King Island Emu (3 Viewers)

[Richard Klim]

Other well-known bird species with quite significant size differences between subspecies include Canada Goose Branta [canadensis] canadensis vs Cackling Goose B [c] hutchinsii (albeit now commonly split), and Western Great Egret Ardea [Casmerodius/Egretta] alba alba vs Eastern Great Egret A a modesta.

 

[Capreolus]

Was size one of the reasons why hutchinsii was split from canadensis?

Also, do you know how much smaller modesta is compared to alba? I didn't find anything in HBW1, Hancock & Elliottt (1978), or Hancock (1999).

 

[Daniel Philippe]

I'd like to know of any bird species whose subspecies exhibit as great a disparity in size as the examples mentioned here, in which the smallest subspecies is at least 25% smaller than the largest.

Wompoo Fruit-Dove Ptilinopus magnificus
Largest race: 48 cm, 500 g
Smallest race: 29 cm, 250 g.

 

[Capreolus]

Excellent example, Daniel; thank you.

According to Gibbs et al. (2001), the smallest and largest races occur at the northern (nw New Guinea) and southern (se Australia) extremes of the Wompoo Fruit Dove's range, with the intervening races linking them in a cline. In the case of the dwarf emus, no such intermediaries existed and they were isolated.

Are such distinctions legitimate?

 

[Richard Klim]

Great Egret

do you know how much smaller modesta is compared to alba? I didn't find anything in HBW1, Hancock & Elliottt (1978), or Hancock (1999).

The relative sizes of alba vs modesta were discussed on BF a couple of years back, eg:
www.birdforum.net/showpost.php?p=1594085&postcount=19
www.birdforum.net/showpost.php?p=1594284&postcount=23

According to various sources, linear dimensions of alba are typically 20-30% greater than modesta, with weight perhaps 50% greater (although if structurally similar, a 30% linear difference would suggest a typical weight double that of modesta!). A striking example here:
www.birdforum.net/showpost.php?p=749244&postcount=1

 

[Capreolus]

Thank you, Richard. Another excellent example. Those photos really do show the striking difference between the two forms.

 

[Capreolus]

After a quick perusal of the Egrets thread and the abstract of Pratt (2011), its seems that the taxonomic status of modesta is still equivocal, and it may yet prove to be a distinct species. Has anyone indicated that the difference in size between the two forms might preclude successful breeding, as was suggested in the SACC proposal to split Pelecanus thagus from P. occidentalis? Is there any evidence of hybridization in Japan and Korea, or in nw India and Pakistan, where alba and modesta come into contact?

 

[Richard Klim]

Great Egret

Is there any evidence of hybridization in Japan and Korea, or in nw India and Pakistan, where alba and modesta come into contact?

Nial Moores reported that there are no known cases of mixed pairs in S Korea:
www.birdforum.net/showpost.php?p=1598658&postcount=39

 

[jurek]

You might also check some widespread gallinaceous birds, like Western Capercaille.

 

[Welsh Peregrine]

Not the same story, but possibly relevant - what about the size disparity between sexes now reported in Moas?

 

[Capreolus]

Good point.

According to Bunce et al. (2003), Dinornis moa showed the most extreme sexual dimorphism of any bird or mammal, with females ca 150% the height and 280% the weight of the largest males. In kiwi, females are typically 120%, and in cassowaries, up to 180%, the weight of males, but sexual dimorphism in plains-dwelling ratites, including the Emu (and dwarf emus), is much less evident or absent.

The mainland Emu would also have been about 150% the height (at the hips) of the dwarf emus, but I don't if or how intraspecific sexual dimorphism in ratites relates to interspecific sexual dimorphism.

 

[Isurus]

On the topic of extremely fast acting island dwarfism might be worth looking at Virgin Island's populations of WT deer. Ridiculously small compared to most mainland populations after only a couple of hundred years or so.

 

[Capreolus]

Thanks, but a quick Google search found that White-tailed Deer were first introduced from the se US to the Virgin islands ca. 1790, but were then re-introduced following the Wildlife Restoration Act of 1937, from Texas and Georgia. Male White-tailed Deer in the Virgin islands weigh ca. 90-100 lbs, females, 70-90 lbs, so they are substantially larger than the Florida Key Deer, males of which only weigh ca. 55-75 lbs. (females, 44-64 lbs).