Greater White-fronted Geese: Flavirostris and Gambelli - Part 2

Greater White-fronted Geese (GWFG): characteristics of Flavirostris and Gambelli - Part 2

This is a continuation of the following blog: 

http://gwfgmdde.blogspot.com/2017/10/greater-white-fronted-geese.html


In this blog we are looking at the following:

4. Extent of black markings on the underparts

Some flavirostris have very extensively black underparts.  This can be matched by occasional gambelli (see photo below) and both forms can have restricted black markings underneath.



Clive Harris © Smithsonian Institution (NMNH #16788)

One feature noted by a number of observers is the tendency for flavirostris to have black markings well into the white ventral area.  I examined skins of flavirostris at the Natural History Museum, Tring, and of presumed gambelli (based on collection location) at the Smithsonian and categorized them into three groups based on whether or not the dark blotching extended past the end of the dark flank marks.  As can be seen below, in nearly all flavirostris, the blotching extends to at least level with the end of the dark flank marks, and in nearly half of skins examined, well past it.  In gambelli almost half the skins had blotching end short of the end of the dark flank marks.   Based on these samples, a bird with no visible  blotching in the white ventral areas, on a typical view, is not likely to be a flavirostris.

	Flavirostris		Gambelli
	Number		Number
Blotching short of dark flank marks	4	8%	11	48%
Blotching level with dark flank marks	25	47%	8	35%
Blotching past dark flank marks	24	45%	4	17%
	53		23

The bird below has fairly extensive black blotching often found in flavirostris.

Clive Harris © Natural History Museum, Tring

Even where the dark blotching is not extensive, it extends well into the ventral area in flavirsotris as can be seen on the 3 skins below.

Clive Harris © Natural History Museum, Tring

5. Bill Shape

Bill shape (and size) is variable in both forms although more so in gambelli. Some flavirostris have almost a convex bulge in the bill (see first photo below) which can give the bill a heavy/blocky feel to it. Others have a straight line from the base to near the tip.  Gambelli has shapes that go from a straight line through to a concave shape with an extended parallel-sided tip to the bill giving a "teat" shape. In general more gambelli have a "longer" looking bill due to this. 

Flavirostris	Gambelli
Clive Harris © Natural History Museum, Tring	Clive Harris © Natural History Museum, Tring
Clive Harris © Natural History Museum, Tring	Clive Harris © Smithsonian Institution (NMNH # 299571)
Clive Harris © Natural History Museum, Tring	Clive Harris © Natural History Museum, Tring
Clive Harris © Natural History Museum, Tring	Clive Harris © Natural History Museum, Tring

In particular the shape that some gambelli have does not seem to be present in flavirostris, namely an the extended parallel "teat" that accounts for a third or more of bill length (measured from tip to gape) and in many of these birds a distinctly concave shape to the bill.  While some flavirostris have longer looking bills they don't seem to have this shape, and often have a fairly bulky look to the middle of the bill.  The top left in the table below is the flavirostris.

Clive Harris © Natural History Museum, Tring	Clive Harris © Natural History Museum, Tring
Clive Harris © Smithsonian Institution (NMHN #88973)	Clive Harris © Smithsonian Institution (NMNH  #367938)
Clive Harris © Smithsonian Institution (NMNH  #528783)	Clive Harris © Smithsonian Institution (NMNH #528784)

6. Shape and extent of white frontal patch

There are also some average differences in the shape of the white face patch. I categorized skins into the following: (1) white face patch forms a straight border with the base of the bill (2) white face patch has a gently curved border with the base of the bill and (3) white face patch has a strongly curved border with the base of the bill.

Straight	Gently curved	Strongly curved
Clive Harris © Natural History Museum, Tring	 Clive Harris © Natural History Museum, Tring	Clive Harris © Natural History Museum, Tring

 The table below shows there is some difference between flavirostris and gambelli.  Over half of flavirostris have a straight border, and a strongly curved border is much more frequent in gambelli.  However there is also a considerable overlap in these features.

Shape of  white border	Flavirostris		Gambelli
	Number		Number
Straight	23	51%	4	17%
Gently curved	17	38%	9	39%
Strongly curved	5	11%	10	43%
	45		23

This is continued here:  http://gwfgmdde.blogspot.com/2017/10/greater-white-fronted-geese_29.html