June 21, 2023 – On June 21, 2021 at 10:57 a.m. EDT, the Summer Solstice officially ushers in the summer season. When people are asked why seasons change on Earth, many answer the question incorrectly. They say the seasons change because of the varying distance between the Earth and the Sun. Distances vary because the Earth rotates around the Sun in an elliptical orbit. But that has nothing to do with the change of seasons. The seasons change because the Earth is tilted on its axis. The tilt is about 23 degrees.

NASA has terrific graphics that explain Earth’s elliptical orbit and the change of seasons. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere.

Spring and Summer – Migratory Birds on the Move!

American Goldfinch (Spinus tristis)

According to the Cornell Lab of Ornithology, the American Goldfinch has a natural, not virtual, tweet that sounds like po-ta-to chip. The summer male is strikingly beautiful with a bright yellow body, black and white wings, and a black crown on its head. The female is duller in color. As a migratory bird, it spends winters in warmer southern climates where the minimum temperature does not dip below 0 degrees Fahrenheit on average. Once north for the summer, the birds are attracted to nesting grounds near milkweed plants and wildflowers to ensure a bountiful supply of seeds, the mainstay of their diet. The species is strictly vegetarian and will only rarely eat an insect.

Goldfinches are also social birds. It’s not unusual to see a group of four to six at one time in a widlflower garden. And with their acrobatic ability of perching on the tip of flowers, the birds put on quite a show, especially if it is windy. Even with strong wind, the birds will be swaying on the flower.

Kirtland’s warbler (Dendroica kirtlandii)
In 1974, the population of singing adult Kirtland’s warbler males was down to 167, according to information from U.S. Fish and Wildlife Service (USFWS). Kirtland’s warblers are counted by listening to the adult male singing. Female Kirtland’s warblers do not sing so are very difficult to count. Based on research, there is close to a 1:1 ratio of males to females so counting adult singing males is an accurate method to count the total population.

The bird species is named after Dr. Jared Kirtland, a doctor who lived near Cleveland, Ohio. In 1851 (yes, the 19th century!), he found the warbler species but the species was not named until a few years later after scientists studied it.

In 1951, the first census revealed 432 adult males. The second census in 1961 reported 502 adult males. By 1971, a sharp decline in population was reported and in 1974, the population reached a low of 167 males. The species had been listed as endangered and in 1973, the USFWS appointed the Kirtland’s Warbler Recovery Team. It was the first time a team was appointed to focus on saving an endangered speices from extinction.

Two reasons for the decline in population were found. Those were: 1) decline in habitat, young jack pine forests in northern Michigan, and 2) cowbirds taking over warbler nests. The team addressed both problems. Cowbirds were captured in cages before the birds could take over warbler nests. Before capturing cowbirds, about 60 percent of warbler nests had cowbirds. But after capturing the birds, that number decreased to less than 10 percent.

The recovery effort was very successful. In 2001, the census revelaed 1,083 males. By 2013, that number increased to 2,020 and then in 2015, there were 2,365 adult males counted. In 2019, the Kirtland’s warbler was removed from the Endangered Species List.

REVIEW QUESTIONS

1. Why do seasons on Earth change?

2. When was the 2021 Summer Solstice?

3. What were the two reasons for the decline in population of the Kirtland’s warbler in the 1970s?

4. What did cowbirds do that caused a problem for Kirtland’s warblers?

5. When was the Kirtland’s warbler removed from the Endangered Species list?

INQUIRY QUESTIONS

1. As the first team that was formed to prevent a bird species from going extinct, what lessons do you think the USFWS took from the team’s success at recovering the Kirtland’s warbler?

2. Do you think there is the potential for the Kirtland’s warbler to decline again in population? If so, why?