Does familiarity with the release site reduce the deflection induced by clock-shifting? A comment to the paper by Gagliardo et al. (2005)
Zoologisches Institut der J.W. Goethe-Universität Frankfurt, Siesmayerstraße 70, D-60054, Frankfurt am Main, Germany
* Author for correspondence (e-mail: wiltschko{at}zoology.uni-frankfurt.de)
Accepted 16 August 2005
When homing pigeons are subjected to a shift of their internal clock, their
vanishing bearings are deflected with respect to untreated controls,
indicating the use of the sun compass. However, the induced deflection is
often smaller than the difference in sun azimuth between the birds' subjective
time and the real time of day (e.g.
Schmidt-Koenig, 1961). This
phenomenon is most pronounced in old, experienced pigeons, which, on average,
show only about 50-60% of the expected deflection
(Wiltschko et al., 1994
;
Wiltschko and Wiltschko, 2001
).
Wallraff et al. (1999
), as well
as Kamil and Cheng (2001
),
hypothesized that the reduced deflection was caused by these pigeons' greater
experience, in particular their familiarity with local landmarks. Gagliardo et
al. (2005
) recently published a
paper claiming that familiarity with the release site is indeed one of the
factors reducing the deflection induced by clock-shifting.
This claim, however, is not justified. An analysis of the response to
clock-shifting must be based on the behavior of normal, otherwise untreated
pigeons, i.e. the data of the control group C in Series I and the data of
Series II (tables 3 and 4 in Gagliardo et
al., 2005). Unfortunately, the pigeons familiar and unfamiliar
with the release sites were from different groups, with possible differences
in pre-experience, and were released in different years. More importantly,
however, the pigeons familiar with the release sites were released as
clock-shifted birds at all three sites used, whereas the pigeons unfamiliar
with the sites were released only once under clock-shifted conditions. This is
a serious flaw in test design, making any comparison problematic. The familiar
pigeons undertook their second and third flights as clock-shifted birds after
they had just experienced that their sun compass provided them with false
information and that they had to ignore this false information to return home.
This affects the sun compass: repeated releases alter the sun compass readings
and eventually lead to a recalibration, as has been convincingly demonstrated
in two experimental series transferring pigeons to a `permanent' clock-shift
(Wiltschko et al., 1976
,
1984
). The test design of
Gagliardo et al. (2005
), by
repeatedly releasing the familiar birds under clock-shifted conditions, thus
tends to reduce the deflection of this group.
The authors claim that this is not the case, referring to Foà and
Albonetti (1980), who released
clock-shifted birds several times and observed larger shifts at two unfamiliar
sites. However, deflections induced by clock-shifting always show considerable
variation. Foà and Albonetti
(1980
) failed to do the
critical test indicating whether the sun compass was still intact, namely to
record the orientation of the formerly clock-shifted birds immediately after
their sun compass was set back to normal. This was done in the two studies by
Wiltschko et al. (1976
,
1984
), with a pronounced
deflection in the reverse direction indicating a recalibration of the sun
compass.
The familiar birds of Gagliardo et al.
(2005) indeed showed their
largest deviation, 98°, on their first clock-shift release, a deflection
that lies clearly within the range of the deflections observed in the
unfamiliar birds - 127°, 116° and 68°. The later deflections of
the familiar birds, 78° and 31°, are smaller and can no longer be
assumed to reflect an intact sun compass.
Yet even so, the claim that familiarity with the release site reduces the deflection is not justified. A statistic based on matched pairs of data cannot be applied on just three releases, and a test such as the t-test does not show a significant difference (t=1.334, P>0.05). Hence, although the test design leads to a decrease in deflections of the familiar birds, one might say that, at best, the case is open.
To answer the question requires more data obtained under equal conditions
for the familiar and unfamiliar birds. We have just completed a series of six
such tests, with birds of equal pre-experience released on the same days at
sites that were familiar to some of the birds but unfamiliar to the others,
with controls and 6 h fast-shifted birds of both groups. Our data clearly
contradict the claim of Gagliardo et al.
(2005): the deflections vary
considerably, but those of the familiar birds range from 39° to 169°
(median, 81°) and those of the birds unfamiliar with the site range from
27° to 124° (median, 68°)
(Wiltschko et al., 2005
). That
is, the size of the deflection induced by clock-shifting is not affected by
familiarity with the release site.
To answer the question of what factor reduces the clock-shift-induced
deflection, we may point out that clock-shifted pigeons show the expected
deviation when their magnetic compass is temporarily disrupted by magnets
(Wiltschko and Wiltschko,
2001): old pigeons seem to fly a compromise between the directions
indicated by their sun compass and their magnetic compass.
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References |
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Gagliardo, A., Odetti, F. and Ioalè, P.
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