabirds are extreme central-place foragers, commuting long distances between colonies and feeding areas. Central-place foraging theory predicts that prey items close to the colony will be preferred over prey items distant from the colony, which can lead to prey depletion near the colony (“Storer-Ashmoles halo”). To investigate the relevance of these ideas to a single-prey loader, we equipped chick-rearing Thick-billed Murres (Uria lomvia) with time—depth recorders (1999–2007) and monitored prey deliveries (1993–2008). Because feeding rates were constant for chicks 3–15 days old, we restricted analyses to that age group. Between-date relationships were examined within individuals to avoid confounding effects of specialization and parental quality. The mass of prey items increased with foraging distance0,5, which suggests that large prey items were depleted by foraging pressure. Foraging distance for pelagic species increased through the season in years without spawning. After accounting for bathymetry, foraging effort decreased with distance from the colony for benthic fish and, in years without spawning, for pelagic fish. Within each season, Thick-billed Murres “fished down the food web”; they began by feeding on large fish, progressed to medium-sized prey (small fish), and finished feeding on small prey (invertebrates). We concluded that pelagic species responded to seabird foraging pressure by moving away from the colony, creating a three-dimensional halo. Benthic species, meanwhile, were depleted from a nearby shelf but remained abundant at a more distant offshore bench. We suggest that populations of central-place foragers are partially regulated by prey depletion.

Pigeon navigation: Effects of wind deflection at home cage on homing behaviour

SummaryFrom fledging time up to the test releases, two groups of experimental pigeons were housed in two cubic cages supplied with deflectors which deviated the winds through to the inside approximately 70° clockwise (CW-birds) or counter-clockwise (CCW-birds). Test releases were made at 9.0, 23.5, and 105.3 km from the loft. With respect to that of control birds, the mean bearing of CW-birds was always deflected clockwise, and that of CCW-birds was always deflected counter-clockwise. Control birds performed better than CW-birds in homing from the first release site, and better than both experimental groups from the second release site.These results agree with the olfactory hypothesis of pigeon navigation (Papiet al., 1972).

Homing performances of inexperienced and directionally trained pigeons subjected to olfactory nerve section

SummaryPigeons not experienced in homing and subjected to olfactory nerve section are generally found to be unable to home from short distances. In nine successive releases from different localities at 3.3 to 11.8 km from the loft, 15 out of 19 experimental birds were lost as well as 2 out of 40 control birds.The homing capacity improves when the birds are released from familiar localities on a line along which they have been trained before undergoing nerve section, but they still perform very poorly in later releases from unfamiliar localities outside the training line. 11 experimental birds showed fairly good performances from a familiar locality on the training line at 43.4 km from the loft (10 returned within the same day of the release, at an average speed of 32 km/h). Only one of these birds was able to home within the same day in a later release in an unfamiliar locality outside the training line at 34.9 km from the loft. Five other birds of this group homed one or more days later, and the remaining five were lost.The initial orientation was poorer amongst the experimental birds. More of the inexperienced experimentals landed near the release point.As the experiments show that olfactory nerve section causes no disturbances in general behaviour and homing drive, it is concluded that olfaction plays an important and specific role in the homing mechanism of pigeons.

Examining the limits of flight and orientation performance: satellite tracking of brent geese migrating across the Greenland ice-cap

Brent geese, Branta bernicla hrota, were equipped with satellite transmitters on a spring stop-over site in Iceland. The brent geese deposit heavy fuel reserves for long-distance flights across the high Greenland ice-cap to breeding destinations in north Canada. Satellite tracking of brent geese on this journey serves the twofold purpose of testing specific predictions about: (i) size-dependent power constraints in bird flight (severe restrictions in climbing capacity with heavy payloads); and (ii) orientation constraints at northerly geographic and magnetic latitudes (different routes depending on which celestial or geomagnetic cues are used). Five individuals were successfully tracked to west Greenland, and they traversed the Greenland ice-cap where it reaches > 2500 m above sea level (ASI). Their climbing rates were exceedingly small, 0.01-0.06 m s-1, indicating critical size-dependent power limitations on flight. The movement up the ice slope was very slow, and the most plausible interpretation is that the geese paused frequently between partly anaerobic flights. The flight tracks were surprisingly irregular, although departure directions from Iceland and across Greenland were similar. The geese’s orientation is probably based on a combination of landmark piloting and a compass mechanism giving a constant geographic course irrespective of longitudinal time and geomagnetic declination (variation) differences along the route.

