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.

Blood samples were collected from the tarsal vein of each bird. Birds were also travked using GPS loggers (i-gotu GT120, mobile action technology) attached to the feathers using waterproof tape, and removed on recapture of the bird.  2-3 breast feathers were plucked for genetic sexing following Griffiths et al.(1998). 60 µl of blood was mixed with an equal volume of EDTA, with a standardised 60 µl of this mixture used for preparing the smears. Blood smears were prepared using the two-slide wedge technique (Coles, 1986), which were then allowed to air dry. Remaining blood samples were centrifuged at 1600rpm for 10 minutes. The red blood cell component was dried at 60 degrees C for 24 hours and then ground to a fine powder and lipids were removed using a 2:1 mix of chloroform:methanol before being sent for analysis of Carbon and Nitrogen stable isotopes.

We provide raw tracking data from the GPS tags and Carbon/Nitrogen stable isotope values from sampled birds.

GPS locations were sampled at 3-minute intervals. We provide raw tracking data so that behavioural annotation can be replicated