Truncated Power Laws Reveal a Link between Low-Level Behavioral Processes and Grouping Patterns in a Colonial Bird

Background: Departures from power law group size frequency distributions have been proposed as a useful tool to link individual behavior with population patterns and dynamics, although examples are scarce for wild animal populations. Methodology/Principal Findings: We studied a population of Lesser kestrels (Falco naumanni) breeding in groups (colonies) from one to ca. 40 breeding pairs in 10,000 km 2 in NE Spain. A 3.5 fold steady population increase occurred during the eight-year study period, accompanied by a geographical expansion from an initial subpopulation which in turn remained stable in numbers. This population instability was mainly driven by first-breeders, which are less competitive at breeding sites, being relegated to breed solitarily or in small colony sizes, and disperse farther than adults. Colony size frequency distributions shifted from an initial power law to a truncated power law mirroring population increase. Thus, we hypothesized that population instability was behind the truncation of the power law. Accordingly, we found a power law distribution through years in the initial subpopulation, and a match between the power law breakpoint (at ca. ten pairs) and those colony sizes from which the despotic behavior of colony owners started to impair the settlement of newcomers. Moreover, the instability hypothesis was further supported by snapshot data from another population of Lesser kestrels in SW Spain suffering a population decline. Conclusions/Significance: Appropriate analysis of the scaling properties of grouping patterns has unraveled the lin

Evaluación de métodos de seguimiento y de manejo de poblaciones de cernícalo primilla aplicados a su conservación

