The crew had an amazing day in Ontario deploying geolocators on 4 Golden-winged Warblers, 1 Blue-winged Warbler, and 3 hybrids including the first Lawrence's Warbler of the year! In terms of numbers, it wasn't the 18-bird day Sean's crew had in Wisconsin earlier this month, but it was the most genetically diverse day you could have working with the Vermivora complex.
One could argue that this bird is even more attractive than a Golden-winged Warbler or a Blue-winged Warbler. It is certainly a much rarer sight. I think the folks at the Braddock Bay Bird Observatory have posted the most straight-forward explanation of how a Lawrence's Warbler comes about (https://braddockbaybirdobservatory.wordpress.com/2013/06/02/comparing-our-blue-winged-golden-winged-and-brewsters-warblers-warning-genetics-included/).
Basically, this guy's parents were both Brewster's Warblers (GWWA X BWWA hybrids), and because the mask and throat patch are coded by recessive alleles, there was only a 1 in 16 chance of him getting this stunning look (1 in 32 if you consider he also had to be a male to get the black).
Gunnar, Kate, and Cassie are pretty happy with marking the Lawrence's Warbler.