The Midget Frigate: Confusion in the Baja Sun

I was birding “El Tanque,” just outside of La Paz, Baja California Sur, Mexico. The afternoon was glorious. The warm tropical sun upon my skin combined with a gentle cooling breeze for the perfect temperature; better yet, I was surrounded by throngs of shorebirds and waterfowl. The weather, amplified by the accumulated fatigue of several days of hard birding, sent me into a nirvana-like trance. I simply stood at the water’s edge, an accidental smile upon my face, gazing absent-mindedly at the avian abundance before me. My mind was still rather unfocused when a group of Magnificent Frigatebirds drifted in off the bay, readying themselves for a drink and a bath (see Bathing with Frigatebirds in the April 2009 BirdFellow archive). The photographer within me finally stirred, re-engaging my brain.

As I positioned myself to capture this frigatebird spectacle “on film,” I noticed that one bird, an adult male, was substantially smaller than the others. I also remembered that identifying adult male frigatebirds could be exceptionally difficult, and worse yet, I had no recollection of the distinguishing marks. I had to choose: take careful notes or bet that I could obtain adequate photos. I picked the latter. Fortunately, the bird cooperated. The one feature I did note carefully before hoisting my camera up was size: its wingspan was 2/3 to 3/4 that of the female Magnificents soaring with it and clearly less than that of the male Magnificent Frigatebirds in the vicinity. 

SmallFrigate-09Mar-ElTanque5a

The size differential here is no illusion. The “Midget Frigate” consistently looked much smaller than its female companions. Female frigatebirds are typically larger than males, and in Magnificent Frigatebirds the average (mean) wingspan of females is approximately 4% greater than that of males; however, the largest female Magnificent Frigatebirds have a wingspan that is about 18% longer than the smallest males (cf. Pyle 2008). The difference in wingspan shown above seems to be more than 18 or 20%. Note: Pyle’s superb tome uses wing chord rather than wingspan. Wingspan is measured from wingtip to wingtip, with the wings flattened against a surface. Wing chord the length of a single wing measured in its natural bowed position. Thus, the two are related but not necessarily equatable. Indeed, the correlation between both measurements and field observation might further vary depending on the bird’s flight posture. Additionally, comparisons using the measurements in birds with differently shaped wings would pose some hazards; fortunately Great and Magnificent Frigatebirds have wings that are rather similar in shape.

After returning to the comfort of my hotel room, I was quickly ruled out Lesser Frigatebird, which always has white stripes on the axilla (the avian equivalent of armpits, where the underwing coverts meet the body). Male Christmas Island Frigatebirds have an oval white belly patch, thus eliminating that species as a contender (James 2004). The remaining three frigatebird species (Magnificent, Great, and Ascension) are completely glossy black plumage. Magnificent Frigatebird is the de facto identification of any frigatebird in southernmost Baja California, where they are fairly common year-round. However, this bird seemed too small to be of that species. Great Frigatebird is a far less likely species, but far from impossible, as it nests on the Islas Revillagigedos off western Mexico and has been recorded once in Baja California (near La Paz, 19 July 2007, Erickson et al. 2008) and twice in California (Hamilton et al. 2007). This species could occur more frequently in southern Baja, lost amidst the swarms of Magnificents. Notably, Great Frigatebirds average smaller in size than the Magnificent, with the wingspan of adult females Magnificent Frigatebirds averaging about 17% greater than that of male Greats. Adult male Ascension Frigatebirds are, essentially, inseparable from Magnificent Frigatebirds, but this species would seem highly improbable in Baja California: it breeds only near Ascension Island off the west coast of Africa in the South Atlantic, with an estimated population of only 20,000 (BirdLife International Factsheet: Ascension Frigatebird).

SmallFrigate-09Mar-ElTanque2a

Despite their name, Great Frigatebirds average smaller than Magnificent Frigatebirds. The average wingspan of a female Magnificent Frigatebird is typically about 17% longer than that of a male Great; however, the largest female Magnificents have a wingspan about 35% longer than that of the smallest male Greats (cf. Pyle 2008). The Midget Frigate seemed at least that petite, or perhaps more so, when compared with its Magnificent female companions.

