Response to `Does familiarity with the release site reduce the deflection induced by clock-shifting?'
Dipartimento di Etologia, Ecologia ed Evoluzione, University of Pisa, Italy
* Author for correspondence (e-mail: annag{at}discau.unipi.it)
Accepted 16 August 2005
The central issue of the paper by Gagliardo et al.
(2005) is that three factors
are relevant in reducing the expected deflection in the initial orientation of
clock-shifted pigeons: (1) familiarity with the release site; (2) anosmia and
(3) the release site features. However, in their comments to our paper,
Wiltschko et al. (2005a
) did
not consider either anosmia or the site effects and disputed that familiarity
can affect the size of the deflection, so determining a correction of the
initial orientation towards the home direction.
In their comments, they state that the size of the deflection is smaller in
aged and experienced birds. It is worth noting that the more homing experience
a pigeon acquires, the higher the level of familiarity with a geographical
area surrounding the release sites and that very often older birds turn out to
be more experienced in terms of homing flights. In the protocol adopted in the
cited experiments (Wiltschko and Wiltschko,
2001; Wiltschko et al.,
1994
), familiarity with the release site area, age and homing
experience was interdependent or uncontrolled, making it impossible to unravel
the role of each factor. To the best of our knowledge, no evidence
demonstrating that the age per se reduces the deflection after
clock-shift has been reported. Also, the protocol adopted by Wiltscko et al.
(2005b) does not help in clarifying the debated question. All the pigeons used
in this experiment were very familiar with an area extending 40 km from home
in the cardinal compass direction, and the `unfamiliar locations' chosen as
test sites were either within or 2-15 km outside of this area. Therefore, the
authors ended up comparing the behaviour of pigeons surely familiar with the
release site with the behaviour of pigeons probably familiar with the release
site. Not surprisingly, both groups behaved in the same way.
The main criticism regarding our paper raised by Wiltschko et al.
(2005a) concerns the
possibility that the observed reduction in the size of the deflection is due
to a recalibration of the sun compass mechanism in permanently clock-shifted
birds (Wiltschko et al., 1984
).
We do not agree with this criticism for the following reasons. (1) The results
reported in Gagliardo et al.
(2005
) are not consistent with
the recalibration hypothesis. In fact, the size of the deflection does not
decrease progressively from the first to the third release (Series I, birds
familiar with the release site). Actually, the deviations of the control
shifted pigeons at familiar sites with respect to their orientation in the
unshifted condition were as follows: first release, 98°; second release,
31°; third release 75°. However, by also comparing only the first
release of Series I (Arnaccio), in which the sun compass recalibration can
surely be excluded, with the corresponding test of Series II (birds unfamiliar
with the sites), a clear difference in the size of deflection emerged, most
likely attributable to the familiarity factor: when the birds were familiar
with the release site, the observed deflection was 92.4% of the expected size;
when the birds were unfamiliar with the site the observed deflection was 126%
of the expected size. (2) Foà and Albonetti
(1980
) provided convincing
evidence against the recalibration hypothesis, showing that the size of the
deflection in permanently clock-shifted birds diminished progressively in the
subsequent tests at the familiar site, but it increased again at the
unfamiliar location. (3) A critical experiment to test the recalibration
hypothesis would consist of performing subsequent releases of permanently
clock-shifted birds from several unfamiliar locations. So far, such a test has
never been done and no convincing evidence of recalibration of the sun compass
is available. In fact, the data reported in Wiltschko et al.
(1984
) can be interpreted
differently than in terms of a recalibration of the sun compass, even
supposing that their pigeons used a site-specific compass orientation (site
recognition and compass orientation): permanently phase-shifted birds,
released many times within a familiar area, might update the association
between the familiar site and the home direction according to the route
experienced during the numerous training flights from the same locations.
Therefore, according to this view, the association `release site-compass
direction', rather than the association `time of the day-sun azimuth', is
actually recalibrated.
Another criticism raised by Wiltschko et al.
(2005a) concerns the comparison
between the data Series I (birds familiar with the release sites) and Series
II (birds unfamiliar with the release sites). We are aware that an optimal
experimental plan would have provided for the test of birds familiar and
unfamiliar with the sites in the same day and we stated in our paper that this
comparison must be considered with caution. On the other hand, Wiltschko et
al. (2005a
) based their
hypothesis on meta-analysis in which they compared orientation of birds
released not only in different years but also from different sites
(Wiltschko et al., 1984
). In
any case, the orientation shown by the pigeons unfamiliar with the release
sites reported in Gagliardo et al.
(2005
) can constitute an
indicative baseline, it being an example of the initial orientation of
clock-shifted birds unfamiliar with the same sites used in Series I.
Wiltschko et al. (2005a)
proposed that older birds deviated less because they tend to rely more on the
magnetic compass than the young pigeons do. It is worth noting that Wiltschko
and Wiltschko (1980
,
1985
) have proposed that very
young pigeons possess only the magnetic compass and use it for a
route-reversal mechanism for homing, learning a sun compass mechanism only
later on. To conciliate the two hypotheses, we should acknowledge that very
young pigeons exclusively use the magnetic compass and, once adult, rely
predominantly on the sun compass. However, when they are older than two years,
their attention to magnetic cues increases again. This certainly does not seem
to be the simplest explanation of the data set on compass orientation in
homing pigeons.
Although a role of the magnetic compass information in correcting the initial orientation of the shifted birds is plausible, the hypothesis that a decrease in the size of the deflection after clock-shift occurs is always and solely due to the contemporary use of the sun and the magnetic compass is unrealistic and not based on experimental evidence. The hypothesis of a role of the magnetic compass in reducing the expected deflection explains neither the behaviour of our anosmic pigeons released from familiar sites nor that of the birds released from unfamiliar locations. Why should the use of the magnetic compass be more important for the anosmic pigeons than for the intact birds? Why should our adult pigeons released from unfamiliar locations totally ignore the magnetic compass information?
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References |
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