What is Phenology?

Plants and animals have life cycle events that seemingly occur like clockwork every year:

  • Frogs and salamanders breed and their offspring go through metamorphosis
  • Flowers bloom
  • Male ungulates (deer, elk, pronghorns, etc.) grow antlers to begin the rut and breeding season
  • Birds migrate back and forth between their breeding grounds and their wintering grounds
  • The leaves of deciduous trees change color and fall off
  • Some mammals, such as bears, groundhogs and ground-squirrels, hibernate through the winter
  • Butterflies go through metamorphosis

How do Animals and Plants Know When to Start these Natural Events? 

Photo of multi-colored Autumn foliage from White Ledges overlooking Lake Solitude in Merrimack County, New Hampshire

The study of how the biological world times natural events is called phenology.  Scientists now understand that plants and animals take their cues from their local climate.  Climate (long-term weather patterns) is impacted by non-biological factors--temperature, precipitation and available sunlight.  Species use the predictable yearly changes in the climate to determine when they start natural events such as breeding or flowering.

Phenology Factors

The three main non-biological factors that affect phenology are:

  • Sunlight
  • Temperature
  • Precipitation (rainfall, snowfall, etc.)

These three factors work together to determine the timing of natural events. 

For example, birds in the Northern Hemisphere begin their migrations to their breeding grounds each spring.  One of the main cues they use is the amount of available sunlight.  In the spring, the amount of sunlight increases a little each day signaling that summer is right around the corner.  Along with sunlight, birds also use the warming temperatures to determine the time of migration. 

Frogs mainly depend on temperature and precipitation to determine when to breed.  Wood frogs use temperature changes to signal when to come out of their winter torpor to breed at vernal pools.  Frogs and toads in the Southwestern United States are completely dependent on rainstorms to create pools where they can lay their eggs.   

Plants use all three factors together--temperature, precipitation and available sunlight--to time their yearly blooming.

Why Should we Care about Phenology?

Phenology is an important subject to study, because it helps us understand the health of species and ecosystems.  Animals and plants do not live in bubbles--every species has an impact on those in its food chain and community.  The timing of one species' phenological events can be very important to the survival of another species. 

If one year has an unseasonably warm winter, frogs might mistake the high temperatures as a sign of spring.  Therefore, several frog species could breed much earlier in the year than expected.  If a late winter storm passes through, then all of the frogs that emerged from hibernation and their offspring could die.  The drop in the number of frogs will impact the predators that feed on frogs, such as raptors and snakes.  The impact will be felt throughout the food chain and the entire ecosystem.  


Farmers depend on insects to pollinate crops, such as blueberries, apples and squash.  There is a delicate balance between the insects and the crops--the crops need to flower around the same time that the insects finish developing in adults.  If the crops flower too early, then the pollinator insects could still be larvae.  There could be no pollinators to make our fruits and vegetables grow! 

Phenology and Global Warming

Global warming is slowly increasing average annual temperatures.  One of the most noticeable ways that global warming is impacting wildlife is by disrupting the timing of natural events.  With warmer temperatures, flowering plants are blooming earlier in the year and migratory birds are returning from their wintering grounds earlier in the spring.

Citizen Science

You can play a part in studying wildlife and global warming by participating in a phenology citizen science program, such as

  • Project Budburst – monitor the first flowering of plants
  • Project Feederwatch – report bird sightings at feeders throughout the winter and spring
  • FrogWatch USA – listen for the calls of frogs and toads during their breeding season
  • Earth Alive – make observations of common phenological events from bird migrations to flowering


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