Identification of the Midget Frigate: Discussion
The size disparity certainly seemed to suggest that the Midget Frigate might be a Great Frigatebird. However, identification of a super rarity should not be based on one mark alone. So, the question arose, “How else does one distinguish an adult male Great Frigatebird from a Magnificent?” When I returned to the United States, I sent photos of the bird in question to Doug Pratt, Peter Pyle, and David James, all of whom were kind enough to provide thoughtful responses. Each person focused on different marks, but in the end (somewhat to my chagrin), all of the marks are mentioned in Howell (1994). This reminded me that a good friend looks at the literature before they pester their buddies…

Howell (1994) and the three gentlemen mentioned above provided the following marks for separating male Great and Magnificent Frigatebirds:

Body Gloss: The body plumage of Magnificents has a purple gloss vs. green in Great Frigatebird. As with gloss in other species, the apparent color can change with lighting and should be used with caution.
Alar Bar: Great Frigatebirds have a paler/brownish alar bar (that is, a bar on the wing’s upper surface that extends from the bend of the wing towards the body) and male Magnificents don’t. Howell (1994) is clear that he believes that this mark is completely reliable, though he does mention that other authors suggest some variability in both species.
Leg Color. Male Magnificent Frigatebirds should have dark legs, while Great Frigatebirds should have legs that are pink to red.
Axillary Barring: Some gray to brownish barring in axillars visible on close view in Great Frigatebird, absent in all or most full adult male Magnificent. 

SmallFrigate-09Mar-ElTanque18a

The purplish gloss of the Midget Frigate is clearly seen here, as is the lack of an alar bar.

SmallFrigate-09Mar-ElTanque22a

From this photo, one can be fairly certain that the Midget Frigate lacked even subtle barring on the axilla. With a little imagination, the dark feet can be discerned. Certainly, no glaringly red or pink feet are visible.

Identification of the Midget Frigate: Conclusion
Sometimes size does not matter. The apparent diminutive size of the Midget Frigate should preclude its identification as a Magnificent Frigatebird. However, its other characteristics all point to it being a Magnificent Frigatebird: no contrasting alar bars, purple gloss to the plumage, black or blackish feet, and no markings in the axilla. This exemplifies the danger of using a single characteristic for identification.

EPILOGUE: Runt Birds
The question does arise, “How common are “runt” or “dwarf” birds?” The problem with that question is that there seems to be no ornithological definition for either term pertaining to adult birds. One could choose a statistical definition, such as 3 standard deviations below the mean size for a given species (meaning the smallest 0.5%), but that definition also has potential for confusion, as size often varies with population and sex. Even if one were to define runt as being the smallest 0.5% within a population, those smallest individuals might be close enough to the norm as to not be obvious in the field.

Many active birders doubt that truly runty birds exist -- in other words, birds small enough to elicit surprise (or misidentification). Certainly, in birds such as warblers, such individuals would be difficult to detect. Exceptionally small individuals would be more apparent among flocks of gulls and waterfowl. I have seen a rare adult Great Basin Canada Geese (subspecies, moffitti) that is distinctly smaller than its cohorts. In geese, it has been established that poorly fed chicks lead to undersized adults (Larrson and Forslund 1991, Aubin et al. 1993, Leafloor et al. 1998). For instance, Canada Geese breeding on Akimiski Island in James Bay average about 8% smaller (using skull length) than geese breeding on the nearby mainland. Leafloor et al. (1998) raised chicks from both the island and mainland on the same diet and, upon reaching adulthood, chicks from both populations were similarly sized. Furthermore, their size approximated that of wild mainland Canadas. It certainly is conceivable that a small female from Akimiski might, on the wintering grounds, appear to be a runt when compared with its comrades, possibly leading to misidentification.

Here in northwestern Washington, the ratty summering California Gull collection seems to show considerable variation in size. On more than one occasion, I have paused to make sure larger individuals were not Herring Gulls. Once, I saw a bird that appeared so small, I initially presumed it a Mew Gull. Some of this variation may be due to subspecific differences, but I am not sure that explains all the variation I see (or at least think I do). However, without catching or collecting these gulls, it is hard to know how much of the apparent size difference is, indeed, apparent (for instance, caused by posture or extreme feather wear) and not real.

