Monday, May 21, 2012

Almost Cain and Abel

Some quick sketches from watching this enchanting webcam: Cornell Herons

Harassed parent scratching chin
Is this a crack I see before me?

Eggs about to hatch

Great Blue Herons (Ardea herodias) nest in rookeries, and lay 2-6 eggs. Parents share incubation duties, with males taking the day shift, and females the night shift.

Unlike many other birds, herons and egrets (and birds of prey) start incubating the moment the first egg has been laidresulting in a large firstborn, and a succession of increasingly puny chicks, as each hatchling has to compete with its older, heftier siblings. Some heron chicks are more equal than others.

Goneril and Regan

While great egret (Casmerodius albus) chicks actively stab their siblings to death, great blue herons may simply starve their younger siblings by outcompeting them when food is scarce. This raises the question of why herons and egrets evolved to have more offspring than they can reliably support. One idea is that by creating an age and size disparity in offspring, parents can allow for environmental unpredictability at minimal cost. If times are good, then all the chicks survive, and if times are hard, the runts perish, but being the youngest, they've had the least invested in them anyway.


Spotted hyenas also appear to be siblicidal, which could explain why females have evolved bizzare mimetic penises that make childbirth a nightmare -- but that's another story.

For an excellent account of familial conflict, I highly recommend "More than Kin and Less Than Kind" by Douglas Mock.
Other sources:

Tuesday, May 15, 2012

Warblers Galore

The spring migration has just reached its peak over the past week, inspiring me to paint some of the wood warblers (Family: Parulidae) that manage to attract throngs of eager birders to places like Central Park in NYC, or the Mt Auburn Cemetery in Cambridge, MA every year.
A true harbinger of spring is the northern parula (Setophaga americana):

Warblers, like most songbirds, are best found by listening. Especially when they are rather hard to tell apart, like these waterthrushes. I've exaggerated the differences in the size and colour of the eyebrow stripe, streakiness of throat, bill size and whiteness of the underparts.
Left: Louisiana Waterthrush (Parkesia motacilla) Right: Northern Waterthrush (Parkesia noveboracensis)

Redstarts have rather trick, variable songs, including a see-sawing, squeaky bicycle wheel version that sounds a bit like an abrupt black and white warbler. I have a particularly soft spot for this zebra-striped warbler, partly because of the unusual way it shimmies up and down trunks and branches.
Othello and Desdemona: Male and female American Redstarts (Setophaga ruticilla)

Black and white warbler (Mniotilta varia)

The common yellowthroat can be endearingly confiding for a warbler, and is widespread throughout most of North America. The male has an amusing racoon mask, and a very strident song that goes "wichety-withcey-wichety".
Common yellowthroat (Geothlypis trichas)
Kentucky warbler (Geothlypis formosa)

In contrast to the very common yellowthroat, the Kentucky warbler is restricted to the deciduous forests of the southeastern US, making it a rare and exciting find further north. Central Park birders were vocalising vigorously at the sight of one of these little birds, with yells of "are you texting or tweeting?", so other NY birders with their smart phones could converge on the unfortunate bird.

Another genus of bright yellow warblers that contains some of my favourites includes the Wilson's warbler, with its little black yamaka, the Canada warbler, complete with eye-ring and black necklace. Surprisingly, DNA evidence now demonstrates that the hooded warbler, which used to be in the same genus, is actually more related to the American Redstart.
Above: Wilson's warbler (Cardellina pusilla), Canada warbler (Cardellina canadensis) Below: Hooded warbler (Setophaga citrina)
For more wood warbler paintings:
Dendrioca warblers

Thursday, May 3, 2012


Most people will be familiar with sexual dimorphism, famously explained by Darwin as a consequence of sexual selection. Many birds provide striking examples of sexually dimorphic plumage.

Wood duck pair (Aix sponsa)

Baltimore Orioles ( Icterus galbula)

In other cases, like the white-throated sparrow, there are two colour morphs, but both sexes can have a tan- or white-striped head. In this species, the head stripe colour is curiously linked with a personality difference (the genes for color and personality are linked on an inverted bit of chromosome that can't recombine). Tan-striped birds tend to be more placid, and make better parents, while white-striped birds are more aggressive, and promiscuous. 

 White-throated sparrow (Zonotrichia albicollis)

 An amusing irony is that white males that spend more time courting other females and less time mate-guarding, are cuckolded at a higher frequency than the tan males. Similarly, white females that spend considerable time engaging in rumpy pumpy away from the nest are subject to higher levels of conspecific brood parasitism (where other females dump eggs in an absent mother's nest) than the stay-at-home tan females. Both white males and females pay the price of extra-pair copulations by running the risk of bringing up someone else's children. 

Why, you might ask, do both colour morphs seem to remain at about 50% in populations of white-throated sparrows? A probable answer is that opposites attract, and tend to produce more offspring than two parents of the same colour morph. Two tan-striped sparrows may be dedicated parents, but they get bullied into making do with paltry territories, while two white-striped birds would spend so much time being asserting themselves that their offspring don't get as much to eat.


L. Y. Huynh;D. L. Maney;J. W. Thomas 2011 Chromosome-wide linkage disequilibrium caused by an inversion polymorphism in the white-throated sparrow (Zonotrichia albicollis) Heredity 106: 537-546.
R. W. Knapton;R. V. Cartar;J. B. Falls 1984 A Comparison Of Breeding Ecology And Reproductive Success Between Morphs Of The White-Throated Sparrow Wilson Bulletin 96: 60-71.
R. W. Knapton;J. B. Falls 1983 Differences In Parental Contribution Among Pair Types In The Polymorphic White-Throated Sparrow Canadian Journal Of Zoology-Revue Canadienne De Zoologie 61: 1288-1292.
E. M. Tuttle 2003 Alternative reproductive strategies in the white-throated sparrow: behavioral and genetic evidence Behavioral Ecology 14: 425-432.