However, it is not so easy for the Bulwer’s Petrel Bulweria
bulwerii of Selvagem Grande (30°09'N, 15°52'W). This very
small petrel, the smallest of the procellariids (body mass
c. 100 g), has access to breeding holes inaccessible, if not
invisible to the observer who can never be sure to have taken
into account all the nests of a colony. If banded birds move to
a new nest site (which is quite frequent because the longevity
of the nests is often much lower than that of the birds) the probability
of further control is rather low. In fact, the recapture rate
is far lower than the survival rate, which means that many birds
escape observation, often definitively if they have moved to
an inaccessible nest. So some calculations and adjustments, the
details of which are given in the following pages, are necessary,
permitting the estimation of the number of birds having
left the nests under study and thus a better knowledge of the
faithfulness to mate and nest site. The first colony (colony A) is
established in a wall made of big stones among which the birds
breed. Owing to the general stability, very few nests disappear
from year to year. The second group (colony B) has colonized
unsteady scree and decaying walls where the longevity of nesting
hollows is often very short. Nest density is variable but in
favourable sites it can be as high as 2–3 nests/m2. During visits
paid every year to the colonies, mostly during incubation
at the end of June and in July, all marked nests are examined,
all the new accessible nests marked and all the adults recaptured
or banded. To avoid unnecessary disturbance, the birds
are handled only once each year, and one bird at each nest site
is marked with white paint on the forehead and tail, to differentiate
it from its mate. Sex is determined by bill measurements
(Mougin 1989).
Preliminary computations
Nest changes
In the study colonies at Selvagem Grande, 19.7% of the banded
birds surviving in a given year disappear during the following
(n=734) – this figure including both dead birds and birds having
moved to an unmarked inaccessible nest. The annual mortality
rate of the adults being 4.1 % (unpubl. data), the rate of
passage from marked to unmarked nests is thus 15.6%. Moreover,
5.6% of the birds back to marked nests (n=323) – 4.5%
of the total numbers – have moved from a marked nest site to
another marked nest site. So, 79.0% of the surviving birds are
faithful to their nests.