I have just finished a rough draft of a manuscript describing the population decline of loons in my study area. That effort forced me to count and calculate, estimate and project. I like math, so the work was not unpleasant. I thought that I would pause and share some of the numbers I produced. First, let me share a complex graph!
This graph shows two sets of values over an 18-year period. The blue line shows the proportion of all adults in the population that are floaters — that is, adults that lack a territory. This group includes both adults too young to have settled and old adults that have been evicted from a territory. The grey bars show the number of chicks banded in the study area three to four years before, adjusted for the number of territories covered those years. In short, the bars show how good the breeding years were in the study area three to four years before. I plotted things this way because three- and four-year-old loons make up the bulk of the new floaters each year, so by comparing the grey bar to the blue line, you can see what impact the breeding success three to four years ago had on the floating population. We expect that lots of chicks produced three to four years before will produce a surge in floater numbers. So the blue line should track the grey bars.
Okay, now let’s see what we can learn from the graph. Notice, first, that the blue line descends overall. This means that the proportion of the adult population made up of floaters declined during the past 18 years. Notice, next, that the grey bars are roughly the same height throughout the graph, which means that chick production was relatively stable over this interval. Already you should be thinking that something odd is going on. The blue line is NOT tracking the grey bars as closely as we expected. The bars and line DO track each other pretty closely at many points, however. For example, the stable chick production from 2002 to 2006 is paralleled by the stable floater population. 2008 went as we expected: good chick production 3-4 years before meant a positive bounce in the floater population. 2013 saw a loss of floaters, as we expected from low chick numbers 3-4 years before.
Now, focus on the last four years. Floater number had fallen to less than 40% of the adult population by 2015. Huge chick production from 2012 and 2013 should have “rescued” floater numbers in 2016 (caused them to spike upwards), but we only see a small bump up in floater numbers that year. And 2017 is worse as, despite big crops of 3- and 4-year-olds, there is a sharp downward turn in floater numbers! Likewise, 2019 should have seen an uptick, but floater numbers actually declined to an all-time low.
I know it is messy to look at this plot. If you find my fine-scaled analysis too picky, forget about the trees I have been discussing and look at the forest. Chick production during the past 18 years has been okay, yet consistently we see fewer floaters than we expect. This is the main puzzling finding of the paper I have written.
It is the mysterious loss of floaters, in fact, that seems to imperil the population of Oneida County loons. We estimate that there are about half as many floaters now as in the late 1990s and that the entire adult population has fallen by 22%. We do not know what is happening to the missing floaters — whether they are dying in their first fall, on migration, or perhaps during winter. But those floaters, which are future breeders, will have to stage a comeback to get the population back on track.