SummaryThe Balearic Shearwater Puffinus mauretanicus is considered one of the most threatened seabirds in the world, with the breeding population thought to be in the range of 2,000–3,200 breeding pairs, from which global population has been inferred as 10,000 to 15,000 birds. To test whether the actual population of Balearic Shearwaters is larger than presently thought, we analysed the data from four land-based census campaigns of Balearic Shearwater post-breeding migration through the Strait of Gibraltar (mid-May to mid-July 2007–2010). The raw results of the counts, covering from 37% to 67% of the daylight time throughout the migratory period, all revealed figures in excess of 12,000 birds, and went up to almost 18,000 in two years. Generalised Additive Models were used to estimate the numbers of birds passing during the time periods in which counts were not undertaken (count gaps), and their associated error. The addition of both counted and estimated birds reveals figures of between 23,780 and 26,535 Balearic Shearwaters migrating along the north coast of the Strait of Gibraltar in each of the four years of our study. The effects of several sources of bias suggest a slight potential underestimation in our results. These figures reveal the urgent need to reformulate the population viability analysis for the species, and then if necessary reconsider its conservation status.
Censusing seabirds from coastal areas requires reliable estimates of bird numbers and the distances of the birds from the coastline. Logistical constraints make visual estimation of distances the only feasible method in many studies. We tested the accuracy of visually estimated offshore distances of six migratory seabird species in the Strait of Gibraltar using simultaneous measurements obtained by radar. Most birds (91%) were detected within 3 km of the coast and we truncated our calibration at this distance. We found a strong correlation between radar and visual estimates (R 2 = 0.83, P < 0.0001). The magnitude of errors in visual estimates was moderate and ranged from 0.08 to 0.20 for different distances and observers. Among the factors potentially affecting the accuracy of visual estimates of distance to seabird in our study were observer identity, bird species, bird behavior, and weather; the most parsimonious model in our study included observer identity as the only predictor, and no model with more than one predictor had a smaller Akaike's information criterion value. Radar can be used to help train observers and to reduce biases in visual estimates of distances by means of calibration. When no other methods are available to accurately measure distances to seabirds, visual estimates of distances, recorded by experienced observers and once calibrated with radar (or other ground-truthing methods), may be acceptable for different species under a wide range of environmental conditions. RESUMEN. Utilización de medidas de radar para la calibración de distancias a aves marinas migratorias estimadas visualmenteCensar aves marinas desdeáreas costeras requiere estimaciones fiables del número de aves y de la distancia de estas a costa. Restricciones logísticas hacen que a veces las estimaciones visuales de las distancias sean elúnico método factible para llevar a cabo estos estudios. Pusimos a prueba, en el Estrecho de Gibraltar, la exactitud de las estimaciones visuales desde costa, utilizando seis especies de aves marinas, concurrentemente, con medidas obtenidas por radar. La mayoría de las aves (91%) fueron detectadas dentro de unárea de 3 km de la costa y ajustamos nuestra calibración a esta distancia. Encontramos una fuerte correlación entre las estimaciones visuales y las del radar (R 2 = 0.83, P < 0.0001). La magnitud de los errores visuales al estimar las distancias fue moderada y variaron de 0.08 a 0.20 para diferentes distancias y observadores. Entre los factores que afectaron a la exactitud de las estimaciones visuales, se encuentran el observador, la especie de ave, la conducta exhibida por el ave y las condiciones meteorológicas. El modelo con mayor parsimonia incluyó la identidad del observador como elúnico vaticinador y ningún otro modelo, con más de un vaticinador, tuvo un valor de AIC menor. El radar puede ser utilizado para entrenar al personal y reduce el sesgo en las estimaciones visuales mediante la calibración. Cuando no hay disponible otros métodos para medir con exactitud la distancia a que se...
The migration strategies of smaller, south European, Mediterranean birds are less well known than those of northern and central European birds. We used geolocators to map individual spatiotemporal migration schedules of three species breeding in the Iberian Peninsula: the White-rumped Swift Apus caffer, rufous-tailed Scrub-robin Cercotrichas galactotes and Bluethroat Cyanecula svecica. The three species crossed the Sahara desert with a westward detour, to reach West African winter grounds in the Sahel (Bluethroats and Scrub-robins) or the rainforest belt (Swifts). Despite the proximity of the breeding grounds to the desert barrier, all but one individual stopped over before the desert crossing during autumn migration. After spending six months on average in sub-Saharan Africa with variable itinerancy, spring migration was faster overall and more direct than in autumn. Autumn migration was of similar duration to that found in related northern European migrants and therefore slower in southern birds. Spring migration was completed in less time than in the northern migrants (data only for Swifts
Previous studies presented two independent models based on the equalization of electronegativity and on an alternative operative formula for the Fukui function. The current paper presents modifications to these models to obtain a new model for calculating the local energy derivatives (electronic chemical potential, local electronic hardness and local hypersoftness) and density (Fukui and dual descriptor indices) in natural bond orbitals for the study of chemical reactivity from the perspective of these orbitals (local electrophilicity and local maximum charge variation have also been deduced). We have radically changed the way of partitioning the electron density because the atomic model cannot distinguish between bonding and non‐bonding electron pairs, or between electrons with different types of bonds, however, this is possible with the model based on bond orbitals. In this respect, this model is ideal for organic reactions because of their correspondence with Lewis structures, which are simple but very useful for representing reaction mechanisms.
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