Evidence of links between haematological condition and foraging behaviour in Northern gannets (Morus bassanus)

Haematological analyses can reveal the physiological condition of birds,
who are known to efficiently disguise symptoms of stress and disease.
However, interpretation of such analyses requires species-specific
baseline data, which are lacking for most free-living seabird species. We
provide baseline reference data for several haematological parameters in
Northern gannets (Morus bassanus) and combine this with telemetry and
dietary data to understand the links between haemotologic condition and
foraging behaviour. Blood samples were collected from breeding Northern
gannets in July 2017 (n = 15) and 2018 (n = 28), which were also equipped
with GPS tags. Smears were prepared for performing blood cell counts,
including immature erythrocyte and microcyte percentages, total and
differential leucocyte counts, heterophil:lymphocyte (H:L) ratio, and
total thrombocyte count, the remaining blood was used for stable isotope
analysis, and foraging behaviours were inferred from the recovered tag
data. Blood cell counts revealed that the sampled birds were highly
stressed and some showed an immune response, evident from the abnormal
leucocyte counts and H:L ratio. There were no sex-related differences in
haematological parameters or diet, in contrast to foraging parameters
where females undertook longer trips than males and spent proportionately
more time in search behaviours. The percentage time spent actively
foraging was negatively correlated with the percentage of eosinophils.
While there was no direct link between haematologic condition and diet,
one bird feeding at a relatively low trophic level undertook exceptionally
short foraging trips and showed abnormal blood cell counts. This suggests
a link between haematologic condition and foraging ecology that can be
employed in assessing seabird health.