Pigeon Homing: Cues Detected During the Outward Journey Influence Initial Orientation

The initial orientation of homing pigeons released from the same site can be significantly different according to the odorous stimuli perceived during the outward journey. In fact, differences in orientation of pigeons transported to the same release site by different routes (“detour effect”) resulted only when the birds were allowed to perceive the odors of the areas crossed during the outward journey. Moreover, birds carried in the same vehicle were different in orientation when subjected to a different pattern of olfactory stimulation during the journey.

Influence of wing loading on the trade-off between pursuit-diving and flight in common guillemots and razorbills

SUMMARY Species of bird that use their wings for underwater propulsion are thought to face evolutionary trade-offs between flight and diving, leading to the prediction that species with different wing areas relative to body mass (i.e. different wing loadings) also differ in the relative importance of flight and diving activity during foraging trips. We tested this hypothesis for two similarly sized species of Alcidae (common guillemots and razorbills) by using bird-borne devices to examine three-dimensional foraging behaviour at a single colony. Guillemots have 30% higher wing loading than razorbills and, in keeping with this difference, razorbills spent twice as long in flight as a proportion of trip duration whereas guillemots spent twice as long in diving activity. Razorbills made a large number of short, relatively shallow dives and spent little time in the bottom phase of the dive whereas guillemots made fewer dives but frequently attained depths suggesting that they were near the seabed (ca. 35–70 m). The bottom phase of dives by guillemots was relatively long, indicating that they spent considerable time searching for and pursuing prey. Guillemots also spent a greater proportion of each dive bout underwater and had faster rates of descent, indicating that they were more adept at maximising time for pursuit and capture of prey. These differences in foraging behaviour may partly reflect guillemots feeding their chicks single large prey obtained near the bottom and razorbills feeding their chicks multiple prey from the water column. Nonetheless, our data support the notion that interspecific differences in wing loadings of auks reflect an evolutionary trade-off between aerial and underwater locomotion.

An experiment for testing the hypothesis of olfactory navigation of homing pigeons

SummaryBeginning from fledging time up to the time of homing experiments, 33 experimental pigeons were kept alternatively for three days in a wire-netting aviary which they could leave for spontaneous flights (O-aviary), and for three days in a closed aviary fenced with plastic sheeting (C-aviary). In the O-aviary they wore masks which prevented or limited their breathing through the nostrils. As controls, 31 birds were kept for three days in the O-aviary without masks and for three days in the C-aviary wearing masks.In eight releases from places situated between 10 and 61 km from the aviary, the experimental birds gave worse homing performances than the control birds. Besides, contrary to the control birds, the initial orientation is mostly either wrong or at random in the experimental ones. These results support the hypothesis of olfactory navigation (Papiet al., 1972), which supposes that the pigeons, during their first months of life, gather information on smells prevailing in surrounding regions through the winds.

Diving behaviour differs between incubating and brooding Brünnich's guillemots, Uria lomvia

Abstract. Bird-borne data loggers were used to investigate the foraging strategies of Brünnichs guillemots breeding in a colony in the North Water Polynya: the flight performance and diving activity of incubating birds were compared to those of chick-rearing individuals. No significant differences were recorded between the bird groups in the potential foraging range. Conversely, clear differences were revealed between incubating and chick-rearing birds in diving behaviour. Chick-rearing birds were generally foraging at a significantly greater depth, and spent significantly higher proportions of the time submerged, than brooding individuals. Despite these differences, the estimated average daily energy expenditure of chick-rearing Brünnichs guillemots was only about 6% higher than that during incubation.