A journey into the eyes and mind of a naturalist.

Fitness is not Strength

For us fitness is being in good shape.  We measure it in pounds, seconds, and calories burned.  Fitness for the rest of the natural world has a much more dire meaning.  Fitness is defined as the ability to produce offspring who live to reproduce.  Humans are the only organism that has to “go work out” just for the sake of staying in shape.  Every other organism either stays “in shape” or they die.

This summer, just weeks after the chickadees moved out, a pair of Easter Bluebirds began building their nest in the same nest box as the chickadees from my previous post.  They built their nest directly on top of the former chickadee nest.

Both the male and female bluebird shuttled in tufts of dried grass and reeds until a small cup nest was formed.  The female then laid five light blue eggs.


The moment the first egg was laid the male bluebird became very territorial of the nest and the adjacent areas.  The male spends a fair amount of energy defending the nest from his own reflection in our patio door window.


His efforts were not only against his reflection but also any other bird, moth, butterfly or dragonfly that ventured too close to the nest.  Sticking your head out the door would often result in dive-bomb attacks by the male, female or both.  They protected their energetic investment with every ounce of their life.  Their only purpose in life lay inside that nest box.

After approximately 10 days the eggs began to hatch.


Fuzz turns to feathers and slowly each chick began to look more like a bird.  Each day showed significant changes and growth of each chick.  The demands of five hungry mouths kept the parents busy ferrying loads of caterpillars, grasshoppers, and other insects to each of the chicks to feed their astounding growth rate.

Each parent diligently supplied a supply of food to each of their growing chicks despite the increasing summer temperatures.   This summer was particularly warm, the second warmest on record.  Late June contained several weeks with high temperatures in the mid 90’s to 100’s.   There was no relief from the heat with the lows only in the mid-80’s.  This heat wave was little more than an inconvenience for humans.  We simply turned on the AC, went to the pool or hung out at the mall.  For five little bluebird chicks in a nest-box the situation was much more dire.

The first several days of temperatures north of 90F claimed its first victim.  One of the chicks died from exposure or lack of food due to the heat.  In the picture above you can just make out parts of the chick’s limp body underneath its nest-mates.  The next morning the lifeless body had been removed by the parents and carried away from the nest as to not attract predators or parasites with the smell of decay.

The heat continued and with each day the chicks died, only those who were able to bear the heat and still have enough to energy to beg for food survive.  Humans have compassion, something that separates us from much of the animal kingdom.  As a compassionate human it was not easy to see these little bluebirds dying but it was nature’s course and it would not be ethical to intervene.


With only three of the chicks surviving the demands on the parents became less intense.  Despite fewer mouths to feed the temperatures continued to increase.  The temperatures prevent the adults from foraging for food for the chicks and increases the heat stress within the nest box.  Unfortunately, the heat had not reached a maximum.  The following picture was taken with a heat index above 115F.

These two chicks are all that remain of the original five.  You can just make out the lifeless body of their sibling on the floor of the nest.  These are birds that are slowly dying of heat exposure.  Their mouths are open to circulate air to cool their tiny bodies.  Unlike humans, birds are incapable of sweating.  The only way they can stay cool is to pant and keep air flowing through their little lungs.  Overheating is a real danger for a tiny bird in a nest box fully exposed to the summer sun.

The picture above was the last picture I took of the bluebird chicks.  The following morning I left on a vacation away from the heatwave at home.

A week later I returned home to find two tiny skeletonized bodies of two chicks on the bottom of the nest.  Only one of the chicks, of the original five, ever made it out of the nest.  This little bird struggled to even get out of the nest,  the trials of life await it in the world outside.  Will it live to create a nest of its own?  The odds are not in its favor.  Selection is a powerful driving force that has shaped the world as we know it and it is the individual fitness of each organism that determines how selection will ultimately affect the entire species.

This pair of bluebird laid five eggs with the potential to raise five offspring to adulthood.  Only one ever made it out of the nest.  From the common definition of fitness these birds are the ultimate athletes but from a biological perspective, they are failures because they were unable to reproduce even enough offspring to replace themselves within the larger population of bluebirds.

Fitness has nothing to do with strength or determination.


3 responses

  1. Anne N

    Except. _We_ humans have contributed directly to these terrible heat waves. And, we built boxes for them, that then smart and adaptable hawks camp out nearby, waiting for them (the bluebirds) to come out. So, to say that you had no responsibility in this scenario? Doesn’t wash. In a case like this, you could have legitimately intervened. We have done so often enough at this point, that we must continue to do so–we are intimately locked into this web of life.

    I certainly appreciate your viewpoint, but it is only one perspective. Fitness may not be strength. Evolve (quickly) or die might be more appropriate in this case. Or hope that a compassionate human takes pity upon you.

    August 15, 2012 at 23:05

    • I certainly agree that humans are intimately locked into a web of life if every other living organism on this planet. And collectively we have altered the natural order of life in subtle and obvious ways on a global scale. I place nest boxes on my property to produce habitat that would otherwise be unsuitable. I would much prefer the natural tree cavities that these birds would use but that is not possible on my small piece of property. Should I have intervened? Absolutely not. I am compassionate but I also cannot let that compassion cloud my judgement and understanding of the natural flow of life. Nests fail, a lot, and not just in nest boxes. Sharp-shinned and Cooper’s hawks can find nests in tree cavities just as easy as nest boxes, they are not evil birds picking off helpless fledglings, they are parents feeding their own hungry mouths.
      The blue birds that nested in my nest box this year didn’t pick the ideal spot. Perhaps the box isn’t placed in the correct spot, perhaps the heat was too much, or maybe they laid their eggs a day too early. Perhaps there is some invisible genetic control that drove this particular pair to choose this particular nest box. If that is the case, and I would argue that it is because there are plenty of other nest boxes in my neighborhood, then I am almost glad the nest failed. Losing the chicks in not a easy event, but ultimately in means that the genes of the this pair will not be perpetuated within the larger population. Evolution does not occur within the individual it can only occur within the population, what I witnessed was evolution in progress. The genetic makeup of the population is changing with each unsuccessful nest, creating a population that is comprised of only those birds who survive the trials of nesting and fledging. Death is essential for life and I cannot pity the natural process that is necessary for life to continue.

      August 16, 2012 at 07:55

  2. Deb

    Sad but beautiful photos; thank you for teaching us

    September 3, 2012 at 22:22

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