The Study of Phenology

“Many of the events of the annual cycle recur year after year in a regular order. A year-to-year record of this order is a record of the rates at which solar energy flows to and through living things.  They are the arteries of the land.  By tracing their response to the sun, phenology may eventually shed some light on that ultimate enigma, the land’s inner workings.” –Aldo Leopold, A Phenological Record for Sauk and Dane Counties, Wisconsin, 1935-1945

Phenology is a segment of ecology focusing on the study of periodic plant and animal life-cycle events that are influenced by climate and seasonal change in the environment. Skunks emerging from winter dens, sandhill cranes trumpeting their return, and seeds ripening are all examples of annual phenological events. Phenology is derived from the Greek word phaino meaning to show or appear, indicating its principal concern with the dates of first occurrence of natural events in their annual cycle.

Phenological first began being recorded at the Aldo Leopold shack and farm by Aldo Leopold in 1936. Leopold took recordings from 1935-1948 in and around this landscape, which inspired Leopold's seminal work on conservation: A Sand County Almanac.

Nina Leopold Bradley continued to carry on her father’s work, compiling a robust phenological database spanning from 1976 until her death in 2011. She found that a substantial number of phenological events occurred much earlier in her data than they did in her father's. Several studies have shown a significant trend for an earlier occurrence of spring phenological events suggesting some species are changing behaviors in response to climate change. Some species are expanding or shifting their ranges. The tufted titmouse, a songbird with an unmistakable call, did not occur during 1935-1948, but became a year-round resident at the Leopold farm in Nina Leopold Bradley’s time. Watch a four minute video of Nina discussing phenology.

Keep track of phenology in your own home!

We have several tools you can use to help you keep track of phenology just like we do here at the Aldo Leopold Foundation. Please download and use the following information for South-Central Wisconsin to help you:

Introduction to the Phenology Project: An overview to get you started. Download PDF.

Day-by-Day Observation Tracker: Use this document to help you know what events you might expect to see on each day of the year and record your own observations. Download PDF.

Phenology Photo Gallery: Not sure you can identify some of the species on the list? Use this field guide to help you. Download PDF.


National Phenology Websites: These websites enable people all over the U.S. to upload and view current phenological observations in their area.

The USA National Phenology Network

Project Budburst

Journey North


New phenology study using data from Leopold and Thoreau shows spring advancing.

Aldo Leopold Foundation Senior Fellow Stan Temple, along with scientists from Boston and Harvard Universities, used data from long-term phenology records initiated by Aldo Leopold and Henry David Thoreau to examine the relationship between spring temperatures and flowering dates. Those historical relationships allowed them to correctly predict how spring wildflowers responded to record-setting warm temperatures in 2012. Their research shows that spring flowering is still coming earlier and earlier as spring temperatures rise. If there are physiological constraints limiting how early plants can flower, they have not yet been reached. These results can help to predict plant responses to a warmer climate, essential for commercial production in spring-flowering fruit trees and other crops. They also indicate that a continued trend of earlier flowering may pose problems for pollination if responses of pollinators to climatic changes don't keep pace.

Learn more about this study:


Phenology Calendars

Each year, the Aldo Leopold Foundation is involved in publishing the Wisconsin Wildlife Phenology Calendar. It incorporates Aldo Leopold and Nina Leopold Bradley's average dates for phenological events in Wisconsin, paired with stunning wildlife photography. This year, the calendar theme is nocturnal animals of Wisconsin. Each month, learn about the members of Wisconsin's ecological communities that occupy the night.


The Wisconsin Wildlife Phenology Calendar is a partnership between the Aldo Leopold Foundation, the Natural Resources Conservation Service, the US Fish and Wildlife Service Partners for Fish and Wildlife, and UW-Extension. It has been produced yearly since 2004.