Flight performance of great cormorants Phalacrocorax carbo sinensis suggests sufficient muscle capacity for adaptive speed adjustment

TL;DR

Greater cormorants can adjust their flight speed in response to ecological needs and wind conditions, suggesting sufficient muscle power despite their large body size.

Contribution

This study shows that greater cormorants can adaptively adjust flight speeds, challenging previous assumptions about their limited flight performance.

Findings

Cormorants can achieve airspeeds beyond minimum power speed during migration and foraging.

They demonstrate partial wind drift compensation by adjusting flight headings along coastlines.

Flight speed adjustments to headwinds were inconsistent in some situations.

Abstract

Power required to fly for a bird generally follows a U-shaped function of airspeed, with higher cost at both low and high speeds. Because power required increases with body mass faster than power available from flight muscles, larger birds may experience restricted flight speed ranges and climbing capabilities. Previous studies found limited flight performance in cormorants. Adapted for both flight and sub-surface swimming, they trade off larger flight muscles for powerful leg muscles used for diving. Our study tested whether the flight performance of greater cormorants is constrained by measuring airspeed under various seasonal and wind conditions. If flight muscles severely limit the range of flight speeds, cormorants would not be able to adopt ecologically relevant speeds between seasons and not increase speed in headwinds to minimize cost of transport. Results suggest that cormorants can achieve airspeeds beyond minimum power speed, selecting speeds near maximum range during autumn migration and exceeding this range on spring migration and during foraging flights. However, expected speed adjustments to headwinds were inconsistent, with some situations lacking the anticipated responses. The cormorants demonstrated partial wind drift compensation by adjusting flight headings along coastlines, though airspeed adjustments were not always observed. Although greater cormorants appear capable of reaching ecologically relevant speeds in many contexts, the overall scope of their flight speeds remains relatively narrow compared with smaller bird species. These findings indicate that greater cormorants have muscle power for adaptive behaviour in some cases, despite the influence of physiological constraints on their flight performance.

Summary: Contrary to previous assumptions, greater cormorants have sufficient muscle power to adjust flight speed in relation to ecological context and winds.

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