Bird populations see ups and downs

  • A Pine Siskin and an American Goldfinch on a feeder. Francie Von Mertens / N.H. Audubon

For the Monitor
Sunday, February 05, 2017

Birds, like all wildlife, depend on the abundance of food, water and shelter. Any change in habitat could cause fluctuations in their populations. But ups and downs of a population often follows a predictable cycle.

Examples of natural peaks and valleys are particularly evident during the winter when finches congregate at bird feeders. The American Goldfinch, Pine Siskin and Common Redpoll are known as irruptive species. That is to say, they are extremely abundant one year and completely absent or rare the next. These cycles can be seen by examining data from the annual Backyard Winter Bird Survey, conducted for over 30 years by New Hampshire Audubon. Such long term surveys are important tools for discovering regular trends.

The American Goldfinch exhibits a peak every two to three years, as illustrated in 2015 and 2016 when the survey count was nearly 12,000 and more than 13,000, respectively. Compare that with 2014 when observers recorded just more than 5,000 goldfinches statewide. By all predictions and seasonal observations, this year’s tally will likely be back down around the 2014 numbers.

If we only look at a few years of data, we might conclude that a dramatic drop in population from one year to the next could indicate some horrible loss of habitat or contamination. However, the long-term trends put the variations in perspective. Steep declines happen regularly with irruptive species, and just as regularly, their numbers rebound.

What causes these deviations? It has to do with the food supply and the fact that New Hampshire is actually the winter home to some of these species – they fly south to be here where weather is less severe than in their Canadian breeding territory.

Though American Goldfinches do nest in New Hampshire and throughout the mid-Atlantic states, there are also breeding populations further north. Their summer food supply consists of seeds from the spruce and fir trees in northern regions. In years when the seed production is good and food is abundant, fewer birds migrate south (to New Hampshire) and the winter population of finches here is less. If there is limited food up north, an invasion of finches into our region boosts the winter population and may even push some of the resident birds further south for a few months.

Though the total observations of Pine Siskins and Common Redpolls are not as dramatic, similar peaks and valleys can be seen with these small, northern-nesting birds that rely on birch seeds for the bulk of their diet. The number of Pine Siskins observed went from 155 in 2014 to 5,938 in 2015 and back down to 521 in 2016. Redpolls trends followed suit with 22 in 2014, 3,475 in 2015 and 56 in 2016. If these patterns continue, 2017 should be a good year for both of these species.

It’s important to note that the backyard winter survey is conducted statewide and the majority of siskins and redpolls are observed in the northern part of New Hampshire.

While it is surprising to many people that our state is the winter home to some types of birds, it is even more astonishing that several kinds of traditionally southern species have ventured into our territory. This movement started long before we began experiencing the record-breaking warm temperatures of recent years.

In the late 1950s, Northern Cardinals started living up to their name and inhabiting New England states. This prompted surprised birders to start surveying them. In 1967, the first Cardinal and Tufted Titmouse survey was conducted in New Hampshire. Mourning Doves and Northern Mockingbirds were added a few years later. As the survey broadened in the late 1980s to include all birds, increases in Carolina Wrens, Red-bellied Woodpeckers, American Robins and even Eastern Bluebirds were recorded.

It is believed that some of these increases are a result of more people feeding wild birds, providing fruit bearing shrubs and suitable habitat. The warming climate may now be a factor as well.

Keeping long-term records of changing populations provides important information for biologists about which species are doing well and which may need conservation assistance. You can help gather that information by participating in the annual Backyard Winter Bird Survey on Saturday and next Sunday. To learn more visit nhaudubon.org/calendar/bird-survey-celebrates-30-years.