|Action F8. Monitoring the population of Phalacrocórax aristotelis and Larus audouinii.|
Analysis of trophic ecology applying methodology of stable
isotopes. Results within the frame of the western Mediterranean, from
where simultaneous data on a considerable number of population groups
Fig. 1 : Reproducing couples of Larus audouinii in the Balearics since 1983.
In the census counts per islands, in the period between 2003 and 2004 it was observed that the decrease in the number of birds nesting in the Balearics in 2004 was of similar proportions on the three islands. (Table 2).
Table 1 : Results of census counts of Audouin Seagull in the period between 2002 and 2004.
In relation to the data compiled on reproductive parameters, the comparison on the proportion of nests corresponding to each different lay size in the different breeding areas between 2002 and 2004 is reflected in the following figure:
Fig.2: Proportion of nests of each lay size counted in the Balearics in the years 2002, 2003 and 2004.
It was clearly observed in the first two years, 2002
and 2003, that there was a progressive increase in the proportion of nests
the more the size of the lay increased. Nevertheless, during the counting
in 2004 it was observed that there was an increase in the number of zero
lays in detriment of the lays of three which are usually the most common
in this species. A delay in the phenology of the laying had already been
observed in 2003 in comparison with 2002 (and was so specified in this
same report corresponding to the year 2003), but this delay does not affect
the proportion of different sized lays. During the 2004 count it had also
been noticed that the laying in some colonies had been more delayed than
on the previous year. The adverse climate conditions of the spring of
2004 may have influenced this delay, although the direct causes have not
as yet been determined. There is sure evidence that some lays were lost
due to storms on the Illa de l’Aire (island), where some groups
did not recover after the loss of the first lays (R.Escandell, personal
Table 2: Comparison of the census results of Phalacrocorax aristotelis in the period comprising 2002-2004 on the island of Majorca.
As data collected throughout a long series of years is not available, it could be that the point of departure , which was the census count of the year 2002, was in fact exceptionally good and that, therefore, the actual present number of couples is more similar to the number counted in the last two years. In any case, and in order to obtain more accurate results, the assessment of more years is required.
During 2002 and 2003 data from the Pityuses (Pine Islands:
Ibiza, Formentera and islets) were forthcoming , but as the areas covered
were not the same, the only comparison available is for the island of
Majorca on which continuous data from the last three years is available.
Information on nest location has been stored in digital form.