Junco hyemalis Bone Microstructure

Migration is the primary strategy that temperate birds use to avoid overwintering under harsh conditions. As a consequence, migratory birds have evolved specific morphological features in their wings and skeleton. However, in addition to varying in overall shape and size, bone can also change at the microstructural level by, for example, increasing its thickness. Such changes are critical to preventing fracture and damage under repeated loading (fatigue), yet it is not known whether migratory behaviour influences bone microstructure. To address this gap in the literature, we performed micro-computed tomography on skeletons of resident and migrant subspecies of the Dark-eyed Junco Junco hyemalis. We investigated the differences in the major wing bone, the humerus, and the major leg bone, the femur. In each bone, we studied the microarchitecture of the two types of bone tissue: cortical bone, the thick outer layer of bone; and trabecular bone, which is the porous network of bone tissue at the ends of long bones. We used linear models to quantify morphological features with respect to body mass and migratory behaviour. Humeri from migratory birds were thinner, wider, and had higher overall geometric stiffness, that is, a higher polar moment of inertia, relative to humeri from resident birds. These features may help keep their bones stiff to maintain their increased body mass during migration. In contrast, migrant femora were shorter, thinner, and had lower geometric stiffness than femora of residents, potentially to reduce total body mass. Tissue mineral density was lower in in both the humerus and femur of migratory birds. In addition, migratory subspecies had less trabecular bone (lower bone volume fraction) due primarily to a loss in trabecular thickness. Migratory behaviour may thus select for improved stiffness and fatigue resistance in the wing bones and reduced mass of leg bones. Our work demonstrates how important insights about morphological adaptation can be obtained by investigating bone microstructure.  

Methods
Specimen data (specimen name, date collected, altitude, etc.) were recorded during collection and obtained by querying the AMNH ornithological collections database. Length was measured during the "scout-view" preview scan for each bone. Micro-computed tomography data were collected for each bone using a high-resolution micro-computed tomography scanner (μCT 35, Scanco Medical, Brüttisellen, Switzerland) operated at an X-ray energy of 70 kVP, an integration time of 300 ms, and a spatial resolution of 6 μm isotropic voxel size. To assess morphological features, regions of interest were first defined using semi-automated region-drawing scripts, and then a threshold of 540 mg cm-3 was applied to differentiate bone from air. Finally, an automatic script (IPL, Scanco Medical, Brüttisellen, Switzerland) employing distance transformation methods was used to measure various morphological features (trabecular number, cortical thickness, etc.).