mercury

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I have pointed out many times how science proceeds not in simple linear fashion but haphazardly. A finding may appear rock solid but subsequent findings modify our understanding, even forcing us to discard earlier conclusions in some cases. Viewed from space — and over large stretches of time — we might seem to advance steadily in understanding a phenomenon. From ground level, progress in comprehending a topic is herky-jerky; we make clear progress for awhile, then reach a dead end, back up, and find a new path forward.

So it has been with loon conservation. Thirty years ago, we feared that methylmercury was a major health hazard for breeding loons and their offspring. At the time, our fears seemed well grounded. Now, having spent millions of dollars measuring mercury levels in loons and looking for its impacts, we must reassess. We now see that, at worst, mercury has the potential to harm loons in the eastern 1/3 of the breeding range — and even there only in acidic lakes. Consequently, where loon populations appear to be declining, we can cross mercury off the list of threats and turn our attention to other potential causes. By the way, it was no mean feat to cross mercury off the list of dangers to loons. Thoughtful, rigorous work by dozens of biologists across North America has made this conclusion possible.

Our study of a much narrower topic — the loon yodel — also seems to be moving forward by fits and starts. (Linda’s gorgeous photo above shows a male yodelling in typical crouched position.) A really cool paper by Jay Mager — a collaborator with the Loon Project for several years — showed fifteen years ago that the frequency of the yodel was strongly dependent upon body mass. Large males, Jay found, had low-pitched yodels. This was fascinating to us, because it meant that male loons were revealing their size to territorial competitors! Now it is well and good to advertise your size if you are a big loon. In that case, the fact that you are yodeling tells opponents that you are motivated to defend your territory, and the low frequency of your yodel informs them that you can back up that determination with beef! But consider the mixed message that a small male sends by yodeling: “I am small but feisty”. Hmmmm. Not such a deterrent to a territorial challenger, it would seem. We are still coming to grips with this so-called “honest-signalling” of body size and trying to learn how small yodelers might benefit from yodeling.

Among males whose ages are known exactly (blue) and males whose ages are estimated (red), older birds have higher-pitched yodels.

But wait! Lately we have had cause to wonder whether body size really does have a strong impact on yodel pitch. Now that a good many males that we marked as chicks have settled to breed, we know the ages of many of our breeding males precisely. Brian Hoover, a postdoc here at Chapman, took these new age data — data unavailable to Jay Mager — and looked to see whether age is correlated with yodel frequency. As you can see from the figure, age is strongly correlated with yodel frequency: old males have high-pitched yodels. Reassuringly, the better our age information is, the stronger the correlation. That is, the blue points show a tighter age/frequency relationship than the red points.

Where does this leave us? I just explained that small males have high-pitched yodels. Now I am telling you that old males have high-pitched yodels. Can it be both? It is possible that both mass and age influence yodel frequency and could exert additive effects. (Imagine the extreme falsetto of a male that was both old and small!) However, Brian examined mass in his more complete analysis and found no evidence that it affects yodel frequency after all. In other words, we might have found a mass/frequency pattern back in 2006 simply because we did not have age data that would have shown us what was really an age/frequency pattern. Statisticians are quite familiar with this frustrating phenomenon; conclusions can change dramatically when a variable that was left out of a previous analysis (age, in this case) is added.

The finding that it might be age, not mass, that affects yodel pitch would be more palatable if high-pitched yodels by old males made more sense than high-pitched yodels by small males. But, intuitively, the new finding makes less sense! This is the delightful pickle we now find ourselves in. “Delightful” because the age pattern is robust and clear, as the graph shows. A “pickle” because we have no idea, at present, why or how old males have high-pitched yodels.

Well…so it goes in science!

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In a recent post, I described how the popularity of loons and the willingness of many folks to pontificate about them without solid data or thoughtful scientific analysis makes loons unique. I tried to point out that this practice can be harmful, if we make misleading statements about loon conservation at a time when some loon populations are declining.

Now, let me give an example. Millions of state and federal dollars have been spent in recent decades in attempting to measure the effect of methylmercury (the toxic form of mercury) on wild animals, especially birds. Without a doubt, more funds have been spent analyzing mercury impacts on loons than on any other aspect of loon biology. What have we learned from this body of work? The major take-homes are that: 1) mercury certainly can affect behavior and survival of adult loons and chicks if it occurs in a high enough concentration in their tissues, 2) high mercury levels tend to occur mainly in loons living on small, acidic lakes, which have negative effects on loons that have nothing to do with mercury, and 3) harmful concentrations of mercury do not occur in most geographic areas within the breeding range. In short, despite an abundance of research and the expenditure of millions of research dollars across three decades, we have no direct evidence that mercury negatively impacts loon populations. In fact, the consensus among loon scientists is that mercury probably has little or no negative impact on most populations.

The situation is dramatically different with lead. Careful analysis of loon carcasses in New England has shown us that lead is quite deadly and affects a great many loons. (The featured photo above shows a deadly lead sinker in the stomach of a loon that died a few days ago in Wisconsin. Photo by Wild Instincts.) In a 2017 study, Grade et al. determined that a whopping 48.6% of the loons they examined had been killed by lead sinkers and lures. The authors estimated that this mortality rate had reduced the New Hampshire loon population by 43%.

The contrast between mercury and lead is stark. Mercury might affect loon survival and breeding success slightly in a few isolated populations. Lead has been shown to cause half of all loon deaths in one state and to make an enormous dent in the loon population of that state.

The contrast between these two toxins goes further. Mercury exposure is pushed to high levels mainly through burning of fossil fuels like coal and oil, which contain mercury. So reducing loons’ exposure to mercury requires a long-term effort to reduce burning of fossil fuels over a large geographic area. In contrast, loons are exposed to lead through our use of lead sinkers and fishing lures. The remedy for lead-related loon deaths is simply to implement use of lead-free fishing tackle in lakes where loons breed. (Steel, tin, and tungsten are common alternatives). Indeed, lead bans are now in place across New England.

Here is the problem. Despite the lack of evidence that mercury affects loons in nature, mercury has become the “go-to” environmental toxin mentioned by many loon researchers. Mercury has become such a prevalent scapegoat in grants, reports, and even published papers that many of us are not keeping its limited impact on loons in proper perspective. A clear-headed, candid, objective review by a loon researcher with a strong background in mercury toxicity would do wonders for loon conservation. At a time when studies have just reported long-term declines in two loon populations, those of us trying to conserve loons would do well to focus our attention on the real enemy.