Great Blue Heron occidentalis (1 Viewer)

[Dave B Smith]

Can anyone summarize the current taxonomy status / definition of Ardea herodias occidentalis?

While it was originally considered a separate specie "Great White Heron, Ardea occidentalis", it was later treated as a color morph of the Great Blue Heron. Currently it appears that several groups treat it as a subspecies of only "white plumaged" G.B. herons while others treat it as a subspecies of mixed (either blue or white plumaged) G.B. herons. Most agree on the range within the US (s. Florida) but differ on the remainder of the range including the West Indies and some islands of Vz.

Clements 2009 shows this as "Great Blue Heron (White form) occidentalis" and implies that it is just specifically the "white plumaged" forms. The AOU appears to use this same approach. BNA online uses this subspecies only for the white plumaged G.B. herons.

'Birds of Northern South America' use the subspecies to include both blue and white plumaged birds.

HBW says that race occidentalis is "mostly white morph birds".

A study by the Florida FWS in 2002 found a size difference in A.H. occidentalis (white plumaged) and the A.H. wardi. They also found that in the main breeding area of occidentalis that about 10 percent of the pairs were "mixed" with both a blue and a white partner. Offspring for these nests could be either blue or white. The "Wurdeman's Heron" is thought to be a cross from a blue and a white parent but this was not confirmed in this study. The final results weren't definitive whether the blue plumaged herons in this breeding survey were "blue" occidentalis or were of another subspecies.

In 2003 the fws published an article on the Great White Heron which says occidentalis is the White and intermediate plumaged birds.

The main reason I started looking at this was trying to determine which subspecies of GBH we had in Trinidad. Most field guides say it is occidentalis.
The birds are listed as non breeding visitors but it appears that some are here year round. However, there has never been a report of a white morph GBH so that's why I'm wondering if they are occidentalis (which appears at a minimum to represent primarily white plumaged birds).

Thanks for any help / clarification.

 

[Richard Klim]

I can't help, but I agree that the literature is contradictory:

• Martínez-Vilalta & Motis 1992 (HBW1) states that occidentalis comprises mostly white-morph birds.
  
  
• Butler 1992 (BNA Online) and Alderfer 2006 (Birds of N America) treat 'occidentalis' as a localised white morph; and Cornell/Clements treats occidentalis as a ssp 'group' (white form).
  
  
• Butler 1992, Martínez-Vilalta & Motis 1992, Dickinson 2003 (H&M3) and Cornell/Clements suggest that birds occurring in the W Indies are occidentalis.
  
  
• But Raffaele et al 1998 (Birds of the W Indies) states that white-phase birds are extremely rare in the W Indies; and Hallett 2006 (Birds of the Bahamas & T&C Is) states that the white form has been reported only sparingly in the Bahamas (on N Andros).

A confused picture. I wonder what Kushlan & Hancock 2005 (Herons) has to say?

Richard

 

[Daniel Philippe]

I wonder what Kushlan & Hancock 2005 (Herons) has to say?

"Occidentalis was erected by John James Audubon for the white birds of the Florida Keys. But these birds pair with dark birds and the subspecies name is now considered to apply to all the dark, white, and intermediate birds of south Florida and the Caribbean. Birds in this population are relatively larger than other populations, altough likely this represents the end of a clinal variation in size down the Florida peninsula that remains to be quantitatively documented. For some time it has been known that there is a tendency for assortive pairing, by colour, among Florida Bay Great Blue Herons. The most likely explanation for this situation is that the white birds evolved as part of a dimorphic coastal population in the Caribbean and this population is now in secondary contact in Florida with birds derived from the continental population (wardi). This interpretation is supported by recent behavioural and molecular studies by Heather (pers. comm.). Based on several criteria (morphological, white-dark pairing, molecular), the geographic variation shown in the southern Florida and Caribbean population is recognizable as subspecific but not species level of divergence."

 

[Richard Klim]

So both Caribbean and S Florida birds are ssp occidentalis; occidentalis is dimorphic, but the white morph ('Great White Heron') is almost exclusively restricted to Florida.

Richard

 

[Kirk Roth]

Arthur Bent, in his Life Histories of North American Marsh Birds, had the view that the species status of occidentalis was unquestionable, due to size and behavioral differences. Very interesting is his account that occidentalis is much more aggressive than wardi.

But to put this in to perspective, Bent also thought that Cory's Least Bittern (the dark morph) was a separate species.

 

[Richard Klim]

BNA Online

Revised species account (26 Apr 2011) for Great Blue Heron (Ardea herodias): Vennesland & Butler 2011.

Recognises sspp fannini, herodias, wardi and occidentalis (following Dickerman 2004 [A review of the North American subspecies of the Great Blue Heron (Ardea herodias)]); plus cognata (tentatively).

The Great White Heron (A. h. occidentalis) has been treated as a species (e.g., Holt 1928, Palmer 1962) or as a color morph—i.e., not a taxon at all (but see Mayr 1956). Its behavior may (Curry-Lindahl 1971) or may not differ from that of A. h. wardi (Meyerriecks 1957). "Wurdemann's Heron" generally is considered to be a hybrid swarm of A. h. wardi and A. h. occidentalis, and A. h. wardi and A. h. occidentalis do interbreed where ranges meet in the Florida Keys (Meyerriecks 1957). That said, Stevenson and Anderson (1994:59) stated that a nest of two Wurdemann's Herons was unknown and therefore concluded that hybrids were inviable, suggesting reproductive isolation.

More recently, McGuire (2002) found that Wurdemann's Heron is fertile and does form breeding pairs. On the basis of her extensive observations of mated pairs, McGuire (2002) concluded that pairing between blue and white birds could not be considered random; nonetheless, hybrid pairs were common enough to prevent genetic divergence at mitochondrial loci, although microsatellite markers differed in frequency (McGuire 2002). Still, most genetic differences separated birds from Florida Bay and the Keys from birds on the Florida Peninsula.

 

[Richard Klim]

Wurdemann's Heron

David Sibley: A Wurdemann’s-like heron from South Florida.