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Aggregation

Aggregation is a dynamical situation induced by food being scarce, patchy, unevenly distributed or unpredictable, related to the species biology such as colonial or solitary breeder, but basically any organism have the potential of showing aggregation behaviour, according to the optimal foraging cost (Smith 1968; Horn 1986; Newton 1979; Davies & Houston 1984) and is shaped by human made disturbances or fires (Brotons et al. 2005).

 

Most studies on aggregation phenomena were mainly in the context of social influences on foraging (Dubois & Giraldeau 2003) or focusing on prey behaviour (Lima 2002; Nelson et al. 2004) but almost no study has focused on  movement dynamic questions, in relation to bird aggregation, breeding, migration, food abundance and long term and short term habitat disturbance together.

 

In the proposed study I will address the above questions, using a non colonial summer visitor/ passage migrant raptor that feeds mostly on reptiles, which I found to have a spatio-temporal large aggregations, where main prey is rodents (both trends never documented for this species before!), by following agricultural activities, mainly ploughing: The Short-toed Snake-Eagle (Circaetus gallicus).

 

I will investigate the effect of agricultural practicing and prey availability, on shaping the Short-toed Snake-Eagle movement and behaviour.

 

My observations suggest that this species, an eagle that its population in Israel seems to be steady, but decreased through most of its north European ranges in the past, studied mainly on its nesting site, previously considered to be an almost exclusive feeder on reptiles, specially snakes (Cramp & Simmons 1980) may serve a unique case study for investigating both local and large scale movement patterns, particularly the patterns of formation and disintegration of aggregations. Thereby, this study will provide better understanding of a relatively poorly-studied species along with better knowledge of decision-making, behaviour and spatial patterns in the context of human related interferences and aggregation in raptors.

 

This study, hopefully proceeds to PhD, will emphasise the importance of Israel and its neighbouring countries to their Short-toed Eagle summering and passage migrant populations and will clarify the potential of this raptor as a beneficial biological controller of rodents and its vulnerability to pesticides, hence, offering new criteria for its protection.

In the Middle East, the Short-toed Eagle is the most common eagle, having a great symbolic value, and its study is planned to bridge between different nations, especially Israelis and Palestinians, through education, stressing its boundless mobility, flexibility, and beauty.

Background on the Short-toed Eagle:

Circaetus gallicus is a medium size eagle of the Accipitridae family. This genus (the snake-eagles) includes six species (C.beaudouini, C.cinerascens, C.cinereus, C. fasciolatus, C.gallicus, and C.pectoralis), five of them restricted to Africa. Only the Short-toed Eagle extends outside this continent, and breeds in southern Europe and Asia, and also in northwest Africa. Individuals were observed in Kenya, after being misidentified as C.beaudouini (Clark et al.2005).

The Short-toed Eagle prefers relatively open habitats for foraging (making it easy to observe its behavior and to follow individual birds). The Short-toed Eagle nests mostly on trees, but also on cliffs. It has one breeding clutch each year and lays one egg. It reaches maturity after four years. No obvious

sexual dimorphism exists.

Status in Israel: Common summer breeder in northern and central Israel, especially in open dry areas of Mediterranean and semi desert habitats. It is also a common passing migrant through Israel (Shirihai 1996).

This is almost the only species who was not affected by the vast poisoning of raptors that occurred in Israel during the 1960's,mainly because of its diet content (mostly reptiles) and the utilization of pesticides mostly during winter (Meir 1986).
 

Study area

The study area is about 350 square kms, characterized by a heterogeneous, complex matrix which includes many agricultural-oriented settlements (the fields are either family-owned small fields or collectively-owned large fields), natural chaparral regions, afforested areas and natural pastures –some of which are heavily grazed by livestock.

This area, which is on the main western migration route in Israel, contains one of the largest Short-toed Eagle concentrations in this country, and is known for large aggregation incidences, occurrences of different numbers of individuals together.

The same area served as the study area of Meir (1986), in the only systematic study of the biology and population ecology of the Short-toed Eagle in Israel (during 1980 – 1985).Thus, working in the same region will provide an opportunity to compare our results with those of Meir (1986)and investigate the changes that may have occurred in the species' ecology and behavior during that time of agricultural intensification.

While the study of Meir (1986)has focused mainly on the breeding success, he studied also the movement dynamics (using wing tags and color rings through simple visual techniques).Therefore, I will also be able to compare and evaluate the different working methodologies.