In precocial species, large brood sizes are usually considered as beneficial and individuals in larger broods grow faster and are more dominant compared to individuals in small broods. However, little is known whether family size also beneficially affects the offspring's physiology.
In the present study, we investigated whether leucocyte profiles in fledgling Greylag Geese (Anser anser) are affected by (1) family size, (2) individual characteristics, i.e. age, body condition or sex, or (3) characteristics of the parents, i.e. previous reproductive
success. From spring 2013 to autumn 2015, we collected blood samples from 100 juvenile Greylag Geese from 20 different pairs. From these samples we determined the absolute leucocyte number, an individual's differential blood cells count and an individual's haematocrit (HCT). The number of
fledglings in a family and therefore the number of siblings a focal individual had, was positively related to the percentage of basophils, negatively to the heterophils/lymphocytes ratio (H/L), and tended to be negatively related to the percentage of monocytes and eosinophils in a sample.
H/L ratio was negatively related to age in days and tended to be negatively related to body condition, whereas the percentage of basophils tended to be positively related to it. Absolute leucocyte number did not differ between individuals depending on family size. However, composition of different
leucocyte types (basophils, eosinophils, H/L ratio) was modulated mostly by the social environment (family size) and not by the characteristics of the individual or the parents. In conclusion, even though we did not find clear evidence of a positive health effect, i.e. a better immune
system, in fledgling Greylag Geese of large versus small families, our results suggest that family size modulates different components of the immune system hinting at its stress-reducing effect.
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