I was able to find almost no documented reports of birds that one might intuitively consider a “runt.” A quick search of the Searchable Ornitholgical Research Archive (http://elibrary.unm.edu/sora/search.php) revealed only one instance: An extremely small adult American Crow among 20 collected in New Jersey; though measurements of the bird in question were provided, no comparison to measurements of the other crows was provided (Kohler 1913).

Final Question: Was this really a runt? As I was discussing wingspan, wing chord, and runtism (runtistics?) with Peter Pyle, he queried, “Could your frigatebird have been a vagrant Magnificent from another, smaller, population?” Great Frigatebirds are known to significantly vary in size from population to population (James 2004, Pyle 2008), but to date the amount of variation in Magnificent Frigatebirds seems unclear (Diamond and Schreiber 2002); so perhaps there is another chapter in this story waiting to be told.

I owe many thanks to Peter Pyle for his incredible willingness to answer my questions, some of which were clever and some of which were stupid. Also, thanks to David James, Doug Pratt, and Dick Erickson for sharing their knowledge and providing stimulating discussions.


Literature Cited

Aubin, A. E., A. Dzubin, E. H. Dunn, and C. D. MacInnes. 1993. Effects of summer feeding area on gosling growth in Snow Geese. Ornis Scandinavica 24: 255-260.

BirdLife International Factsheet: Ascension Frigatebird. http://www.birdlife.org/datazone/species/index.html?action=SpcHTMDetails.asp&sid=3844&m=0

Diamond, Antony W. and Elizabeth A. Schreiber. 2002. Magnificent Frigatebird (Fregata magnificens), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology.

Erickson, R. A., S. G. Mlodinow, R. Carmona, G. Ruiz-Campos. 2008. The Nesting Season: Baja California Peninsula. North American Birds 61:644-0646.

Howell, S. N. G. 1994. Magnificent and Great Frigatebirds in the eastern Pacific: A new look at an old problem. Birding 1994:400-415.

James, D. J. 2004. Identification of Christmas Island, Great and Lesser Frigatebirds. Birding ASIA 1:22-38.

Hamilton, R. A., M. A. Patten, and R. A. Erickson, eds. 2007. Rare Birds of California. Western Field Ornithologists, Camarillo, California.

Kohler, L. S.  1913. A runt crow (Corvus brachyrhynchos) at Pompton Lakes, N.J. Wilson Bulletin 83:97-98.

Leafloor, J. O., C. D. Ankeny, and D. H. Rusch. 1998. Environmental effects on body size of Canada Geese. Auk 115: 26-33.

Larsson, K., and P. Forslund. 1991. Environmentally induced morphological variation in the Barnacle Goose (Branta leucopsis). Evolution 45: 235-244.

Pyle, P. 2008. Identification Guide to North American Birds, part II. Slate Creek Press, Point Reyes Station, California.

All photos taken by Steven Mlodinow


1

While I can understand how the alliterative quality of the “Midget Frigate” title of this article rendered all other options moot, I nonetheless think that “The Little Person Frigate” was the only proper choice for a heading, especially given the historic persecution of this tiny (no pun intended) subset of the birding community.

2

Greetings

Chris Hill (currently at Coastal Carolina University) was kind enough to forward an article to me in which 10,000 Eurasian Starlings were captured and measured. Seven males were rated as “giants,” averaging ~10% longer (not due to tail-length differences) than the mean (for males), while 6 (5 of which were females) were labelled as “pigmy,” averaging ~10% shorter than the mean for females (again, not due to tail length). These birds would likely be detectable in a flock, though clearly they make up a very small percentage of total individuals. The paper in question also lists other abnormalities including a number of bill abnormalities, which are a topic of current ornithological study re: toxins in environment. The article is Hicks (1934) in Bird-Banding 5:103-118. Please note, I intend no offense by my use of the terms “pigmy” and “giant” above.

Steven Mlodinow

3

gPQDJd I am so grateful for your blog post. Want more.

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