Almost four years ago, I wrote a blog post about the importance of Florida’s Gulf Coast as a wintering area for loons breeding in the Upper Midwest. I pointed out that 75% of loons breeding in Wisconsin and Minnesota winter along the Gulf Coast of Florida from Pensacola to Fort Myers. And I suggested that conditions in Florida were likely to have substantial impact on the loons we see on our lakes in the north.
My thoughts have returned to Florida of late. Why? Because recent findings suggest that the answer to our greatest riddle might be found there.
Followers of the blog may recall that the single most worrisome pattern related to the Upper Midwest loon decline is the mysterious disappearance of juvenile loons before they reach adulthood. In Wisconsin, the survival rate of juveniles to adulthood plummeted from above 50% to less than 20% between 1993 and 2016, as the graph from our statistical test shows.
The problem has gotten worse since 2016. Only 13 of 209 chicks (6.2%) banded in Wisconsin between 2017 and 2020 have returned as adults. That is a ghastly statistic.
What might be the cause of this massive die-off of young loons? We have good data from the breeding grounds. If juvenile mortality were high during this interval, we would have detected it. Death during migration is another possibility, of course. We do not have good data from that period. But it seems implausible that the varied array of aquatic habitats used by young loons along the migration route have suddenly become a death trap for them.
So it was with increased urgency that I turned my gaze to Florida two weeks ago with our juvenile return data in hand. My hope was to take a second, more thorough look at the likelihood of spotting loons two to four years after we marked them as chicks. I had taken a preliminary glance at this pattern 12 years ago. This time I had: 1) twice as much data, and 2) measurements of physical and biological patterns from ocean water along the Florida Gulf Coast that might help explain the decline. Among physical and biological ocean attributes that I could examine this time were water clarity, temperature, pH, salinity, and concentrations of Chlorophyll A, dissolved oxygen, nitrogen, and phosphorous.
What do the data show? After accounting for observation intensity 2-4 years after banding (which has varied during the study) and location of lake where the chick was banded (because juveniles from central lakes are spotted more often), three variables strongly predict the probability of resighting of a color-marked juvenile loon. In order of decreasing importance, they are:
- Year — The likelihood of spotting a banded chick as an adult has decreased by an average of 8% from one year’s “crop” to the next. This is the alarming pattern that I seek to understand.
- Body condition at banding — Chicks that are heavy for their age when banded are much more likely to be resighted as adults.
- Chlorophyll A level in Tampa Bay in December of the first year — Chicks are much less likely to return if Chlorophyll A levels in Tampa Bay were high in December of their first year (see graph below).
Wait! Does this last finding make sense? First, it is vital to understand that Chlorophyll A is a measure of aquatic phytoplankton — the microscopic algae that can make water appear green to us. A very high concentration of Chlorophyll A can indicate an “algal bloom”, which reduces water clarity and can lead to loss of oxygen and release of toxins. Second, it is important to understand that the negative impact of Chlorophyll A on loons occurs only in December of a loon’s first winter. Return rate of juveniles is not associated with Chlorophyll A levels the month before (November) nor the month after (January). Third, we must be familiar the the migration schedule of juveniles. Juveniles reared in Wisconsin and Minnesota typically reach their Florida winter quarters in late November. Stitching all of this together, the negative impact of Chlorophyll A in December on return rate to Wisconsin suggests that many young loons perish in their very first month on the wintering grounds in Florida if algal levels are high at that time. We would expect these neophytes to be at risk during this period, because they must suddenly find new aquatic prey in wholly unfamiliar habitat. High algal levels add another layer of difficulty to foraging, likely reducing the abundance of prey as well as a loon’s ability to find them.
Of course, this is a hugely important finding. This is the first evidence — to my knowledge — where an occurrence in winter affects a demographic pattern detected on the breeding ground. The effect, moreover, is strong. And the pattern is evident despite the fact that Chlorophyll A data represent only one small part of the winter range (Tampa) that is as far as 350 miles from where some Wisconsin loons spend the winter. We can reasonably surmise that the pattern would be far stronger if we had complete data from all along Florida’s Gulf Coast and could match locations of wintering loons up with Chlorophyll A data from their exact location.
Unfortunately, this striking finding cannot explain the steep decline in juvenile survival rate over the past quarter century. Why not? First, Chlorophyll A levels have remained roughly stable in Florida — maybe even falling slightly — across the two decades or so when juvenile survival has been getting steadily worse. Second, the statistical decline in annual return rate among young adults remains strong even after we have accounted for the effect of Chlorophyll A.
In short, many of our first-year loons do appear to die in Florida, shortly after arriving there. But we are still utterly in the dark with regard to the continuing yearly decline in young adult survival. Young adult loons are future breeders — essential to the stability of the Upper Midwest loon population. It is urgent to learn what is killing them. I will keep looking.
The featured map is from Google Earth. It shows lines drawn between where a loon was banded in summer and where it was recovered or spotted during winter months. Green end points indicate Wisconsin breeders and red indicate Minnesota birds.