Equal Time for the Oversized

In two earlier offerings in this journal ("Runtism Revisited" December 2009, and "The Midget Frigate..." July 2009) Steve Mlodinow discussed "runts" in bird populations, so we felt that in the interest of equal time we should take a closer look at a bird that appeared to be oversized. On 20 December 2009, while taking part in the Coos Bay, Oregon Christmas Bird Count, Shawneen Finnegan and I found a very large winter-plumaged adult California Gull at a public boat ramp in Empire, Coos County, Oregon. This individual appeared to be significantly larger than all of the other California Gulls gathered at this site. We made observations and took photos for about twenty minutes before moving on. To our eyes, this bird looked longer-legged, larger-headed, longer and thicker-billed, and bulkier overall than any of the dozen or so California Gulls hanging around the boat ramp parking area. 

Aside from having a bit more dark feathering on the hindcrown and nape, the plumage of the larger bird appeared to match that of the other similar-aged California Gulls. In a few of the many images that I captured the mantle color on the larger bird appears slightly paler than the mantles of the smaller birds. However, we did not notice any consistent difference in mantle color in the field, thus we have concluded that these slight variances (seen only in photos) can be attributed to slight changes in light angle. Comparing the relative darkness or lightness of mantle colors on gulls is problematic at best and often misleading due to the way slight light angle changes affect our perceptions of gray tones. 

There were some subtle structural differences between the larger bird (in the foreground) and other adult California Gulls in the flock. The right hand bird in this image is representative of the other adult Californias we were seeing. Note the length and particularly the thickness of the bill on the front bird. Also, it seemed to consistently show a flatter crown profile, a more squared-off hindcrown, and a longer, flatter slope to the forehead. The other Californias (like the one in the background in this image) appeared to have more rounded crown profiles, shorter, steeper forehead slopes, and more rounded hindcrowns. The image below offers a better size comparison, while further illustrating some of the structural differences. Note the overall bulk of the right hand bird, as well as the bill length and thickness.

After consulting The Birds of North America Online, it is hard to say whether this bird exceeds or falls within the normal range of size variation for California Gull. Male California Gulls average larger (mass in grams), are longer-legged, longer and thicker-billed, and longer-winged than their female counterparts (Winkler 1996). Further, it is important to point out that there are two subspecies of California Gull, both of which are known to occur along the Pacific Coast during the non-breeding season.

According to Howell and Dunn (2007), the interior L. c. albertaensis, which breeds primarily in Prairie Provinces and farther north in central Canada, averages approximately 5-12% larger than the more westerly L. c. californicus, which breeds in the Great Basin and a few isolated colonies in California. In the measurement ranges presented by Winkler (1996), size differences between the smallest female californicus and the largest male albertaensis were much more significant. In addition to averaging larger, L. c. albertaensis is typically described as paler above. However, depending on lighting conditions, differences in mantle tone between the two subspecies may or may not be perceptible (Howell and Dunn 2007).

The front four birds in this image are all California Gulls. As it appears in this image, the left most bird was larger, bulkier, and noticeably heavier-billed than any other California Gull that we observed at this site.

Howell and Dunn state that the wintering ranges of these two subspecies are poorly known. Generally speaking, the majority of California Gulls that occur along the Oregon coast are presumed to be L. c. californicus, though most observers do not make any effort to identify individual birds to subspecies. It should be noted that there is enough overlap in overall size, bill length and depth, tarsus length, and wing length (Winkler 1996) that many individuals cannot be safely identified to subspecies in the field. When observing this bird in the field, we did not perceive it to be lighter-mantled. However, the bill length and thickness, and obvious larger size would suggest that it is a male and likely a male L. c. albertaensis. Even if this bird represents the larger subspecies, visually it seemed to approach outlier proportions.

Literature Cited: 

Howell, Steve N.G. and Jon Dunn. A Reference Guide to Gulls of the Americas. New York: Houghton Mifflin, 2007.

Winkler, David W. 1996. California Gull (Larus californicus), The Birds of North America Online(A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:http://bna.birds.cornell.edu/bna/species/259

All photo by Dave Irons