By Anita Tendler, Cornell Class of 2019
Have you ever chosen a hotel because it got great reviews on Yelp, only to find that the room was terrible and the service was worse? You probably felt tricked out of your hard-earned money. You, my friend, were the victim of misleading information. But, did you know that birds, and other animals, also have to rely on cues and information gleaned from the environment before making important decisions about where to spend their time? They don’t simply choose a place to nest arbitrarily…they rely on signals from the environment, such as cavity size, food availability, and abundance of predators nearby.
Though it may seem like birds should be skilled at reading the landscape they depend upon, the cues in a human-modified environment can be deceptive. In a natural habitat, cues are usually positively associated with the quality of the habitat. But, in a human-modified habitat, such as urban areas, indicators of habitat quality can become misleading (e.g., a bird feeder provides ample food, but nonnative vegetation reduces insect availability). When the habitat is giving off good vibes, but there is some underlying reason why birds won’t succeed there, we call this an “ecological trap.” If an ecological trap is pervasive enough that it causes a negative population growth rate, ecologists refer to this area as an attractive sink. The opposite of a sink population is a source population, or one in which the population is productive, and stable or growing. Everyone wants their property, whatever its size, to be a source habitat, but the truth is that it’s very hard to tell, and even harder to prove experimentally.
Bigger Isn’t Always Better
In a recent study from the UK, researchers were able to experimentally show a preference by Great Tits (Parus major) for a nest box design that was not in their best interest. As we know, nest boxes have numerous designs, accommodating different species depending on size and shape of the box and its entrance hole. Historically, a larger natural cavity would have been found only in a larger tree with a big canopy, and thus the number of arthropods available for feeding the future nestlings would have been high. However, a presumably attractive, larger nest box may have led birds in the UK study to breed in a subpar environment.
When Great Tits were given a choice among three progressively larger nest boxes, they actively occupied the largest size, regardless of other visual cues within the surrounding environment. In this species, larger cavities are linked to larger clutches, suggesting that the birds are being stimulated to increase their clutch size in anticipation of a rich supply of insects. In urbanized environments, naturally insect-rich vegetation is often replaced with its decorative, exotic counterparts that cannot support caterpillars or other insects that birds commonly use to feed their nestlings. Because the birds were attracted to a misleading cue in the environment (i.e., larger nest box size), and then laid more eggs in them than the habitat could support, they were subject to an ecological trap.
Avoiding The Trap
So how can responsible nest box landlords avoid the ecological trap phenomenon? Examples of potential ecological traps include:
- Using inadequate nest boxes.
- Not using predator guards in areas where predators are concentrated (e.g., outdoor cats in your neighborhood).
- Letting your grass grow long enough to attract ground-nesting birds, and then mowing it mid-nesting season.
- Spraying insecticides in your yard, thus eliminating the food base for birds and potentially poisoning nestlings and adults.
Your Data Hold the Answers
As mentioned, it’s very hard to tell if your property or nest box trail is acting as an ecological sink, but here is a quick informal way to think about it. If you are a NestWatcher, download your “species summaries” report from NestWatch.org, leaving “year” blank to retrieve all years. Look at the “nesting success rate” column and scan through your list of species. Are there any species for which the nesting success rate is particularly low? If so, download the “breeding data” spreadsheet for the year(s) in question and review it. Was there one bad-luck event that is driving your low success rate, or do most nests seem to have less than a 50/50 chance of survival? Is there anything you can do to improve the situation (e.g., add predator guards; avoid feeding birds if it is attracting predators such as raccoons or House Sparrows; change nest box design to avoid temperature fluctuations; etc.)? Demonstrating an ecological trap is data-intensive and complex (even for professional scientists), but exploring your NestWatch data to find any potential problems is a great first step.