White-throated Sparrow - Bruant à gorge blanche
After posting another image of what some call a ‘tan-morph’ White-throated Sparrow from the spring, a few people asked about this form of the bird. So I am posting this ‘mug shot’, which I secured because I had my tripod set up on the shore of the Ottawa River for shorebirds and this Sparrow just landed and stared. I pivoted as gracefully as I could - thankfully it was low to the ground - and did my best. But it actually captures most of the distinguishing features of the ‘morph’.
Sibley calls the tan-morph version the ‘brown stripe’, in contrast to the ‘black stripe’. This naming practice picks up on the key distinguishing features: the dark lateral crown stripe is brown on this bird (as are the cheeks), while it is black on the other version. Other related differences include the breast (mostly gray on the black stripe, mottled as it is here on the brown stripe) and the yellow loral markings: mustardy on the brown stripe and bright yellow on the black stripe.
Where the species lives, its population is split approximately in half between the two kinds of markings, and almost every mating pair includes one of each (hence the population distribution). Until the early 70s the brown stripe markings were attributed to young birds or to females, based on field observations. Once that was cleared up (both sexes have both colourings in adulthood) scientists started noticing that the black stripe are more aggressive and the brown stripe better at guarding the nest. Which explains the pairings: assertive females (black stripe) going after homebody males (brown stripe), and then homebody females getting stuck with aggressive males.
This kind of 50/50 split in the colouring of a species is very rare. ‘Genetically based plumage polymorphism’ doesn’t turn up very often in birds. As the early birders assumed, plumage variation is normally based on age and sex. This is one exception.
White-throated Sparrow - Bruant à gorge blanche
After posting another image of what some call a ‘tan-morph’ White-throated Sparrow from the spring, a few people asked about this form of the bird. So I am posting this ‘mug shot’, which I secured because I had my tripod set up on the shore of the Ottawa River for shorebirds and this Sparrow just landed and stared. I pivoted as gracefully as I could - thankfully it was low to the ground - and did my best. But it actually captures most of the distinguishing features of the ‘morph’.
Sibley calls the tan-morph version the ‘brown stripe’, in contrast to the ‘black stripe’. This naming practice picks up on the key distinguishing features: the dark lateral crown stripe is brown on this bird (as are the cheeks), while it is black on the other version. Other related differences include the breast (mostly gray on the black stripe, mottled as it is here on the brown stripe) and the yellow loral markings: mustardy on the brown stripe and bright yellow on the black stripe.
Where the species lives, its population is split approximately in half between the two kinds of markings, and almost every mating pair includes one of each (hence the population distribution). Until the early 70s the brown stripe markings were attributed to young birds or to females, based on field observations. Once that was cleared up (both sexes have both colourings in adulthood) scientists started noticing that the black stripe are more aggressive and the brown stripe better at guarding the nest. Which explains the pairings: assertive females (black stripe) going after homebody males (brown stripe), and then homebody females getting stuck with aggressive males.
This kind of 50/50 split in the colouring of a species is very rare. ‘Genetically based plumage polymorphism’ doesn’t turn up very often in birds. As the early birders assumed, plumage variation is normally based on age and sex. This is one exception.