Behavioral ecologists are human. Although we try hard to view biological events critically – to look for confounded factors, biased samples, untested assumptions – we miss a lot. So it is when we look at the nesting behavior of birds.
Ecologists around the world have made a simple, elegant discovery about how birds respond to nest failure. Once they have settled on a breeding territory and reared young successfully once, breeding birds get conservative. They reuse the same nesting site again and again. On the other hand, if they try to nest in one location and the nest fails, they shift to a new location. We call this simple strategy the “win-stay, lose switch” rule.
Let’s think a bit more about the win-stay, lose-switch (”WSLS”) nesting rule. What is it about a nest’s location that links it so critically to success or failure? The main answer is predation. Most predators are long-lived mammals (raccoons, squirrels, foxes), reptiles (mainly snakes), or birds (crows, jays, hawks, or gulls) that travel within fixed small ranges looking for food throughout their lives. If a bird’s first nesting attempt is not found and gobbled up by a vertebrate predator, a second one at the same site will likely escape predation as well. On the other hand, a raccoon, blue jay, or rat snake that found and ate your eggs at one site in mid-May will likely do so again in June, if you reuse the nest site. By moving away from the site of a failed nest, you might find a new site that does not fall within the predator’s home range – or is better hidden or otherwise inaccessible to the animal – and the prospect of successful breeding is renewed. That is the simple beauty of the WSLS rule.
While predation is the most obvious and important reason for using the WSLS rule, there are other reasons why moving a nest might be beneficial following failure. A species like the cliff swallow, whose nests become infested with swallow bugs – blood-sucking insects that attack and kill nestlings – should (and does) respond to infestations by moving the nest. The key point: vertebrate predators and tenacious parasites are persistent and location-specific nesting threats. To place a new nest at the same spot shortly after losing a first one to such a threat is to court disaster.
The WSLS rule has been confirmed as a logical and successful nesting strategy by ecologists around the world. Numerous theoretical papers have been written about it (including one by me). The rule is so widespread that scientists often think of it as “the way” that birds respond to nest failure. But closer inspection of nest failures shows that we have oversimplified the picture.
Nest failure can also occur owing to threats that are fleeting and non-location-specific. Fleeting, non‑location-based threats are those that occur at a brief moment in time, are not likely to recur soon, and are no more likely at one location than another. Examples are “freak” weather events, like early spring snowstorms or heat waves. Fleeting threats of this kind usually end quickly – so quickly that they abate before the nesting pair can even lay a new clutch of eggs. Fleeting threats make very different demands on nesting birds than do persistent threats and should be countered with a different strategy. Why? Think of a pair of loons whose nest has been flooded by a 6-inch rainstorm. If the pair were to use the WSLS rule to respond to this fleeting threat, they might move their nest away from a traditional nesting location (say, a favorite island) that they had used to produce many fledglings in years past and choose a new, untested nesting location. In so doing, the pair would discard years of accumulated knowledge about their territory and dim their breeding prospects.
What is the proper response to a fleeting threat of nest failure? Nothing! That is, the logical and adaptive response (i.e. that which maximizes the chance of breeding success) is to ignore fleeting causes of nest failure and consider the next nesting attempt a “do over”. Do birds have the capacity to respond differently to different causes of nesting failures? It is too soon for a general statement, but loons can do so. If a predator gets their eggs, loons use the WSLS rule (i.e. they move the nest). If a fleeting threat causes them to abandon their eggs, loons ignore that nesting attempt, often placing a new set of eggs right back in the same nest they started and abandoned a week or so before.
Followers of the blog will know that loons face a fleeting (but very severe) threat to nesting that most other birds do not: black flies. Perhaps their vulnerability to black flies — which typically only cause nest failure for a week or so in late May — has caused loons to evolve a more sophisticated response to nesting setbacks than other birds. I have begun combing through the literature on avian renesting behavior in order to determine if, indeed, the nuanced renesting behavior of loons is unique. Since I have just started, we can bask for the moment in the possibility that loons are a cut above the rest.