Costly Winters

on

In recent blog posts, I made the point that the course of a young loon’s life is more affected by its early experiences in Wisconsin or Minnesota than by conditions during its first winter in Florida. Winter happenings along Florida’s Gulf Coast do affect youngsters, but the amount of food they receive in their first several weeks of life makes an indelible imprint on their well-being.

One might have expected established breeders to show even greater immunity to winter conditions. Once an adult has claimed a territory, reared chicks to fledging, and survived several trips from the Midwest to Florida and back, what challenge is left that can threaten it? Can’t adult loons begin to “coast” a bit after these achievements? And if so, might the four months spent along Florida’s shoreline simply be a period of rest and recovery from the stresses of territoriality?

To some degree, established adults can coast. Having settled on a breeding lake at the age of 6 years or so, they have surmounted life’s greatest obstacle. Since senescence does not take hold until they are in their mid teens or 20s and since annual survival of loons in their prime is 94%, newly-settled breeders stand a good chance of holding their territories for a decade or more. But long-term ownership is never certain. The simple act of raising chicks exposes an adult loon to territorial challenges, because nonbreeders bent on claiming a territory use the presence of chicks there as a badge indicating its quality. So proud parents in one year pay the steep cost of increased territorial defense — and the risk of eviction — the next.

Recently we have learned an incredible thing. Ocean conditions that loons face during winter — whether they are 8, 11, or 15 years old — can reduce their body condition subtly such that they are prone to eviction from their territories several months later, when they return to the breeding grounds. Specifically, loons that have spent the winter in cold, dilute ocean water are much more likely to get booted from their summer territories than those that spent the off-season in warm, salty water. Here is what those patterns look like.

Fig. 1. Territory holders that experienced cold ocean water during the previous winter are more likely to lose their territory in a battle than those that spent the winter in warm water.
Figure 2. Breeders that encountered ocean water of low salinity were more likely to be evicted from their territories than breeders that had wintered in salty water.

An obvious question is this. Why is warm salty water beneficial to loons? Sadly, the answer is not obvious, although loss of salinity can be linked to increased runoff from rivers into the Gulf of Mexico, which reduces water clarity and can spawn phytoplankton blooms. (Both low clarity and increased phytoplankton are harmful to loons.) The negative impact of cold ocean water is also hard to interpret, but cold water forces loons (and other warm-blooded aquatic creatures) to expend energy just to maintain a high and stable body temperature. Perhaps the energetic hit that loons face in keeping warm in a cold ocean puts them at a long-term disadvantage.

We are not the first to discover that environmental factors in one season can impact animals in another. In fact, such “carry-over effects” are now known in several species of songbirds. Understanding carry-over effects is crucial to conservation, because they reveal the interconnectedness of the seasons. If the quality of a bird’s winter habitat limits when it can migrate in the spring, how successful it is at finding a territory on the breeding grounds, and the number of offspring it raises, then clearly we must take a holistic view to understand avian conservation.

From a territorial loon’s standpoint, poor ocean conditions in winter pose yet another challenge. It is bad enough that raising chicks puts a great big target on your back. We now realize that loons that encounter cold, dilute ocean water during a winter after rearing chicks will face a double whammy in holding their territory the following spring.

Our discovery of carry-over effects in loons might help us understand how the species’ odd system of territory eviction evolved in the first place. Perhaps natural and inevitable fluctuations in the quality of the winter habitat guarantee that some adult breeders will be vulnerable to takeover each year. If so, winter-weakening sets the stage for the evolution of territorial eviction as an effective behavioral strategy for claiming a territory.

We eagerly await the 2025 field season and have a very strong team in both states. However, field costs have mushroomed unexpectedly by a whopping $28,000. As it stands, we are $1,800 short of our goal of raising $20,000 to earn an additional $20,000 in matching funds from the Walter Alexander Foundation. If you are able, please consider helping us cross this threshold so that we can defray most of our field costs. Thanks so much to those who have already given!

The beautiful photo of the male is one of Linda Grenzer’s. It shows the Deer Lake male (B/S,P/R) becoming airborne during a takeoff run. Love that pink band!