[spa] La biología de la conservación, nacida para dar respuesta a la crisis de biodiversidad creciente durante el siglo XX, está volviendo su atención actualmente hacia la necesidad de mejorar el entendimiento y la colaboración entre la ciencia y la gestión del medio, para mejorar la conservación de éste. En este contexto, es interesante optimizar los métodos de seguimiento de las poblaciones y analizar de manera cuantitativa sus riesgos y posibilidades de intervención. Hemos tomado como modelo de estudio el cernícalo primilla, especie colonial facultativa catalogada en la categoría de vulnerable. La población modelo es la del valle de Ebro, estudiada desde 1993 a 2002. Se han analizado los métodos de seguimientos y factores de riesgo para el periodo reproductor y el periodo de dispersión premigratoria. El cernícalo primilla nidifica en huecos de edificios en colonias de tamaño variable, desde parejas solitarias a varias decenas. El seguimiento de la población reproductora implicaba hasta el momento un esfuerzo muy alto, inviable para un seguimiento continuado en el tiempo, además del riesgo para la especie que supone el acceso a los nidos. Obtuvimos un protocolo de censo poblacional basado en el censo en dos tiempos (localización de las colonias más conteo) que consiste en uno o dos conteos breves en el momento de mayor visibilidad de las aves en las colonias previamente detectadas. El seguimiento de la productividad se basó en el estudio de al menos nueve colonias de cuatro o más parejas, obteniéndose una información fiable con las mínimas molestias para la especie. La principal causa de fracaso reproductor encontrada fue la depredación, que afectó a todas las fases y a ejemplares adultos. El depredador más frecuente fue la rata. La presencia de organoclorados en huevos fue notable, aunque no afectó a la tasa de eclosión; en vista de la relación entre DDE y dieldrín con los índices de espesor y ultraestructura de las cáscaras, queda abierta la posibilidad de emplear estos índices como biomarcadores. Entre las causas de mortalidad adulta se identificaron importantes factores antrópicos. Durante el periodo premigratorio se encontraron dormideros que concentraban un elevado número de aves, algunos en zonas con escasa presencia en periodo reproductor. La mayor parte de ejemplares (hasta un 95%) se congregaban en los ubicados en subestaciones eléctricas, superando los mil individuos, siendo este máximo en los primeros días del mes de septiembre. Se identificó como método óptimo de censo el conteo de las aves que entraban en las subestaciones a principio del mes de septiembre. Las observaciones de vuelos y búsqueda de cadáveres mostraron un bajo riesgo de mortalidad de los cernícalos en las subestaciones ocupadas. La lectura de anillas mostró una importante presencia de ejemplares foráneos. Se empleó el análisis de isótopos estables (δ13C, δ15N y δD) en plumas para caracterizar poblaciones reproductoras y asignar los ejemplares dispersantes. De acuerdo a nuestros resultados, a escala peninsular esta técnica puede no ser idónea, ya que no ha sido posible diferenciar suficientemente poblaciones. Resultados relevantes han sido las diferencias interanuales y entre pollos y adultos, observados en los tres isótopos analizados. Estos aspectos deberán ser tenidos en cuenta en el diseño de estudios de ecología espacial como éste. Mediante un enfoque multidisciplinar y a partir de una serie larga de datos, hemos optimizado las metodologías de seguimiento del cernícalo primilla en el valle del Ebro durante los periodos reproductor y premigratorio e identificado los principales factores de riesgo sobre los que se puede intervenir de cara a la conservación, valorando su importancia y analizando los aspectos que pueden permitir su gestión. Las vías propuestas pueden ser exportables a otras poblaciones y especies coloniales.[eng] Conservation biology is an interesting discipline for science and management. Collaboration between these areas provides useful methods to conserve biodiversity. We have studied monitoring and management efficient methods that provide feasible data about a Lesser kestrel (Falco naumanni) population in Ebro Valley. Lesser kestrel breeds in colonies from one to around 50 pairs, under the tiles of farmhouses roofs. As a migratory species, we focused in both breeding and premigratory periods. We have developed methods for monitoring the number of breeding pairs and their productivity with the minimum effort. To estimate the breeding population, we need to census one or two times each colony, employing 5 minutes per visit, during the period of maximum visibility, and consult a table. The population productivity may be estimated by monitoring a minimum of 9 colonies with 4 or more breeding pairs per each one. Moreover, we have quantified the importance of different factors affecting breeding success and survival, being predation (mainly by rats) the most important. We have detected an important presence of organochlorin compounds in eggs, showing a significant correlation between both DDE and dieldrin in Ratcliffe and Cooke indexex. Thus, we highlight the possibility of using these indexes as biomarkers. Lesser kestrels use communal roosts during premigratory period. The main congregations in our area are in electric substation, so we propose monitoring these places during the period of maximum presence, that is, between the end of August and the beginning of September. Flight observations show a low collision risk using these power installations. A high proportion of birds are not natives of the area of study, as it is shown with the absence of ringed birds. We have analyzed stable isotopes in feathers (δ13C, δ15N y δD) and demonstrated this is not the adequate technique to describe a gradient at this geographical scale. Useful results, when studying ecology questions by stable isotopes, are the differences observed between adults and chicks and interannual variations. We have provided monitoring methods and management proposes integrating different approaches from a long-term population monitoring and suggest their use to work with other populations and/or other colonial species

Proximate causes and fitness consequences of hatching failure in lesser kestrels Falco naumanni

Hatching failure is a pervasive phenomenon in birds, but factors affecting hatchability remain poorly understood. We studied proximate causes and fitness consequences of hatching failure in a long-monitored population of the colonial lesser kestrel Falco naumanni. We investigated whether hatchability was related to clutch characteristics, parental traits, and social or environmental features. Hatching failure represents a cost for the parents in terms of immediate fitness, since it reduced both their number of young fledged and recruits in the breeding population, even when controlling for clutch size. Hatching failure showed a non-linear relationship with clutch size, clutches of four eggs showing higher levels of hatching success than larger or smaller clutches. Hatchability could therefore play a role in the evolution of optimal clutch size in this species, at least constraining the maximum number of eggs the parents can afford to incubate. Contrary to most studies, the mean volume of the clutch and the individual egg volume were negatively related to hatching failure, indicating that large eggs have thermoregulatory and/or nutritional advantages. Mean daily maximum temperature during incubation affected hatching success negatively, but only for females in poor condition. This result seems to indicate that females are reluctant to jeopardize their own condition, but instead sacrifice incubation effort by paying the costs of a lower hatching success in circumstances of high temperatures. There was no evidence that hatching failure was related to the intrinsic properties of individuals or genetic similarity between the parents as indicated by low repeatabilities of: (1) males that bred with different females, (2) females that bred with different males, and (3) pairs breeding together in different years. Neither colony size nor subpopulation size affected hatchability. All these findings show how hatching failure is simultaneously influenced by several factors acting in a complex way, which could in part explain the apparently conflicting conclusions of empirical or even experimental studies carried out to date. © Journal of Avian Biology.Peer Reviewe

