Signs of Fall

Although signs of the fall season are usually evident enough from changes in the weather, there are many clues from nature that a seismic shift in the seasons is occurring.  Even if you did not have a calendar, you should be able to predict the month if you study the many signs that nature provides.

During a recent walk on our farm I picked up a handful of nuts from the forest floor.  Can you identify  the acorns, shagbark hickory nuts, black walnut, buckeye, hazelnut, and chinquapin?  These provide a bountiful crop for wildlife to harvest and hide away for the future.  Many such nuts are never recovered by rodents and germinate.   

There are many soft fruits in the forest including this nannyberry.  It is Viburnum lentago and is one of many viburnums which produce "haw" fruit which are prized by animals of all kinds.  The variation in fruit color is interesting since it illustrates how plants communicate the ripeness of their fruits to frugivores.  It would be disadvantageous for the fruits to be eaten before they are ripe, since the seeds are not mature, so unripe fruits are often unpalatable or even poisonous.   The change in color from green to red, yellow or black that signals ripeness is easily recognized and is so familiar to us that we often fail to understand the evolutionary meaning. 

Grasses are also producing seeds and I here illustrate this with one of my favorites, Indiangrass.  Grasses do not have pretty flowers and they are wind pollinated so we do not always consider them to be beneficial to animals.  But the seeds are eaten by a wide variety of birds and over an extended period.  So it is important to allow grasses to flower and mature seeds in the fall and leave them in the fields during the fall and winter as wildlife food.  Grasses can also be quite beautiful at this stage of growth.

One distinct sign of fall around our ponds is the rapid decline in insect activity and a change in the species present.  For example the large common green darner migrates south and is replaced by the similarly sized shadow darner.  The autumn or yellow legged meadowhawk is the last dragonfly to emerge in the north and likely the last one seen before winter.  The male shown here is a beautiful red color to advertise its virility.  The appearance of odonate species that specialize in cooler conditions seems to be a classic case of avoidance of competition by a temporal shift in activity.

Eastern tiger swallowtails are a common butterfly in our area but we almost never find the caterpillars which feed on black cherry and tulip poplar.  This caterpillar found its way to our porch and has changed from green to brown as it approaches the time for pupation.  It has two false eye spots which may confuse avian predators into thinking it is a scary snake.  In addition when disturbed it protrudes an osmeterium which looks very much like the tongue of a snake.   These mechanisms to avoid predation illustrate how intense the efforts of birds are to find and eat caterpillars, and how gullible birds can be when confronted by these elaborate ruses.  

When I see large golden garden spiders on their webs I know it is fall.  These spiders over-winter as eggs or young spiders and gradually grow to an adult size over the summer.  The large female builds a characteristic orb web with distinctive  zig-zag patterns called stabilimenta that strengthen the web, and may attract insects and warn birds not to fly into the web.  

The migration of hawks is a characteristic fall phenomenon and I recently noticed this juvenile red shouldered hawk in a tree on our farm.  The juvenile plumage pattern seems to be good camouflage and signals a lack of maturity to adult hawks.  We do not see red shouldered hawks during the breeding season so this must be a bird that is migrating south.  They make use of rising currents of warm air or thermals to minimize their energetic cost of long distance flight.

Although the arrival of fall signals the end of the growing season for most animals and plants, it is a time of many changes in the world of natural history that are of great interest.  So get out and enjoy the beauties of fall and observe how a few species wax while most others wane in abundance.