A female American redstart. Amy Sancetta/Associated Press

Earth Day, April 22, is just around the corner. The brainchild of Senator Gaylord Nelson of Wisconsin, the first Earth Day occurred in 1970. As a high schooler, I was excited by this new day of significance and worked with a few other students to promote the day at our school. Who knew how important this day would become as a flashpoint for environmentalists and conservation?

So many activities were planned on Earth Day that April has been designated Earth Month to spread those activities around. April, a time of renewal and growth in the northern hemisphere, is a particularly appropriate time.

In the 1970s, a catchphrase in the environmental movement took hold: Think Globally, Act Locally. There is much to recommend from this pithy phrase. We all can recite a litany of how we can help protect Mother Earth locally in our daily lives. Recycle paper and metal. Cut down on our use of electricity. Install solar panels. Buy local vegetables and fruits. Plant trees.

Think Globally is an acknowledgment of the fact that we share one atmosphere. Water and nutrient cycles are global in scope.

However, I think this phrase is overly simplistic. The underlying assumption is that if everyone can improve their own patch of the earth, collectively world environmental health will be assured.

To me, the catchphrase embodies two extremes: our backyard and the globe. But we see interconnections between parts of the globe that fall in between. As environmentalists, we need to be mindful of these relationships to be effective stewards of our natural world.

Here’s an example. The American redstart is a common warbler, nesting broadly across North America. This species is a neotropical migrant, wintering in the Caribbean, Central America, and South America. Most of the redstarts nesting in New England overwinter in the Greater Antilles.

Peter Marra studied overwintering redstarts in Jamaica, one of the Greater Antilles, as a part of his doctoral research at Dartmouth College about 25 years ago.

The warblers are territorial on their wintering grounds, with each individual, male or female, maintaining an exclusive feeding territory. Older male redstarts dominate food-rich mangrove habitats while females and younger males must make do with territories in drier, second-growth scrub habitat with less food.

Redstarts in wetter habitats gained weight through the winter while those in scrub habitat lost weight. Peter sought to determine if wintering habitat had an effect of nesting success. Did individuals wintering in mangrove habitats have better nesting success than those in drier habitats?

Because the redstart population is nearly 40 million birds, the odds of finding a nesting bird in North America banded in Jamaica is essentially zero. But Peter had a clever way of gauging the winter provenance of breeding birds.

Most chemical elements occur in several forms, called isotopes. They differ by the number of neutrons in their nucleus. Carbon provides a useful tool because two isotopes – carbon 12 and carbon 13 (with one extra neutron) – are common and stable. The proportion of the two isotopes varies in plants among different habitats.

The proportion of carbon 12 to carbon 13 is passed up the food chain to herbivorous insects to birds. So, a blood sample of a redstart tells us in which type and quality of habitat a bird overwintered. Peter did just that by capturing redstarts shortly after their arrival on their breeding grounds and taking a blood sample.

Redstarts arriving earliest had wintered in the mangrove forest. These birds had the best choice of territories and mates. Birds with blood signatures indicating wintering in scrub habitat arrived later with less weight and other signs of poor physiological condition. Those birds had to take lower quality territories.

So, we see an interconnection between wintering grounds and breeding grounds that is not on a global scale, complicating conservation efforts. Sometimes, acting locally is not enough.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu