Detection of Individual Replacements in a Long-Lived Bird Species, the Bonelli's Eagle (Aquila fasciata), Using Three Noninvasive Methods
Accurate estimations of adult mortality are essential for understanding population dynamics and achieving efficient management actions that are directed toward long-lived species. Several noninvasive methods may be used to monitor endangered long-lived birds like raptors, but their performance in real-world scenarios remains poorly studied. We used eight Bonelli's Eagle (Aquila fasciata) breeding pairs (1) to compare the efficiency of direct observations with binoculars and telescopes (2004–2016), camera trapping (2012–2016), and feather genetic analysis (2004–2015) for detecting and identifying individuals over time, (2) to conduct a cost-benefit analysis of methods, and (3) to evaluate the accuracy of the direct observation method for estimating survival by comparing it with camera trapping and feather analysis. Both the feather genetic analysis and camera trapping approaches allowed us to successfully identify individuals and to detect replacements that we did not notice during direct observations. The classic direct observation method overestimated adult survival, which could hamper an accurate understanding of the demographic dynamics of this population. Feather analysis gave a low detection rate of individuals (47%), probably due to difficulties in finding feathers in the rugged habitat of this species. However, identification success at the individual level with this method was 96%. Remote camera surveys had both a high detection rate (100%) and identification success (93%). Given that costs associated with camera surveys were lower than the costs of genetic procedures, we concluded that camera trapping is the most efficient indirect method to assess adult survival in this and other raptors with external features that make individual identification possible. However, because no method showed complete efficiency, genotyping of feathers collected at the end of the breeding season could be a useful additional method, especially when camera trapping is not possible or when it fails. Detección de Reemplazos Individuales en una Especie de Ave Longeva, Aquila fasciata, Utilizando Tres Métodos No Invasivos Las estimaciones correctas de la mortalidad de aves adultas son esenciales para comprender la dinámica poblacional y lograr medidas eficientes para el manejo de especies longevas amenazadas, como es el caso de las rapaces. Si bien se cuenta con varias metodologías que podrían ser adecuadas para ello, su utilidad en situaciones reales no ha sido muy estudiada. Usamos ocho parejas reproductoras de Aquila fasciata para (1) comparar la eficiencia en detectar e identificar individuos a lo largo del tiempo mediante observaciones directas realizadas con prismáticos y telescopios (2004–2016), foto-trampeo (2012–2016) e identificación genética a partir de plumas (2004–2015), (2) realizar un análisis comparado de costes y beneficios y (3) evaluar la precisión del método de las observaciones directas para estimar la supervivencia en relación al foto-trampeo y el análisis genético para el seguimiento poblacional a largo plazo. Tanto el sondeo genético como el foto-trampeo detectaron e identificaron con éxito los individuos, permitiendo también detectar reemplazos que hubieran pasado desapercibidos por medio de la observación directa. El método clásico de observación directa sobreestimó la supervivencia adulta, afectando una adecuada comprensión de la dinámica demográfica de esta población. Las plumas mostraron una baja tasa de detección de individuos (47%), posiblemente debido a las dificultades de muestreo en el hábitat rocoso de esta especie, aunque el éxito de identificación de individuos fue del 96%. Las cámaras-trampa mostraron elevadas tasas de detección (100%) y de identificación (93%). Dado que los costes del foto-trampeo fueron inferiores a los costes de laboratorio, concluimos que el foto-trampeo es el método indirecto más eficiente para estimar la supervivencia adulta en esta especie y en otras rapaces con diseños de plumaje que permitan la identificación de individuos. No obstante, teniendo en cuenta que ningún método fue totalmente eficiente, el genotipado de plumas recogidas al final de la nidificación puede ser un método adicional útil, especialmente si el foto-trampeo no es posible o cuando falla. [Traducción de los autores editada]ABSTRACT
RESUMEN
History of individual detection by feathers (circles) and camera trapping (squares) of Bonelli's Eagles (Aquila fasciata) in Granada, Spain. Open symbols show new individuals (i.e., those assumed to have joined the territory after the previous territory holder had died or migrated to another territory). Broken lines show the period when replacements took place, whereas continuous lines show the period during which the same individual was assumed to be present (e.g., the female of T1 was confirmed to be the same individual in 2006 and 2008 and assumed to be present in 2007, when no feathers were analyzed). A ? indicates an uncertain identification due to failure to extract DNA or absence of suitable pictures. These eight territories were monitored by direct observation during 2004–2016.
Examples of individual identification through unique plumage traits, specifically by the striped pattern of flight feathers of Bonelli's Eagles (Aquila fasciata) in Granada, Spain, at three territories. For a given territory, the figure illustrates the same individual across consecutive survey seasons. See Fig. 1 for territory identification.
Comparison of the efficiency of the three monitoring methods of Bonelli's Eagles (Aquila fasciata) in Granada, Spain. DOM (direct observation monitoring), FGM (feather genetic monitoring), CTM (camera trap monitoring). Sample sizes are given above the bars.
Contributor Notes
1 Email address: jmgilsanchez@yahoo.es
Associate Editor: Christopher J. W. McClure