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By Root 785 0

influence of artificial short days, whereas another will flower only if subjected to long days. Similarly, a constant laboratory photoperiod induces birds to either lay eggs or shut off egg production. Wild birds breeding in the north start their reproductive cycles, including all their behavior of migration, courting, and nest building, in an orderly progression influenced by photoperiod. Our chickens lay throughout the summer when the natural photoperiod is on average at least thirteen hours of light and eleven of darkness. But to keep the hens laying in the middle of the winter all we have to do is keep a light on in their shed for several extra hours. At the spring equinox, near the end of March, the photoperiod is rapidly approaching thirteen to eleven, and that is also the time when many organisms are preparing for summer. Since they use the photoperiod to inform them of what season they are in, how does the same photoperiod, at around the fall equinox in late September and the spring equinox in late March, allow them to differentiate spring from fall?

Temperature is too variable to serve as a reliable cue to differentiate the beginning from the end of summer. Plants and animals not only need to know when summer is or is coming, but also need to predict when it will begin and end. Any given photoperiod as such is not the only answer. It seems remarkable enough that any organisms can measure photoperiod and almost universally respond appropriately to it, but they need an added mechanism to determine the direction of the changing photoperiod. That seems a lot to ask for. When plants flower at an inappropriate time, this effect is often attributed to “stress” or to unusually high temperatures. Stress could indeed be a contributing factor, but perhaps the springtime photoperiod in the fall is in itself a stressor.

Fig. 41. One bud (of eight on this section of twig) of a honeysuckle bush that has opened to extend a twig with leaves and flowers in October; normally the plant blooms in late May.

Mistakes or imperfections provide variety for natural selection to work on, to permit evolution. There is even a mechanism whose only “purpose” is to produce variation. It’s called sex. If there were no variation, there could be no evolution. If species had been magically created, they would all be clones. It was a heritable mutation that caused central European blackcap warblers to misorient, so that they ended up flying west in the fall rather than south to Africa. These birds got a new population started, and they are now thriving in Great Britain. Some possibly misguided phoebes who abandoned cliffs and started nesting on houses became the norm because houses were safer. Salmon that did not find their home streams on their spawning migration and by chance ended up in other streams eventually colonized the new streams, expanding the populations. Possibly confused woodpeckers that wasted a lot of time and energy hammering pseudo nest holes in the fall found them to be useful places for overnighting during very cold weather, and they had a slightly better chance of surviving than those who seemed to be less misguided. Similarly, the rare blue violet that blooms in the fall reminds me that nature is not always proximally perfect, though it evolves and ultimately persists because of its imperfections.

FOR YEARS ON WARM NIGHTS AND SOMETIMES ALSO ON warm days in late summer and fall, I have heard strange, generally isolated high-pitched cheeps and chirps coming from our woods. Whenever I have stalked near to determine the sources of these birdlike calls, they have invariably stopped and I have seen nothing. It has taken me a while to finally establish that I was hearing the voices of apparently misguided spring peepers and wood frogs. In the spring these frogs collectively make a deafening din in their breeding ponds, and afterward they hop back onto land in the nearby woods, where they remain silent all summer. However, by September and early October, in Indian summer, when you again begin to hear their voices, they are never