Nests, Important for Bird Survival and Reproduction

It is a great time of year to be in our area.  Those of us who keep track of bird activity call this time of year migration season. Of course we are referring to the large numbers of birds that breed farther north of us returning from their wintering grounds farther south and making a short stop here with us.  Those birds will not be breeding for some time yet as they have to arrive in their breeding grounds which in some cases are several thousand miles farther north.  However, this time of year is already breeding time for many of our resident birds. If you pay attention to our year-round resident birds you will observe birds in courtship displays.  For example the doves who will gain height with exaggerated flapping then make a shallow arch and descend close to their prospective mate.  It is also common to see many birds and species flying around with long grass stalks and shorter twigs in their beaks.  Al these items are construction materials that will be used to build their nest.  Some nest are incredible feats of engineering and I have always marveled at the different construction methods and techniques used by birds to develop such elaborate and functional structures.  I also have always marveled at the apparently insignificant and incredibly flimsy construction of some species nests, such as that of the white-winged dove.  If you have not a seen a white-winged dove it is not surprising as it is a flat platform of twigs so thin one can actually see the eggs when looking at a nest from below.   I always wondered how those twigs stay together as a unit during incubation and chick rearing.

Nests come in many shapes and complexities. Our beach nesting birds do not actually build a nest rather they scrap a bowl on the sand or gravel and lay eggs directly on that substrate.  Swallows will build elaborate mud constructions, whether bowls or tunnel shaped nests depending on the species. The most common nests are bowl shaped structures constructed from local vegetation material such as twigs and grasses or both. We can marvel at the diversity of nest types out there but why are nests so prevalent in the bird world? Regardless of shape and construction type nests have important function for all birds.

Nests are important for birds because the embryos in the eggs must develop outside the bird’s body exposed to the changing conditions of the environment.  Optimum temperature for embryonic development is 37-38 degrees C. lower temperatures can disrupt embryo development and temperatures above 42 degrees C can be lethal.  So the nest in some cases provides conditions to keep eggs at higher temperatures than ambient temperature and in some cases to lower temperature than the environment.  A      nest not only has to provide the appropriate conditions for egg development it must in some cases also provide concealment and protection for eggs and later chicks. It also helps hide the incubating adults, which in many cases attend the eggs 100% of the time during incubation. Because the nest is stationary it could be become easy target for predators if not adequately concealed. Some nests, such as those of grassland birds are incredibly difficult to see, even when you know where the nest is after seeing an incubating adult fly up.  Some nests from our local forests such as that of the white-eyed vireo are cup shaped and hang near the end of branches not high from the ground, yet they are not easily seen among the leaves and vegetation.  Nesting season means there is not long to wait for chick rearing season and we will soon be seeing many young birds learning to fly and begging for food throughout our area, even before the migration season is over.

By: Felipe Chavez-Ramirez

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