I have had a lot to say about male loons and their experience. Indeed, the fact that the male decides where to place the nest means that he develops a tight bond to his familiar territory and fights hard — harder than his mate — to keep it.
But females too play a vital role in breeding success (like the pictured female photographed by Linda Grenzer on Bear Lake). How might female experience affect the outcome of a nesting attempt? Now that I have begun a detailed analysis of causes of breeding success and failure, I have started to ferret out the difference that female experience makes. I am only halfway done, but I can already see that the number of years a female has spent on her territory strongly affects the date her chicks hatch. As the graph below shows, females that have just arrived on a new territory — because they have evicted the previous female owner or replaced a dead one — have an average hatching
date of 22 June. In contrast, 4-year veterans on territories hatch their eggs, on average, 5 days earlier — June 17th. Now this might not sound like much of a difference in hatching date. But when you are tasked with stuffing your voracious chicks with fish, watching them grow rapidly to adult size, and hoping they get proficient enough at foraging, flying, and avoiding trouble to eke out a successful migratory flight to Florida, you take every extra day you can get!
You might ask: “Does this delay occur because of female inexperience with breeding in general or does it come about because of lack of experience on a specific territory?” The delay seems to be associated with lack of familiarity with a specific territory, because females that pick up stakes and move to new territories show delays in hatching date just like the ones they suffered on their first territories.
The cause of the improvement in nesting schedule with experience is likely to be energetic. That is, a female that knows how and where to find food on a lake is able to recover from spring migration quickly and begin the lengthy and arduous process of raising chicks. A female that is still learning where to find food on her new lake spends extra time before she reaches a suitable body condition to commence breeding.
Don’t male loons have just as big a problem restoring their body condition and thus readying themselves for a nesting attempt? Perhaps. But males experience minimal breeding costs until they begin joint incubation duties with females. Apparently males’ energetic deficit from migration does not hinder the breeding schedule.
You probably noticed that the graph depicts a curve, not a line. Older age-classes of females — those that have spent 5 or more years on a territory — actually begin to nest later than females that have spent 4 or fewer years there. What on Earth could explain this peculiar pattern?
Since it makes little sense that a female’s experience on a territory could begin to work against her after several years, we must look beyond experience for an explanation. The later hatching dates of more experienced females probably arise from reproductive senescence — a decline in reproductive performance that occurs with advancing age. Senescence is well-known in mammals and also many birds. We should not be surprised to see such a pattern in loons.
It is exciting to discover and ponder the reproductive quirks of female loons. Like many of our findings, this one only became visible because we studied thousands of nests, by hundreds of marked loons, across decades of their breeding lives. That, of course, is our bread and butter.