DOI: 10.6078/d1wt5h
title: Junco hyemalis Bone Microstructure

description: 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.  
description: 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<sup>-3</sup> 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.).
description: General notes: All specimens are
 included twice in the spreadsheet: once for data on the humerus, and once
 for data on the femur. There are some missing values
 for elevation of collection (11 out of 87 specimens).
 Full definitions for all data fields:
 Museum: Institution from which the specimen was
 loaned. Here it is always the American Museum of Natural History (AMNH) in
 New York, NY, US SpecNum: Specimen number of the
 skeleteon in the museum Name: Full species name of
 the specimen (Genus species subspecies) Subspp:
 Subspecies name Sex: Specimen sex
 Location: Details on where the specimen was collected
 Elev: Altitude (in m) at which the specimen was collected, if
 available. Date: Month, day, and year at which the
 specimen was collected Day: Julian date: the integer
 day of the year where 1 corresponds to January 1st and 365 corresponds to
 December 31st (in a non-leap year) Mass: Weight at
 collection in grams Side: Right or left wing / leg
 from which the bone was taken Bone: Femur or
 humerus CtDOI: DOI number for cortical computed
 tomography data available at MorphoSource
 (https://www.morphosource.org/projects/0000C1143)
 TbDOI: DOI number for trabecular computed tomography data
 available at MorphoSource
 (https://www.morphosource.org/projects/0000C1143)
 Length: Bone length in mm, taken during the
 scout-view TbBV: Trabecular bone volume (mm^3):
 Volume of mineralized trabecular bone tissue within the total volume
 analyzed BVTV: Trabecular bone volume fraction (%):
 Ratio of trabecular bone divided by the total volume of tissue
 analyzed ConnDens: Connectivity density (mm^-3): The
 degree of connectivity between trabecular, normalized by the total volume
 assessed SMI: Structure model index (SMI): Trabecular
 structure assessed from 0 to 3, where 0 is plate-like and 3 is
 rod-like TbN: Trabecular number (mm^-1): Average
 number of trabeculae per unit length. Calculated as the inverse of the
 mean distance between the mid-axes of the trabeculae
 TbTh: Trabecular thickness (mm): Average thickness of a
 trabecula. Determined by fitting a sphere to the trabecula and averaging
 the diameter of the sphere for all trabeculae TbSp:
 Trabecular separation (mm): Average distance between trabeculae. Found
 using the same process as trabecular thickness but spheres are fit to the
 empty space rather than the trabeculae CtAr: Cortical
 bone area (mm^2): Average cross-sectional area within the tissue that is
 mineralized bone TAr: Tissue area (mm^2):
 Cross-sectional area within the outer edge of the diaphysis (e.g. the
 whole bone cross section), including both mineralized bone and interior
 bone marrow. Averaged across all cross-sections in the analyzed
 volume MaAr: Marrow area (mm^2): Average
 cross-sectional area of marrow tissue, equal to T.Ar – Bn.Ar
 CtArTAr: Cortical bone area fraction (%): Fraction of total
 tissue area occupied by mineralized bone CtTh:
 Cortical thickness (mm): Thickness of the cortical bone averaged across
 all cross-sections J: Polar area moment of inertia
 (mm^4) Geometric resistance of the cortical bone to twisting. For a hollow
 cylinder, this is , J = π(D4 – d4)/32, where D is the outer diameter of
 the circle, and d is the inner diameter CtTMD:
 Cortical tissue mineral density (mg cm^-3): Average amount of mineralized
 tissue within a volume of cortical bone, calculated from the grayscale
 value and converted to a physical density using a calibration phantom.
 This includes small (5 μm) pores containing cells and their dendrites, but
 not larger pores containing blood vessels