Amateur Naturalist: The Change Of Seasons

A lone hummingbird remains at a feeder. Perhaps it is still storing away food before its flies south. A week earlier several hummingbirds were at the feeder at the same time. The first snow of winter has fallen, as seen in the background. Photo by Robert Dryja

Los Alamos

The first distinct change of the seasons for this year occurred during the last week of August and first week of September.

The temperature had been reaching toward eighty degrees in August. It then dropped to freezing, bringing rain and snow at the start of September. A cold front from the north brought this all about. The weather brought an immediate indication that summer was coming to an end. However, plants and birds have been providing indications for this change in their ways.

Plants provide a visual clue that summer is coming to an end but this clue is spread over several weeks. Aspen trees have had their leaves slowly change from darker green to lighter green and finally to yellow. This change has been occurring for the past month. Groves of aspen will become completely yellow in a few more weeks. Rather than showing current weather conditions, plants show the long-term seasonal trend.

Plants sense the change in the length of day and their growth is based on this. Shasta daisies bloom in the spring when daylight increases to about thirteen hours compared to about ten hours in winter. An occasional Shasta plant is confused in late summer and blooms again when the daylight hours decrease back to thirteen. Perhaps plants can sense whether daylight is lengthening or shortening as well the length of a day itself.

Birds in contrast provide clues to the changing of the season over a few days instead of over weeks. The rate at which hummingbirds sip sugar water from a feeder is an example. A feeder contains eight ounces of sugar water. More and more of this sugar water is consumed on a daily basis as summer progresses from June through July. The high point is reached in August when a feeder is drained completely each day. Hummingbirds weigh about 1/10 of an ounce and may drink up to six times their body weight each day. A little math suggests that possibly up thirteen birds are coming each day for the sugar water. Seven birds can be seen arriving at the same time at a feeder and this occurs several times in a day. The estimate of thirteen birds therefore is reasonable.

The daily refilling of a feeder continues until the final few days of August. The rate then slows significantly. A feeder now is drained slowly over several days. Typically, two to four hummingbirds came at the same time and only in the early morning or evening. The annual hummingbird migration south in anticipation of winter evidently is beginning.

White winged doves provide another sign that summer is ending. These doves spend the summer at higher mountain elevations but then move toward lower elevations as cool weather arrives. Up to ten white winged doves start arriving together at a bird feeder. They become in effect the replacements for the hummingbirds. This also occurs in same last few days of August. Steller’s jays provided a third clue that summer is ending. They tend to live at forested, higher elevations for the summer. They remain for the winter but move to lower elevations. An occasional Steller’s jay starts visiting a bird feeder at the end of August and may remain for the winter. A Woodhouse scrub jay joins it.

But what stimulates the birds to start migrating at a particular time in late summer? Several variables coming together may be the cause. Days become shorter in the last half of summer but this involves a change of a couple of minutes from day to day. Can birds sense such small daily changes or do they sense the combined change that has accumulated over several days? The decline of plant and insect food may stimulate migration. However, a decline in food by itself does not define which direction to fly. A decline in temperature is like a decline in food. It may stimulate migration but does not define which direction to fly. The position of the sun or stars can provide for a sense of direction, but are the small changes in position from day to day sufficient to stimulate the urge to migrate?

The movement of weather systems in contrast may indicate specifically when to migrate and which direction to fly. The clockwise rotation of air in a high-pressure system coming from the north can help carry birds southward. The counter-clockwise rotation of a low-pressure system may be an opposite stimulus to not fly in order to avoid being carried northward.

All of the preceding together can explain why, when and where the different species of birds decide to fly for the winter. The effect occurs in a few days rather than spread over months.

Nine white winged doves look for food by a feeder. They start arriving daily in September whereas none had come in the earlier months of summer. Photo by Robert Dryja

A Steller’s jay checks out a feeder at the start of September, similar to the white winged doves. Photo by Bob Walker