Does land irrigation actually reduce foraging habitat for breeding lesser kestrels? The role of crop types

The lesser kestrel is a Globally Threatened Species which large decline has been related to recent agricultural changes in European pseudo-steppes. Irrigation is considered as one of the major threats for this and other steppe birds, but the actual effects of irrigation on foraging habitat selection have been scarcely examined. We studied the selection of traditional dry cereal farming and irrigated habitats by foraging lesser kestrels during the breeding cycle, paying especial attention to possible differences among crop types. Field margins were the scarcest but the most positively selected habitat, and different stages of cereals cultivated following traditional practices were selected depending on the breeding and agriculture cycles. Effects of irrigation were dual. While irrigated maize and other crop types were avoided, alfalfa was used in proportion to its availability and later highly selected after harvesting. Moreover, field margins in irrigated land were selected in a similar way than in traditional dry farmland. Therefore, although maintaining low-intensity farming is still the main recommendation for this species, new management options arise when social pressure makes irrigation unavoidable. Further agri-environmental schemes in these circumstances should thus promote cultivation of alfalfa with a low input of biocides while avoiding maize, together with increasing field margins, to make compatible irrigation with lesser kestrel conservation. © 2004 Elsevier Ltd. All rights reserved.Peer Reviewe

Colony size selection determines adult survival and dispersal preferences: Allee effects in a colonial bird.

Avian coloniality traditionally has been investigated by examining how breeding success varies with colony size, but other crucial fitness components rarely have been examined. This may lead to wrong conclusions because unmeasured parameters may change the final fitness balance. We used multistate capture-recapture models to investigate adult survival and dispersal in relation to colony size within a long-term monitored population of lesser kestrels (Falco naumanni). Nest predation probability decreases with colony size, and adult survival is predicted to show the same trend because adults are exposed to the same suite of predators. As expected, survival probability was higher in large colonies (0.72 +/- 0.015; mean +/- SE) than in medium or small colonies (0.65 +/- 0.02). Additionally, dispersal persal probabilities were higher going from small to large colonies (0.02 +/- 0.01) than from large to small (0.08 +/- 0.01), as predicted by theory of habitat selection shaped by fitness maximization. These asymmetries are likely to generate size-specific colony population dynamics, so they should be taken into account in studies of colonial birds and other metapopulation-like systems. Allee effects, that is, positive density dependence, appear to be the cause of the evolution of dispersal behavior and may explain the maintenance of coloniality in this species.Peer Reviewe

Scaling in the distribution of Lesser kestrel nests spatial distribution.

<p>A, Box-counting plot of the spatial distribution of nests in the most populated year in the Ebro Valley. B, Distribution of nearest neighbor distances between nests (black) and between colonies (white). All distances are in meters. Note the match between the mean distances and the inflection points (departing the pattern from a straight line, and thus from a fractal-like nest distribution pattern) in the box-counting plot. The small number of large between-nest distances corresponds to solitary nests for which the nearest nest is far away (in the closest colony).</p

Location of Lesser kestrel colonies (farmhouses) in the study area in year 2000.

<p>The rectangle encompasses the initial subpopulation named Sastago. Inset pictures show the location of the study area (Ebro Valley, NE Spain), and an example of one of the farmhouses where these small falcons bred under roof tiles.</p