Skull measurements from 42 great-tailed grackles (Quiscalus mexicanus) from Central and North America

In this study, we explored whether it is possible to validate a field proxy for brain size in great-tailed grackles (Quiscalus mexicanus) using the linear measurements of skulls.

We collected data from February through September 2014 on 42 great-tailed grackle skulls (see more information in the associated journal article: Logan & Palmstrom in review) obtained from the Museum of Southwestern Biology and the Ornithology Division of the KU Biodiversity Institute and the Santa Barbara Museum of Natural History.

We obtained endocranial volumes using three methods:
1) Linear measurement method - Linear measurements were collected using callipers as would be measured on a live bird in the field. We recorded skull length from the base of the bill to the back of skull along the occipital crest (Figure 1). Skull height from posterior edge of the foramen magnum to the top of skull along the frontal region (Figure 2). And skull width at the widest part of the brain case along the squamosal bones (Figures 3) to the nearest 0.1mm.

2) CT scan method - Skulls were CT scanned at the Pueblo Radiology Medical Group in Santa Barbara California using a Siemans 16-slice Somatom Sensation 16. Endocranial volume (cm3) was calculated using the DICOM viewer OsiriX v5.8.5 (Figure 5) for 1x1mm slices (regular) and for 1x1mm slices that were taken with the CT scanner bed moved 0.5mm forward (offset). We used the average endocranial volume in analyses. The offset was added to increase the precision of the endocranial volume measurements since grackle skulls are small (approximately 20mm in length). This resulted in about 20 slices per scan (one slice every 1mm). The offset allowed us to measure more area (one slice every 0.5mm) by increasing the number of slices to approximately 40 per skull.

3) Bead method - Endocranial volume was measured by pouring 1mm diameter glass beads (BioSpec Products, catalog number 11079110) into the cranium through the foramen magnum until full. The skull was repeatedly shaken to settle the beads and then filled again until the beads reached the posterior foramen magnum without falling out (Figure 4). The volume was calculated by pouring the beads out of the skull and into a graduated cylinder (5ml in 0.1ml graduations).