Ten out of 90 birds exhibited vocalisations during LAPS. Potential
vocalisations were observed in a small number of birds in a previous
LAPS study, but the nature of the sounds generated by birds indicated
that they were caused by involuntary movement of air through the
body during convulsions [Martin et al., in press]. In the current study,
nine of the 10 birds vocalised after loss posture and of those that had
EEG implants showed slow-wave activity, however repeated
vocalisations resembling ‘peeping’ were apparent. Marx et al. describes
vocal responses to isolation in chicks and distinguishes between distress calls (characterised by declining frequency and high energy) and short
peeps (characterised by declining frequency, low energy and short duration)
[34]. Thus peeping-type vocalisations may signal social or isolation
distress, but have also been claimed to be a common pattern in the
regular vocalisation of chicks in non-isolation situations [31]. The occurrence
of vocalisations during LAPS raises questions about the conscious
state of the birds and also the welfare implications of the process. In
general, vocalisation after stunning is thought to indicate consciousness
and possibly distress [51], though vocalisations have been reported in
pigs during stunning after loss of posture [22] and in dogs during anaesthesia
when apparently unconscious [e.g. 3]. Peek et al., reported that
the same nerves and muscles were active during both vocalisation and
respiration in chickens there was strong neural interaction between
these systems during stimulation of the respiratory system with carbon
dioxide [43]. The respiratory effects of hypoxia therefore could possibly
be responsible for the vocalisations observed, especially since their frequency
and timing exactly matched abdominal respiratory movements;
this may explain the presence of this response in apparently unconscious
birds. The welfare relevance of these vocalisations is difficult to assess, but
vocalisations occurring following loss of posture in a subset birds does not
fit with the idea that they are caused by negative sensations as a direct
consequence of undergoing LAPS — which we would expect to affect all
birds and be exhibited in the conscious phase. Recently, a study subjecting
birds to LAPS with and without administration of an opioid analgesic
(butorphanol) showed that head-shaking and vocalisation were not reduced
with provision of analgesic agent [Martin et al., in press].