Beak dimensions affect feeding performance within a granivorous songbird species

Beaks of granivorous songbirds are adapted to dehusk seeds fast and efficiently. This is reflected in the large variety of beak shapes and sizes among species specialized in different seed types. Generally, larger beaks improve the dehusking of larger seeds by transmitting and withstanding higher bite forces. Meanwhile, smaller beaks are better suited for processing smaller seeds by allowing faster beak movements and better seed handling dexterity. These patterns are presumably the result of a trade-off between force and velocity inherent to lever systems. Since beak shape also varies among individuals of the same species, we investigated whether beak shape relates to variation in feeding performance and beak kinematics in the Domestic Canary (Serinus canaria). We analysed beak morphology of 87 individuals through both traditional size measurements and 3D-landmark analysis to capture metrics such as beak depth, length, width and curvature. We related these metrics of morphology to data on feeding performance and beak kinematics during feeding on smaller canary seeds and larger, tougher hemp seeds. We found that individuals with larger absolute beak depths were faster at dehusking the large seeds. Despite that individuals with shallow or long beaks displayed higher beak opening-closing frequencies, this did not result in a significantly shorter processing time of the smaller seeds. Our data are therefore in line with the presence of a force-velocity trade-off within a species, but without a velocity-related drawback of beak-size adaptations for increased bite force on the handling performance of a smaller and easier-to-crack seed.

Methods
Image data of birds with their beaks held in a closed resting position were selected from the recordings of Andries et al. (2023).

Image data was calibrated using synchronized images of a calibration object with known dimensions in XMAlab.

Landmarks were annotated on the calibrated images in XMAlab.

3D-coordinates of the landmarks were extracted and used to calculate beak size metrics in Microsoft Excel.

To analyze beak curvature, 3D-coordinates of the landmarks on the upper beak's curve were further processed and analyzed in R (version 4.3.3.).

Morphological data were related to feeding performance, beak kinematics and seed handling skill metrics, directly taken from Andries et al. (2023).