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Multidimensional differentiation in foraging resource use and competitive interactions over nesting sites between two sympatric top predators

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Multidimensional differentiation in foraging resource use and competitive interactions over nesting sites between two sympatric top predators

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One of the influential periods in avian life-cycles is the annual breeding season and thus competition over foraging resources, suitable nesting sites and territories is a key factor which can determine fitness and distribution, especially for raptors which are highly selective regarding their breeding habitats. However, ecologically-similar species were found to develop specific strategies to partition their resources, leading to niche differentiation and divergence, in order to avoid interspecific competition. In this study, I analyzed the foraging habitats, foraging activity, diet, nest site characteristics, breeding success and competitive interactions between two top predator populations. Whereas the short-toed eagle (Circaetus gallicus) (hereafter STE) naturally breeds in the Judean Foothills (Israel), the long-legged buzzard (Buteo rufinus) (hereafter LLB) only recently (over the last two decades) invaded the nesting habitat of the STE during their breeding season. These two newly sympatric species have similar nesting ecology, share the same areas for their nesting territories and, in some cases, even alternate use of nests in consecutive years. They are therefore expected to compete over foraging habitats, food, nesting sites and territories. Regarding their foraging resources, my analysis determines multi-dimensional differentiation between these two sympatric top-predators. By combining information from comprehensive diet and GPS movement analyses I found four dimensions of differentiation: (1) Geographic foraging area: LLB tended to forage relatively close to their nests (2.35±0.62km), while STE forage far from their nest (13.03±2.20km); (2) Foraging-habitat type: LLBs forage at low natural vegetation, avoiding cultivated fields, whereas STEs forage in cultivated fields, avoiding low natural vegetation; (3) Diurnal dynamics of foraging: LLBs are uniformly active during daytime, whereas STEs activity peaks in the early afternoon; and (4) Food-niche: while both species largely rely on reptiles (47.8% and 76.3% for LLB and STE, respectively), LLB had a more diverse diet and consumed significantly higher percentages of lizards, while STE consumed significantly higher percentages of snakes. My results suggest that this multidimensional differentiation allows the spatial coexistence of these two dense populations in the study area. Regarding their nesting sites and territories, my analysis suggest interspecific competition over nesting sites and territories. I analyzed interspecific interactions between these two species by combining information from comprehensive observational and experimental methods with GIS analysis and remote sensing data. I characterized 65 variables from 381 nests along with their breeding success during four consecutive breeding seasons and found that: (a) nest site characteristics substantially overlapped between species; (b) LLBs took over 21% of all STEs' nests; (c) intraspecific aggression rates among LLBs were higher than their interspecific aggression rates with STEs and also higher than intraspecific aggression among STEs; (d) LLB and STE breeding densities (1.59±0.11 and 2.96±0.11 pairs/10km2, respectively) are likely the highest across their respective breeding distributions with a maximum productivity of 0.96±0.01 and 0.56±0.05 (young fledged/breeding pair) for LLB and STE, respectively; and (e) intraspecific interactions among both species play an important role in determining their breeding success and the spatial distribution of nesting sites. My results suggest interspecific competition over nesting sites and territories between both species, and that LLB dominancy has both direct and indirect impacts on the spatial and demographic distribution of STEs, as seen through the establishment of new LLB pairs in the Judean breeding area. In addition, I present novel insights concerning avian movement and challenge the traditional perspective of avian migration, by revealing an exceptional movement pattern: Migration is a fascinating phenomenon, which in the northern hemisphere involves directed movements between northern breeding grounds and southern off-breeding grounds. Reverse migration - a migratory movement in the opposite direction, had been seldom studied and considered to be an exceptional behavior of animals. I show that the Judean long-legged buzzard population is migratory rather than sedentary, and that this population performs a true reverse migration from their southern breeding grounds in Israel to their northern off-breeding grounds in Syria, Turkey and Russia. I suggest that this unusual phenomenon is likely related to food availability in their breeding and off-breeding grounds. I used the remote sensing based Enhanced Vegetation Index (EVI) to provide support to the hypothesis that LLBs track ecological productivity along latitude gradients and maintain consistent low EVI levels across their annual cycle. These low EVI levels are linked to the foraging ecology of LLBs, which forage in relatively open areas characterized by low vegetation. To the best of my knowledge, no similar study has shown a true reverse migration pattern and provide a reliable suggestion for this behavior. Moreover, this species had never been tagged before and its annual movements were previously unknown.
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