Why A Dawn Chorus?

Anyone who has ever gone birding, and lots of people who have not, know of the phenomenon of the dawn chorus; the time when it seems like every bird makes itself known with a song.  It is a wonderful time to go birding as you are likely to see and hear the maximum diversity of birds in your area.  But, why does this occur?  What is it about dawn that makes birds want to sing?  Why not at noon, or any other time for that matter?  Well, there are three main reasons that have been identified that all contribute to making dawn the best time for birds to spend some time singing.

One has to do with the physics of sound transmission.  Cold air is stiller than warm air.  This means that there is less distortion of sound waves that are traveling through cold air as compared to warm air.  Since the air is coolest just when it is starting to get light, it is the best time for a bird to sing and have its song heard clearly at the longest distances.  One issue this brings to mind is: why don’t birds sing at night?  The air temperature is even lower at night than it is at dawn, so shouldn’t that be an even better time to sing?  Most birds are diurnal.  They can see better in the daylight, there are fewer predators during the day, and their food is active during the day.  So they sing during the same daylight hours that they are awake during, making dawn the best option.

Speaking of food, this is a second reason for singing at dawn.  For insectivorous birds, and to a lesser extent birds that feed on anything else, their food is not available until they warn up a bit and start to move around.  This means that  later in the day, foraging is a much better use of time.  It is more efficient time management to sing when there is no food to be found anyway, and to not sing later when food gathering becomes the high priority.

The third reason for a dawn chorus is to maintain territories.  Since nights are dangerous for most birds with harsh temperature conditions and higher predation risks, the first thing in the morning is a good time for a bird to advertise that it survived.  This tells its neighbors that they should not try to move in on its territory.  If there is silence coming from a territory at dawn, there is a good chance that it is open for the taking.

All three of these factors add up to make dawn the smart time for a bird to sing, and a wonderful time for us to go out and hear them do it.  So get out early and listen to the dawn!

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The Lek Paradox

A lek is an area where many males of a species come together and display.  These displays can include very elaborate vocalizations, plumage, and behaviors such as the complex dances of some birds of paradise.  The displays play a very important role in mate selection in these species because they are the only trait that the females who visit the lek have to judge which male to mate with.  Males of lekking species do not hold territories where the female can raise her young and find food or provide parental aid in any other form.  The females provide all the care for their young, so what are the females choosing in a mate?  They are choosing genes.  Males that have traits that make them attractive have the genes that code for those traits, and so the offspring of these males should also have those traits and be attractive.  So, by choosing attractive mates, the females are maximizing the odds that their own genes will also be passed on to the next generation.  This setting results in some theoretical complications in terms of sexual selection.  One important one is that if females pick traits over and over again for generation after generation all the males will end up having the same traits.  If all the males are the same, there will be no way for the females to tell which males are the fittest and therefore who to pick to mate with and the whole system will collapse.  Yet this does not happen.  So how is variation being maintained in the face of strong selection?  This is the lek paradox.

Several solutions to the lek paradox have been proposed.  One is that females may not choose the same trait every breeding season.  This is often referred to as the fluctuating selection hypothesis (Jia et al. 2000), and it states that during one breeding season large males are favored, but then next breeding season it is males with the longest tails that are preferred, and the breeding season after that it is the loudest males that do the best.  This basically describes fads in fashion.  Also, preferences may not change breeding season to breeding season, but instead the large males may be favored for several seasons and then the shift to a different trait may be more gradual.  Either way, these changing preferences would result in the maintenance of genetic diversity.

Another possible resolution is that females may disagree with one another as to the most important traits.  This has been shown in Guppies by Brooks and Endler (2001) where females were allowed to chose between different males.  Most females seemed to agree that large males were the most fit, but when females were presented with males of equal size, some preferred males with large amounts of orange on their tails, while others preferred males that more black spots on their sides.  This disagreement would also maintain genetic variability.

A third proposed solution is that females are making an overall assessment of male fitness taking into account many traits.  This idea is called the genetic capture hypothesis (Rowe and Houle 1996), and it states that overall fitness is dependent on many morphological and physiological features.  In other words, for a male to be fit he must be able to find lots of food and escape predators and resist parasites etc.  All these abilities are coded for by many many genes, and it is the sum total of these genes that the females are choosing.

These hypotheses are not mutually exclusive, so in all likelihood there is a combination of factors at work.  But determining which is the dominant force is still an interesting goal, and still very much in dispute.

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The Corpse Flower

I recently saw, and smelled, a blooming Corpse Flower (Amorphophallus sp.) for the first time, and it was amazing!

The genus Amorphophallus is comprised of about 200 species that generally inhabit secondary forests of in the tropics and subtropics of Africa and the Pacific Islands.  The plants grow from a stem that is underground, called a tuber or corm, and from this stem they grow one leaf.  But, this is no ordinary leaf.  It can grow vertically up to 2 or 3 meters tall.  The petiole can get to be around 15 cm across, and is cool and smooth to the touch.  At the top it then splits into three horizontal blades that each have many leaflets.  It is a very beautiful leaf with the petiole a mottled pattern of light and dark green, and the leaflets a bright fresh green.  The overall impression is that it is a tree, and not a single leaf.  Each leaf lasts only one growing season before it dies back, so these plant are deciduous.  The plant will grow several leaves, over several growing seasons, to accumulate nutrients in the corm.

Once it has enough stored energy, which generally takes 3 to 6 years, it will bloom.  The single blossom can be huge!  It is a spectacular deep, dark red and black flower and it smells like something between rotten meat and spoiled fish.  The smell can be oppressive.  The one that I got to see was in the UC Davis Plant Conservatory, and the back third of the greenhouse was filled with smell.  Flies are attracted to this smell and are the insect pollinator for the flower.  The flies are so convinced that they have found a wonderful food source that they often go so far as to lay their eggs in the flower.  Flowers of the species in this genus must be pollinated the same day they open.  Once the female parts of the flower have been pollinated, the male parts (which had been concealed) open and begin to release pollen.

If anyone is on the Davis area, I would highly recommend paying the Plant Conservatory a visit and checking out the Corpse Flower while it is still blooming.  If you do not make it to see this plant’s flower, there is another Corpse Flower plant that is  likely to bloom next year and that one is even bigger!

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