In the wake of my litany of negative findings related to loon breeding success, one burning question presents itself. What is the cause? I am sure that this obvious question occurred to readers of my blog more than once. I feel now as if I have been dodging it. Although we are still gathering information, I will tell you what we currently know. I think I owe you that.
First of all, we must remember that this is a longitudinal decline. By this I mean that the declines are not based data from one, two, or even five years. Rather, these declines have been occurring since the beginning of my study in 1993. Look, for example at the increase in the number of singleton (1-chick) broods over the course of the study.
It is a noisy pattern, but the pattern marches steadily upwards. There have been good years and bad years for 2-chick broods, but the overall pattern is clear. Progressively fewer pairs have been able to rear 2-chick broods as the years have passed. In the mid-1990s, about half of all pairs had 1- and 2-chick. Now almost all broods are singletons. Even more striking is the decline in returning chicks (below). In the mid-90s, we could expect about half of all chicks that we banded to come back to the study area as adults a few years later. Now, only about 1/6 of all banded chicks are ever seen again as adults.
The longitudinal nature of the decline makes it difficult to dismiss the results as a blip resulting from a bad winter here or a severe outbreak of black flies there.
Secondly, the patterns connect logically in a worrisome way. There are not only fewer chicks these days but lighter chicks, leading to the obvious conclusion that low chick mass is a reflection of harder conditions for chicks that leads to lower survival. Furthermore, fewer chicks surviving to fledge should result in fewer young returning as adults, and indeed there are now far fewer young adults in the population than was the case two decades ago. Finally, the smaller young adult population should mean less pressure on territorial breeders to defend their territories. Consistent with this expectation, we now see significantly fewer intruders into territories than we did years ago, and, as I pointed out recently, territorial breeders are evicted much less frequently than they used to be. In short, the entire set of measurements paint a stark and coherent picture.
Third, the lower fledging rate of chicks together with their decreased mass over time points rather strongly to food as the likely cause. Starvation of chicks, if it is occurring, would by itself explain all of the “downstream” patterns (i.e. fewer returning adults, lower intrusion and eviction rates in territories) we have detected.
Pulling threads together, we have a consistent picture of long-term decline in loon breeding success across a broad swath of lakes, probably owing to a fall in food levels. This summary leads to the most vexing question: Why should food levels fall? To be more specific, what factor or factors might result in populations of small fishes falling gradually over the course of a 27-year study? The answer, it is clear, has to be some environmental factor that acts like a slow, incessant march, not a lightning strike.
What environmental factor influences fish communities broadly but gradually? Could a recent increase in recreational fishing be the cause, which has led to fewer large fish capable of producing the small ones on which loons depend? Possibly. If overfishing is to blame, it must be true that the problem has gotten especially severe in the last few decades and was not a great problem in the 1980s and 1990s. Yet small fish (“panfish”) populations apparently declined most during the 1960s through 1990s and have somewhat rebounded during the past 20 years. Could the problem be instead that anglers are now more apt to practice “catch and release” of large fishes, whereas they caught and kept their fish in the old days? If so, these released large predatory fishes might be competing with loons for small fishes and driving small fish populations down so much that loon chicks suffer. Again, the “catch and release” explanation would only work if this practice has intensified across all kinds of lakes in Oneida County over the past quarter century. Yet catch and release increased most sharply in the 1980s and had hit a plateau by the time I began my loon study in 1993. So, upon quick inspection, neither obvious explanation for a reduced small fish population over the past quarter century passes muster.
Our search will continue until we learn what factor — probably related to a decrease in small fish populations — explains loons’ recent reproductive slide. I feel confident that we will ultimately learn what is causing our loons to struggle. We must all hope that the cause is reversible.