Mammals under a colony of great cormorants: population structure and body condition of yellow-necked mice

Investigations into small mammals within the territory of a breeding colony of great cormorants (Phalacrocorax carbo sinensis), carried out in 2011-2014 near Juodkrante (West Lithuania), demonstrated the colony's impact on the dominant rodent, yellow-necked mouse (Apodemus flavicollis). The age and sex structure of the sampled mice (n = 432), along with body weight, body condition index, and residuals from the linear regression, were used to analyse data from five zones of the colony and the surrounding forest. We found that in the most active zones of the cormorant colony, the age structure of the population was tilted towards a prevalence of juveniles, while sex structure was towards a prevalence of males (P < 0.0001). Despite males being significantly longer in body and heavier, body condition index was the same in both sexes. The effect of the zone of the colony was confirmed for body weight and its residuals; body length and body condition index (main effects ANOVA) were negative in the zones where cormorants were nesting actively.