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Assessing reintroduction outcome: comparison of the juvenile post-fledging dependence period between wild and reintroduced Bonelli’s eagles in two Mediterranean islands

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  • Ecologia applicata Italia
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Abstract

Islands are key areas for biodiversity; however, they are extremely sensitive to anthropic actions. This has led to local species extinctions, especially large predators such as raptors. Consequently, reintroduction and conservation projects aimed at reversing population decline of endangered species have recently gained popularity. Nevertheless, their relatively elevated cost and chance of failure make them controversial, hence assessing their effectiveness is essential. Within the early stages of raptors, the post-fledging dependency period (PFDP) is the one in which individuals must face dangers without having completely developed their skills. Thereby, comparing PFDP patterns concerning reintroduced and wild individuals is of major interest as it would help to plan and improve future conservation actions. We analyzed the behavior of 38 juvenile Bonelli’s eagles (15 reintroduced and 23 wild) tracked through GPS telemetry, tagged as nestlings in two insular environments. The study period encompassed a total of nine-year movement data from reintroduced chicks in Mallorca (Spain) and wild chicks from Sicily (Italy). Movement parameters (i.e., age of first flight, age of dispersal, length of the PFDP, revisits to the natal or release area, and residence time in them) were analyzed together with their behavior during the PFDP for reintroduced and wild individuals. Similar movement patterns were obtained for both origins, although wild individuals revisited the natal site more often and dispersed earlier. Behavior was also similar, it varied throughout the PFDP, observing a more abrupt progress in wild individuals and an earlier development of travelling and hunting behaviors. Observed differences are probably related to food availability, which can improve body condition and thus delay onset of dispersal, together with parental presence, which can prompt an earlier ending of the PFDP by encouraging juvenile independence.

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Abstract: We report details of two PCR-based molecular sexing techniques for the Bonelli's Eagle (Hieraaetus fasciatus) and evaluate the reliability of morphometric measurements to predict the sex of nestlings in the field. Blood samples taken from 63 nestlings in southwest Portugal (1994-99) were analyzed using the intron polymorphism method (M1) , and 56 of these were also analyzed with the single-strand conformation polymorphism approach (M2). Contamination or poor preservation of samples precluded one sex determination with M1 and six others with M2. Sexing by both methods was concordant for 98.0% of samples. Linear discriminant analysis was used to determine whether any single variable or combination could provide reliable sex determinations, using 10 body measurements from 43 nestlings aged 35-50 d, sexed unambiguously by both molecular methods. Models were evaluted by cross-validation of the original data and from the classification of an external sample (N = 12). Females were significantly larger than males. The greatest separation between sexes occurred in body mass, but differences were also noted in tarsus diameter and the lengths of the hind claw, foot, culmen, and forewing; no differences were detected in the lengths of tarsus, fore claw, seventh primary, and central tail feather. A discriminant model including body mass, hind claw length, and age provided the maximum separation between sexes and it correctly sexed 96% of the nestlings. A model including tarsus diameter, hind claw, and age showed similar accuracy. Both models were satisfactory in determining the sex of nestling Bonelli's Eagles between the ages of 35-50 d in the field, but combination with molecular techniques may be preferable in studies requiring absolute precision for every individual handled.
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Tools for performing model selection and model averaging. Automated model selection through subsetting the maximum model, with optional constraints for model inclusion. Model parameter and prediction averaging based on model weights derived from information criteria (AICc and alike) or custom model weighting schemes. [Please do not request the full text - it is an R package. The up-to-date manual is available from CRAN].
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Despite recent efforts to develop the science of reintroduction biology, there is still no general and broadly accepted definition of reintroduction success. We investigate this issue based on the postulates (1) that successful reintroduction programs should produce viable populations and (2) that reliable assessments of ultimate success require that populations have reached their regulation phase. We assessed if the viability of these reintroduced populations could be evaluated using the same criteria as for remnant populations, such as the Internation Union for Conservation of Nature (IUCN) Red List criteria. Using modeling, we projected the viabilities of theoretical populations with various life history and environmental characteristics and we tested whether population sizes (criterion D of the IUCN) and other potential predictors are relevant proxies of the risk of extinction (criterion E of the IUCN) in the case of remnant populations with an unknown past history and in the case of reintroduced populations that have reached their carrying capacity. We found that, as for remnant populations, population size can be used as a relevant indicator (although subject to considerable uncertainty) of the viability of reintroduced populations. However, the results demonstrate the importance of the reintroduction failure filter, that is, the fact that the reintroduced populations that have successfully reached their carrying capacity are those with the highest and more stable growth rates, especially if populations have been reintroduced with a few individuals. As a consequence, the general relationship between the current size of a population and its projected viability will, most likely, differ considerably between remnant and reintroduced populations. Overall, our results demonstrate that there are no theoretical limitations on the application of some of the criteria widely used for remnant populations to define reintroduction success, although these criteria are very conservative for reintroduced populations and might be rescaled to account for the demographic filter that early extinction constitutes for these populations.
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To investigate fledging behaviour and proximate causes of dispersal, 221 radio-tagged goshawks were studied in the post-nestling period during 1980-1987. Offspring left the nest tree from 39 days after hatching. Only 2% of observations were more than 300 m from the nest when hawks were 40-65 days old, while they finish growing their main flight feathers, compared with 26% of pre-dispersal records in the following 25 days. Dispersal from the nest area was abrupt, at 65-90 days old for 90% of hawks and by 95 days after hatching for 98%. Females dispersed a week later than males. Young hawks given supplementary food dispersed later than others in the same area, but not later than 95 days after hatching. All hawks that were fed artificially after the removal of their parents also left by this age. It is concluded that dispersal was enabled by completion of feather growth and was accelerated by food shortage, but probably resulted from behavioural maturation if food was abundant.
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Captive American kestrels (Falco sparverius) were used to model the effect of low prey availability on growth and survivorship potential in nestling raptors. The experimental design consisted of 4 treatments: nestlings were fed ad libitum (100% diet) or on increasingly restricted diets (90, 80, and 70% of the ad libitum diet). Nestlings fed a reduced diet grew significantly more slowly than those fed the ad libitum diet, as shown by the body mass growth constant. The restricted diet had no significant effect on the asymptotic size of the kestrels, i.e., body mass and tarsus and antebrachium lengths. However, young kestrels fed reduced diets (80 and 70%) had a lower body mass at fledging than those fed ad libitum and the 90% diet, although they fledged at the same age. The growth of the ninth primary was slower for birds on restricted diets than for those fed ad libitum. Sexual dimorphism was observed for body mass and antebrachium growth constants, with males growing faster than females. However, females reached a higher asymptotic body mass and antebrachium length, as well as a higher body mass at fledging, than males. Thus, when prey availability is reduced by as much as 30%, nestling kestrels grow more slowly and store less fat, which could lead to poorer postfledging survivability.
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We studied factors influencing the length of the postfledging period in the Montagu's harrier,Circus pygargus . Fledging date influenced the date of departure, for both wild birds and captive-reared birds (for which food supply was constant throughout the postfledging period), suggesting that departure time was constrained by migration. However, this relationship was absent in good food years, when some birds had short periods of dependency even if fledged early in the season. The end of the postfledging period was associated with a decrease in the food provided by the parents, coinciding with an increase in the fledglings' flying and hunting abilities and in aggressive food solicitation. The postfledging period was shortest when food was scarcest (when controlling for fledging date), and was also shorter for younger siblings (when controlling for brood effects), even though the parents provided less food in poor food years, and for nestlings in larger broods, particularly the younger ones. These results suggest that departure time is an adult's choice. However, we also predicted that if fledglings control parental investment, they should attempt to prolong the period of dependency when they receive little food. Length of the postfledging period was affected by food abundance, with shorter periods in better food conditions for equivalent hatching dates. In addition, the postfledging period was longer for birds from larger broods (when controlling for hatching date and food abundance), and for wild birds than captive-reared birds except in peak vole years. These results suggest that fledglings try to compensate for a deficiency in the food supply with longer periods of dependence, provided there is enough time before migration.
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Peregrine Falcons (Falco peregrinus) of seven subspecies from four continents were bred in captivity, and approximately 1173 of their progeny were released in the midwestern United States and adjacent regions of Ontario and Manitoba in an attempt to replace the original population that was extirpated by chlorinated hydrocarbon poisoning in the 1950s. We analyzed the success of individuals of the different subspecies introduced to the Midwest. Five of the seven subspecies released have contributed to the current breeding population. Subspecies of breeding Peregrine Falcons were equally represented when breeding birds of high productivity were compared with less prolific breeders. The subspecific makeup of the breeding population did not differ significantly from that of the released population, suggesting that adaptability in this species was sufficient to override genetic differences between subspecies. Peregrines of widely different genetic stocks have thrived after release, making substantial genetic contributions to the new population.
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The behaviour of five broods of radio-tagged Spanish Imperial Eagles Aquila heliaca adalberti was studied in Doñana National Park, Spain during the post-fledging period. The distance between perching sites and nest, the mean flight duration and distance, the percentage of time spent flying and the home range all increased exponentially with age. As the young got older, the parents spent less time in their vicinity. Young were not seen hunting, but depended on their parents for food. They begged and chased their parents throughout the post-fledging period, with higher intensity at the end. Nevertheless, the adults became progressively more reluctant to feed them, as reflected in the decrease in feeding frequency and in the number of approaching flights towards the young. At the end of the post-fledging period, adults often performed aerial displays and frequently chased their offspring. The age of independence of the different young studied varied between 123 and 145 days. The correlations between individual independence and the dates when the young were last fed by their parents, and when the highest intensity of parental aggressive behaviour occurred, were higher than correlations with the variables related to the maturation of flying. Therefore, it is suggested that parental ‘meanness' and aggressive behaviour may be the factors determining the date of juvenile independence and dispersal from the home territory.
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The movements of 96 radio-tagged juvenile Common Buzzards Buteo buteo were studied during 3 years in Dorset, southern England. Contrary to previous studies which implied that Common Buzzards are territorial, we found that 39% foraged within 1 km of the nest during their first winter, presumably within their parents' home range. Most (72%) of those that did not disperse were recorded making brief excursions during August and September, before opting for a “stay-at-home” strategy. There was no significant difference between the sexes in the tendency to make excursions or disperse, but females dispersed further than males. Buzzards which dispersed early tended to settle significantly farther from their nests than did late dispersers. Long-term radio-tracking has shown that 72% of the dispersing birds returned towards their natal area in the following breeding season. Such philopatry may be an important hindrance to avian recolonization following local extinctions.