I was reviewing some pictures from 7 July 2013 and noticed that some of the Marbled Godwit bills were pink at the base, while others were orange (see image below). [There are also Willets (with shorter, gray bills) in these images, but ignore them for now.]
After a brief perusal, I can't seem to find much information about bill color in Marbled Godwits. The Birds of North America account states that males have a brighter orange base of the bill during the breeding season. Are the individuals with orange bills males and those with pink bills females? Can anyone provide some insight?
If it's true that you can use bill color to separate males and females at this time of year, you can try your hand at it with the next picture. For the godwits with visible bills, how many females (pink bill bases) and males (orange bill bases) do you see? [You can click on the image to see a larger version.]
I would say 4 females and 4 males, i.e., 4 with pink bill bases and 4 with orange bill bases. In the upper right corner, there are 3 with pink bills and 1 with an orange bill. In the lower left corner, there is 1 with a pink bill and 3 with orange bills.
I'm not sure you can use bill color to separate males and females, but it's an interesting question and exercise! And if bill color isn't an indicator of sex, why does it vary?