Autumn around New York City is a good time to find migrating Blue Grosbeaks. Possible identification problems with this bird, however, is in its similarity to the Indigo Bunting or when viewing either females or young males. Both the Blue Grosbeak and Indigo Bunting males are brilliant blue birds with relatively large bills. On closer examination, though, there are some fairly obvious differences.
• The male Blue Grosbeak has two rusty wingbars. While not as pronounced as on the males, the tan-colored females will also show two brownish wingbars.
• Neither the male nor female Indigo Bunting has wingbars.
• Blue Grosbeaks have a large, heavy bill, the top edge of which nearly forms a straight line into the forehead.
• Indigo Buntings have a much smaller bill.
• The large, squared off head of Blue Grosbeaks give them a block-headed appearance.
• The head of Indigo Buntings are rounded and more delicate.
• At 28 grams, the average Blue Grosbeak is nearly twice the weight of an Indigo Bunting (14.5 grams).
So how does one determine whether a backlit bird, seen in silhouette, is a Blue Grosbeak or an Indigo Bunting? The bill and head should be a dead giveaway.
Here's a good comparison page of the two species.