with increasing endemism level of the island focal
species. For instance, the Canarian endemic blue
chaffinch (Fringilla teydea) has a mean clutch size of
2.0 eggs (Martín and Lorenzo 2001), the endemic
subspecies of chaffinch Fringilla coelebs canariensisa
mean clutch size of 2.6 (Martín and Lorenzo 2001),
whereas the northwest African subspecies of
chaffinch (F. c. africana) shows a value of 3.9 (Cramp
and Perrins 1994). Canarian endemic laurel-forest
pigeons (Columba bolliiandC. junoniae) lay one egg,
instead of the two eggs laid by the native but non-
endemic rock dove (C. livia). The Canarian kestrel
(Falco tinnunculus canariensis) has a mean clutch size
of 4.41 eggs, whereas Moroccan kestrels, at the
same latitude, have an average clutch size of 4.80.
Several hypotheses, more or less interconnected,
have been suggested in explanation of the decrease
in fecundity of island birds.
●The energy reallocation (Cody) hypothesis. This
is a refinement of David Lack’s hypothesis on the
evolutionary significance of clutch size, and has
been especially advocated by Cody (e.g. 1971). It
posits that greater environmental predictability on
islands will lower mortality, resulting in lower pop-
ulation fluctuations than in mainland regions. This
might result in selection for smaller clutch size,
with reinvestment of the saved energy in other
components of fitness, such as better quality of
young or increased longevity for parents through
better foraging efficiency, predator avoidance or
competitive ability (Blondel 1985).
●The resource predictability (Ashmole) hypothe-
sis. First postulated by Ashmole (1963) and sup-
ported by Ricklefs (1980), this hypothesis predicts
that clutch size is determined by the differences in
food resources between the non-breeding and the
breeding season, increasing the clutch size in direct
proportion to seasonality. In stable island environ-
ments, where seasonality is buffered by the ocean,
there is less seasonal fluctuation in food supply,
thus favouring a diminution of the clutch size.
●The density compensation hypothesis. Island
isolation precludes the arrival of many mainland
competitors, which drives density compensation in
the island species (MacArthur et al. 1972), resulting
in higher population sizes and thus in higher
intraspecific competitiveness for resources, favour-
ing higher investment in fewer young (Krebs 1970).
●The reduced predation and parasitism hypothe-
sis. Island isolation prevents many groups of conti-
nental predators and parasites arriving, thus the
insular bird populations have higher survival rates,
and competitiveness, which results in a diminution
in clutch sizes.Relaxation in territoriality
Insular lizards, birds, and mammals often exhibit
reduced situation-specific aggression toward con-
specifics. This relaxation in aggressive behaviour
can be expressed in the form of: (1) reduced terri-
tory sizes, (2) increased territorial overlap with
neighbours, (3) acceptance of subordinates on the
territory, (4) reduced aggressiveness, or (5) aban-
donment of territorial defence (Stamps and
Buechner 1985). These changes are often associated
with unusually high densities, niche expansion,
low fecundity and the production of few, competi-
tive offspring. Two main non-exclusive hypothesesNICHE SHIFTS AND SYNDROMES 191Table 7.2Alterations in nesting sites in some Orkney birds first noted by David Lack (after Williamson 1981, Table 6.4).
In all cases the normal mode is also found in Orkney
Species Normal mode Orkney mode
Fulmarus glacialis(fulmar) On cliffs On flat ground and sand dunes
Columba palumbus(woodpigeon) In trees In heather
Turdus philomelos(song thrush) In bushes and trees In walls and ditches
Turdus merula(blackbird) Woods and bushy places Rocky and wet moorland
Anthus spinoletta(rock pipit) Sea cliffs Out of sight of sea
Carduelis cannabina(linnet) Bushes and scrub Cultivated land without bushes; reedy marshes