If you have been following my blog, you are aware of a worrisome set of circumstances. Survival of Wisconsin adults is as high now as ever, but breeding success has fallen sharply. And the negative silver spoon effect I detected this fall compounds the reproductive downturn. Underfed chicks are unlikely to survive to adulthood. Those that do survive are unlikely to claim a territory and raise chicks themselves. In other words, Wisconsin breeders produce fewer and poorer quality young than before, which contributes to population decline.
And matters are worse in Minnesota. Our data from northcentral Minnesota show: 1) consistently lower adult survival than in Wisconsin, 2) breeding success on a par with Wisconsin, and 3) a more severe silver spoon effect. That’s right, adult loons in Crow Wing and Cass counties experience lower overwinter survival than their Wisconsin counterparts and have been rearing young of poorer quality in recent years. Here is a figure showing the latter pattern.
As you can see, Wisconsin breeders have actually bounced back to raise healthier young in the past few seasons! This is excellent news, and we hope the pattern persists. But Minnesota breeders have gone the opposite direction. Why is this a concern? As the figure below shows, the probability that a loon reaches adulthood depends upon its mass as a chick.
Thirty two years’ of data in Wisconsin have shown that loon chicks with a mass/age value above 80 have about a 50/50 chance of returning to the study area as adults. Roughly 42% of those with mass/age values of 70 to 80 come back as adults. Chicks in mass/age classes of 60 to 70, 50 to 60, 40 to 50, and below 40 return at rates of 36%, 28%, 18%, and 14%, respectively. Now look at the mass/age data from Minnesota for the past two years:
The future looks bright for the Big Trout-West chick we banded in 2023, the larger Cross-South of Happy Bay chick from 2023, and the Fawn-E chick from this past summer. Things look fairly rosy for both Island-Channel chicks from last year, the Goodrich-SE chick from 2023, the Kimball-West chick from 2023, last year’s Little Star/Star chick, two of three Lower Hay chicks from 2024, the alpha Margaret-North chick from 2024, the chick on North Roosevelt, the Lower Cullen-SW chick from 2024, the larger of two 2024 chicks from Cross-Arrowhead, this year’s Roy-South chick, the alpha chick from Ossie-Island, and the Little Pelican-South chick from 2023. The remaining 51 Minnesota chicks were below average size (56 grams/day). Hence, 75% of 2023 and 2024 chicks from Crow Wing and Cass counties stand very little chance of reaching adulthood. Even if they beat the odds and do so, they are unlikely to produce a single chick.
In light of these patterns, I am getting a sinking feeling about Minnesota loons. Of course, the worrisome data come from only the past two years and one part of the state. Perhaps these years were atypical, and Minnesota loons will bounce back this summer. I hope so!
The featured photo shows the female from Rush Lake-Channel in 2024 as she alertly approaches an intruder. Photo by Isaac Pavalon.