A White-crowned Sparrow. And then the fledgling popped up:
It isn't. It's a usurper, a baby Brown-headed Cowbird. The incidence of cowbird nest parasitism is low in British Columbia, but increasing as this species expands its range and increases its numbers. According to Campbell et al., 2001,
"In British Columbia, 2% of 469 nests found with eggs or young were parasitized by the cowbird. There were 5 additional records of a White-crowned Sparrow feeding a cowbird fledgling. On the coast, 7 of 338 nests (2%) were parasitized. Friedmann and Kiff (1985) note that the Brown-headed Cowbird has greatly extended its area of sympatry with the White-crowned Sparrow, making the latter a frequent host of the parasite."
So there it was, in the flesh. Mother bird, annoyed at me, flew off across a field with bulky baby in hot pursuit.
Campbell, R.W., N.K. Dawe, I. McTaggart-Cowan, J.M. Cooper, G.W. Kaiser, A.C. Stewart and M.C.E. McNall. 2001. The Birds of British Columbia. Vol. 4. UBC Press.