Fig. 4. (A) Stroke frequencies (strokes s^–1) during deep and shallow diving in double-crested cormorants. Values are grand means established from individual bird means ± 1 s.d. (the number of birds used is indicated above each column) and are based on 10 observations per bird and category. Birds ascended passively during deep diving. *Significantly different from shallow diving. (B) Modelled changes in mechanical work against buoyancy per s during descent for a 2.1 kg double-crested cormorant (assuming a vertical descent speed of 1 m s^–1). The horizontal line indicates the point of neutral buoyancy. Note that change is greatest within the first 10 m.