L’aquila dei serpenti. Extracts from the summaries of some Chapters

Francesco PETRETTI
This book deals with the results of a long term study on the behavioural ecology of the Short–toed Eagle in Central Italy.
I began field surveys in 1972 and I have been monitoring the breeding population for more than thirty five years, collecting data on the habitat selection, the population size, the home range distribution, the feeding ecology and the growth and development of the nestlings.
The specific objectives were to study the reproductive cycle and the breeding success, the population structure and dynamics, the hunting strategy and prey composition , the social behaviour within the eagle population, the ecological niche of this species through a comparative analysis of other raptors, its adaptation to the Mediterranean environment and its future.
The Short-toed Eagle is a summer visitor in Italy and Europe as well.
The arrival of the Eagles on their Italian breeding ground from their African wintering quarters usually takes place in the first half of March, although arrival dates can vary significantly according to latitude and in part weather conditions.
In my study area in central Italy all pairs are on their territories by the 15th of March, while in the Alps the first individuals usually arrive in the last 10 days of the month.
By the end of March, the vast majority of Italian breeding pairs are already on their territory. The breeding phenology was assessed through the observation of the nesting attempts, when laying and hatching dates or both were known with a good degree of accuracy. The incubation period was calculated as 46 ± 1 days and the period was assumed to be the same for all other nesting attempts followed. The median laying date was 7 April (range 31 March – 16 April) and median hatching date was 23 May (15 May – 1 June). Fledging dates were calculated according to the first recorded flight of the eaglet from the nest-tree and were spread between 17 July and 17 August (median 27 July).
Nestling period averaged 65.8 days (± 6.09 range 54-80 days). Observations of the behaviour of a pair were made in 1980, during the pre-laying period. The pair was first seen in the breeding territory on 7 March; copulations were recorded from then on the nest area and hi the tree which was eventually selected for nest building. The building of the nest was started and concluded in less than 18 days (between 17 March and 3 April) and the egg was laid between 4 and 6 April. The timing of nesting in Central Italy did not differ appreciably from that reported by Cramp and Simmons (1980), although my laying dates seem advanced with respect to those of other continental and northern populations probably due to the different climatic conditions of the Mediterranean area.
Laying dates and the consequent nestling period are timed to allow chick growth in the months with minimum rain fall, which are most profitable for hunting snakes .
In my study area the cycle of breeding pairs was probably linked to the activity of snakes which come out of hibernation in early spring. The average nestling period was a little shorter than the period of 70-75 days, reported by Cramp and Simmons (1980), who summarize the results of all the relevant studies on this species.

Population and territory

In my study area I recorded an average density of 48.6 km2/pair, including an average of 25.9 km2 of open land per pair. Mean nearest neighbour distances between nests ranged between 2.4 and 6.3 km in 1980 and between 2.4 and 6.4 km in 1981 (mean 4.4 km). The shortest distance between two active nests was 1.2 km in 1985.
Home ranges, measured from the final map connecting the outermost plotted points, ranged from 13.0 to 31.0 km2 (polygon method) and from 28.2 to 63.6 km2 (circle method).
Although cited authors do not specify if density figures refer to woodland and open land combined or just to one of them, the data suggest that the populations breeding in wetland and plain areas, such as in the Guadalquivir and Evros deltas, are the densest. My results lie between the values reported for these highly productive ecosystems and those for barren and less productive maquis.
The pairs used to build a new nest each year: the average distance between nest-trees in the same breeding territory (n=20) in 1980 and in 1981 was 327.8 m, ranging from 150 to 450 m (± 97.18).

Daily cycle

The main activities of adults were classified as follows:
– nest attendance (incubation and/or brooding, feeding chick, platform building);
– roosting on perches;
– time spent away from the nest area (hunting, travelling, interacting with other individuals).
The relative lenghts of activities expressed as the percentage of daylight hours from sunrise to sunset (n=20 days) are summarized in figures and tables.
The male spent roughly 50% of the day time perched on roosts and 50% away from the nest area during the whole breeding season, while the female spent the incubation period almost completely attending the nest, though in a few instances I recorded females moving from the nest and leaving the egg unattended for a few minutes in warm hours. The time spent at the nest by trie female during the nestling period decreased from 59.8% to 3.2% as the chick grew, while time spent away from the nest increased when both partners were involved in foraging trips and agonistic interactions. Summing up the contributions of both sexes, the time spent away from the nest area ranged from a minimum of 38.2% of daylight hours during the incubation to a maximum of 98,6% of daylight hours during fledging. In the pre-laying interval, the female seemed to spend more time in the nest area than the male.
Departure and arrival times of partners in the nest area were positively correlated with sunrise and sunset .The birds began to move from night perches two or three hours after sunrise and ceased flying activity generally two hours before sunset. Morning activity began with preening on night roosts and with short-distance flights from perch to perch. I recorded the time when preys were brought to the nest .Most observations (70%) occurred between 09:00 and 13:00 h.
In Tolfa hills the nests were usually located in dense timber, near the tops of the hill; usually well away from roads. The altitude of the nest tree and that of the were positively related (p<0.001) (Out of 21 nest sites, 6 (28.5%) in evergreen oak woodland and maquis, 6 (28.5%) in deciduous wood scattered with evergreen oaks and 9 (43.0%) were in pure deciduous wood).
Nests were built by both sexes. In 1980, one pair occupied the nest built by a pair of Black Kites (Milvus migrans), forcing them to move before laying.
The pairs rarely used the same nest in consecutive years; out of 39 nesting temps, the same nest was used for two consecutive years only in two cases and r three years in one case.
Nests were usually placed right in the top of the tree (57.2%) or on lateral anches (42.8%) overlooking steep slopes. Their features are summarized in table 4.
The platforms were made of twigs; green shoots were added during the incubation and the nestling season by both parents (green material was carried to the nest in six instances during 20 days of observation). The raptors were seen collecting nest material both from the canopy of trees and from the ground in small clearings near the nest area. Twelve different species of evergreen and deciduous trees and bushes were identified in the nest material.