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Posts Tagged ‘Great Blue Heron’

A few years ago I wrote a post on niche partitioning among herons and egrets. That post was inspired by watching several species of herons and egrets foraging for food along Putah Creek near Davis, CA, and the resource they were partitioning into niches was food.

Recently, as part of my work at the Sacramento-San Joaquin Delta Conservancy, I encountered another example of niche partitioning by herons and egrets. This time, the resource these birds are partitioning into niches is nesting trees.

One of the grants that I manage at the Delta Conservancy is at the Cosumnes River Preserve and it includes and grove of large Valley Oak trees that many herons, egrets, and cormorants use as a rookery (a rookery is a colony of breeding animals, generally birds). One way that the various species have evolved to utilize the same trees, and yet avoid directly competing with each other, is for each species to utilize a different part of each tree to nest in.

Great Blue Herons typically nest on the very tops of the crowns of trees, Great Egrets typically nest only in the upper one-third of the canopy, Snowy Egrets typically nest in the middle one-third of the canopy, and Black-crowned Night-Herons prefer to nest in the lower one-third of the canopy (see the image below).

Niche partitioning of nesting locations within a tree by heron and egret species

I think this stratifying of nesting locations is amazing! Species have evolved to fill so many different niches, and so many niches can be divided into finer and finer gradations. I wonder if there is really any limit to how many species can evolve, and how complex an ecosystem can develop, in a give location.

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I went for a walk along Putah Creek yesterday at the South Fork Preserve.  The early morning was windy and not yet hot, but the sky was clear and the sun was bright.  When I got to the first creek access point, I was treated to a view downstream of the creek scattered with egrets and herons (all members of the family Ciconiiformes).  Usually there are one or two, but I have never seen this many along the creek before.  About half-a-dozen Black-crowned Night Herons were perched in some dead and fallen branches just above the water line.  Three Snowy Egrets were just beyond them foraging in the shallows skittering their feet around in the mud and silt and darting off after food when it was disturbed by the egret’s agitated foot movements.  A bit farther into the channel, stood 5 Great Egrets.  They were all hunting through an area of the creek that was thick with aquatic plants and alga.  They stood almost motionless, waiting for some unsuspecting prey to swim by.  Out in the deeper water in the center of the channel were the 3 Great Blue Herons using the same hunting strategy as the Great Egrets, they stood waiting.

Watching these birds all crowded into the same area got me thinking on how each species utilizes a different part of the habitat and has characteristic behavioral differences that help then to do so, a concept known as niche partitioning.  But which came first, the different species or the different behaviors?  Perhaps populations of ancestral Ciconiiforms became somehow reproductively isolated and so became different species, and then these different species developed different behaviors.  Alternatively, perhaps different behaviors arose in that ancestral species that allowed some members to use one part of the habitat better, and other members use a different part of the habitat better, and this then led to assortative mating and speciation.  And the answer is not necessarily the same for all species, in fact it is almost certainly not.  But how often does reproductive isolation come first?  How often is behavioral differentiation the inciting incident?  No one is really sure, but I find it fascinating to think about.  Gaining a greater understanding of how these different processes work and interact will give us a greater understanding of evolution and natural selection as a whole.

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