Saturday, September 24, 2011

The geese are gathering

Flock of geese on the Rideau (photo taken this time last year)
It seems that the geese have begun to prepare for migration again, gathering in large flocks near the Ottawa River now, and practising their formation flying skills in family groups.

Bird strikes are a hazard for pilots at this time of year. There were many geese at Rockcliffe airport today and while Chris was flying today he heard Air Traffic Control reporting signals from skeins of geese on their radar equipment. I spotted a long line of geese flying above the airport myself, pointing this out to some people from Germany, for whom it's a novel sight.

One of the ways in which the birds navigate, it seems, is to follow the course of rivers.

The weather is still warm so we were wondering how the geese know that winter is coming closer. We assume it is the shorter days and longer nights that make them feel restless to get going. The tree tops are changing colour now as well. Do geese have colour vision?


  1. Yes they do.
    "... many species of birds, fish, reptiles and amphibians, as well as some invertebrates, have more than three cone types and probably superior color vision to humans.... In the birds, tetrachromacy is achieved through up to four cone types, depending on species. Brightly colored oil droplets inside the cones shift or narrow the spectral sensitivity of the cell. It has been suggested that it is likely that pigeons are pentachromats." (Humans are only trichromats.) I never knew that!

  2. A simplified version: In the time of the dinosaurs, mammals were little nocturnal scurrying creatures. They had no need for colour vision, but strong need for night vision. Mammals developed rods (night vision) to supplement simple cones (bright vision). Many mammals are therefore (still) completely colour blind (no sense of colour at all).

    The dinosaurs meanwhile were ruling the daytime. For them colour vision was beneficial and they developed good quality 4-cone colour vision with four evenly spectrally spread cones, peaking in what we'd call UV, blue, green and red. The birds came from them.

    Once the mammals "took over", some of them developed a two-cone system: red and blue vision. Later some early primates realised the advantage of separating red and green (being able to make out ripe fruit). The red cone "split" into a red and green cone. The spectral responsivity of those are almost overlapping - the eye gives a slightly stronger red signal or a slightly stronger green signal and the brain does a lot of signal processing. (See how overlapping they are at:

    This was a relatively recent evolutionary step, which is why it's not perfectly expressed in humans (red-green colourblindness is far far more common than blue blindness - and blue blindness is more likely to be due to a yellowing of the eye's lens from sun damage, rather than a lack of blue cones).

    There are several types of red-green blindness. A complete lack of one or other cone (no green cones or no red cones). Or, more commonly, the responsivity curves of the two are even closer together than normal and the brain finds it too hard to distinguish.

    If you have no red cones you can have difficulty seeing light beyond 650 nm (I have a friend who can't see red laser pointers), but most sources are not so monochromatic, so your green sensors still sees that light. If you have no green cones, then the red cones take over and everything still looks just as bright, just the colours aren't distinguishable.

    I think Dad (and maybe Thomas) has the no-green-sensors style.

    There's a type of shrimp that has 12-cone vision. And most cones are in the spectral region we call blue or green.

  3. ps. Isn't it funny how much this family likes to teach people things. I've caught Alexander teaching his friends recently.