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Sexual Dimorphism
Category: Birding at the Chico

 Outside the breeding season, many of Chico Basin Ranch's male and female mammals look similar.  For example, male and female spotted ground squirrels look identical to the untrained eye. However, in many bird species the plumage differences between males and females is striking. Think ducks and game birds. Taking this a step further, it makes sense for all ground nesting birds, where females perform the majority of egg incubation duties, for the female to be dull brown in order to avoid detection by predators.  

Blue Grosbeaks, a fairly common breeder where there are dense trees or shrubs on the Chico, is a good example of sexual dimorphism in birds.  First, look at the beaks for identification purposes and you can see why the French name gros meaning large is a good name to describe the size of this grosbeak species' bill. As expected, the female (right) has a dull brown plumage but the male is strikingly colored. (This bird is actually a month old male. Young birds need to be dull regardless of gender to avoid detection. After a molt, he replaces his brown feathers with the bright blue ones) There is a reason. During the mating season males compete for available females. Male bird songs say to females "look at me, I am hot" and if the females show any signs of interest for the singing male, he then strut his stuff, showing off his fabulous plumage, and sometimes flying in with a takeout grasshopper to the female.  Sometimes it all seems to work.

Posted by Bill M. on 09/13/2017
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