Keeping Track: Frequently Asked Questions About Phenology

By Dr. Stanley Temple, Beers-Bascom Professor Emeritus in Conservation, University of Wisconsin-Madison, and Senior Fellow, Aldo Leopold Foundation

Keeping records enhances the pleasure of the search, and the chance of finding order and meaning in these events.” Aldo Leopold

The annual Wisconsin Wildlife Phenology calendar is published by the Aldo Leopold Foundation. Many of you may have your 2018 calendar prominently placed to conveniently track when various events in nature occur during the year. Here, I’ll try to answer some of the frequently asked questions about phenology, in general, and the calendar, in particular.

What is Phenology?

Phenology is the study of when seasonal events happen in the natural world. The arrival of migratory birds or the blooming of plants are just a few of the conspicuous seasonal events that often attract our attention. More recently, phenology has become a more rigorous field of study as naturalists have formally recorded their observations and tried to make sense of the patterns they observed in the resulting data. Typically, phenological records focus on dates of the first and last occurrences of an event during the year. Interest in phenological records has increased recently because man-made climate change has influenced the dates of first occurrence for many species.

How to Use the Information on the Calendar?

Pasque Flowers Blooming

The 2018 calendar is an attractive way to anticipate and keep track of when natural events occur in Wisconsin. The calendar can be a gentle reminder to venture out on a sunny winter day in early January, for example, to catch a Northern Cardinal singing for the first time, or to visit a dry prairie in late-March to find the first Pasque Flowers blooming

In most instances the 2018 calendar notes the earliest date on which an event has been recorded since 1935 in south-central Wisconsin. But, depending on the year’s weather and climate conditions, the event may occur days or even weeks later. Also, if Wisconsin experiences another record-breaking warm season, the event might occur earlier than it’s ever been observed before. In northern Wisconsin many of the events in the calendar can be expected to occur later.

Factors Influencing Arrival Dates

The observation of the very first occurrence of an event can be influenced by two factors: when the event actually occurred and when the event is noted and recorded by an observer.

Observer behavior can influence the first recorded date. Casual observers tend to record the date later than more diligent observers. Knowing ahead of time when an event is expected tends to result in anticipation and earlier detection. Observers who make observations daily tend to detect events earlier than those who observe irregularly. Also, observations by weekend naturalists are consistently late because, for example, an event that actually happened on Monday wouldn’t be detected until they ventured out on Saturday.
More critical are the environmental factors influencing the actual date. Temperature is one of the most obvious. Many, but not all, events tend to occur earlier in years when seasonal temperatures are warmer. Many spring wildflowers like the Pasque Flower, for example, bloom about 4 days earlier for each 1 degree (C) rise in average spring temperature. Temperatures, of course, can be highly variable from year to year, and that variability results in year-to-year differences in when temperature-dependent events occur.

Another major influence is length of day or photoperiod. Many plants and animals time their seasonal activities closely with day length. For those species there is much less year-to-year variation.

Geographical factors can also influence the actual date: Events typically occur later at higher latitudes and at higher elevations (as a rule of thumb, about 1.25 days earlier for each degree of latitude). Since there are 4 degrees of latitude difference between southern and northern Wisconsin, events can be expected to occur about 6 days later up north. Also, events on south-facing slopes tend to happen earlier than on north-facing slopes, and events in urban areas that trap heat tend to happen earlier than in rural areas.

Phenological Records Reveal Climate Change

Canada Goose Flying
Gary Shackelford

We can examine historical phenological records of hundreds of natural events for evidence of how climate change is affecting the timing of seasonal events. Over the past decade, for example, American Robins are arriving on average 22 days earlier that they did in the decade 1935-45 when Aldo Leopold was recording. In fact, many robins are no longer even bothering to migrate at all and now overwinter in Wisconsin. So, spring arrival of robins is losing its significance. Already, Aldo Leopold’s essay, “The Geese Return,” has lost much of its significance since Canada Geese are now year-round residents in southern Wisconsin. If trends like this continue, some historical seasonal events may even become extinct.

Our historical records show many species of plants and animals are adapting to climate change by shifting the timing of their important seasonal events. That’s okay for those species that can cope with the changing climate. However, other species that rely on seasonal changes in photoperiod to time their seasonal activities are not so fortunate. Many of our long-distance migrant birds that winter in the tropics (e.g., warblers, tanagers, vireos) rely on photoperiod to time their northward migration and are arriving on the same dates observed by Aldo Leopold over 80 years ago.

Problems arise if these species get out of sync with other environmental factors that are also changing in Wisconsin. Late-arriving migrants might lose out for scarce resources with earlier arriving species that are adapting to Wisconsin’s changing climate. Or, they might be arriving too late to raise their young at the peak of availability of food like insects that are very temperature dependent.

For example, Great Crested Flycatchers can’t excavate their own nesting cavity and must find an existing cavity in order to breed. Being long-distance migrants, they consistently arrive around the first of May by which time they have lost out in competition with European Starlings that have already claimed available nesting cavities several weeks earlier. “The early bird gets the worm,” or in this case the prime nesting cavity.

Great Crested Flycatcher (Left; and European Starling (Right;

How Can You Make Your Phenological Observations Available to Researchers?

Fortunately, if you keep records of phenological events there are now several citizen science projects that are eager to add your observations to their databases. The largest of these projects is eBird. This project run by the Cornell Lab of Ornithology allows bird watchers to upload their observations directly to the central database where the data is promptly analyzed and made available for inspection by the public. Data from Wisconsin can be added and viewed at Thousands of bird observations in Wisconsin are added each year from individual birders and from participants in the ongoing Wisconsin Breeding Bird Atlas project

For plants and other natural events, there is the National Phenological Network at Their “Nature’s Notebook” project is a treasure trove of information about the timing of seasonal events for hundreds of species. Learn more at

The 2021 Phenology Calendar is now available! Get your copy today! Supplies are limited.