James and team tap into the behavioural dynamics of red-breasted nuthatches and their use of conifer resin (sap). In their paper “Behavioural co-option of plant secondary compounds by a cavity-nesting bird is an adaptation against competition and predation”, James studies the how and why of nuthatch nesting behaviour and their apparent love for sticky, smelly conifer resin! On top of a delightful explanation on the “In-s and Out-s” of nest predation by squirrels (pun intended), James reminds us all to not give up on projects, even if it takes 9 years or more to get them done!
🐦⬛ About the paper
Plants and other organisms produce many unique chemical compounds that can be useful for animals. Humans are great at taking advantage of such substances (e.g. rubber, medicine), but many other types of animals also co-opt plant secondary compounds for their own benefit. For example, some birds and mammals purposefully wipe their bodies with certain plants (or insects) to apply chemicals to their fur and feathers in a behavior known as ‘anointing’. Other animals seek out and use certain plants in building their nests that have anti-microbial or anti-parasitic properties. This sort of behavioral co-option of compounds from un species is probably more common than is currently recognized and may have important ramifications for fitness.
We studied the evolution and function of the behavioral co-option of conifer resin (aka sap) by nesting red-breasted nuthatches. These small cavity-nesting songbirds will collect sap from live conifer trees (e.g. ponderosa pine, Douglas fir) and fly back to their nest hole with it. They smear the sap all around their nest entrances, coating it in a sticky, smelly mess. If you’ve ever got sap on your clothes, skin, or hair, you probably understand that it’s nasty stuff. It is sticky and can be very difficult to remove. In fact, red-breasted nuthatches have been observed dead stuck to the sap outside their own nest! Why do these birds do this?
One possibility is that the sap helps protect eggs and nestlings from nest predators or other birds that would steal the nuthatch’s nest. Predators like squirrels would have to get their fur coated in sap if they were to enter a sap covered nest. Small birds like house wrens might even get stuck in the sap and be unable to fly away. This would mean that the application of sap to nest entrances evolved as an adaptation to ameliorate predation or competition. Alternatively, applying sap to nest entrances may just be an evolutionary relic that has little current function. Many other species of nuthatch around the world use mud to make their nest entrances narrower. So sap application might just be random modification of mud plastering that occurred in a recent ancestor of red-breasted nuthatches. These ideas had been long discussed, but tests have been lacking.
We used phylogenetic analysis and two field experiments to test these alternative hypotheses to show that sap use in red-breasted nuthatches likely functions to mitigate nest predation and competition. First, we found that it was relatively unlikely that any recent ancestor of red-breasted nuthatches exhibited mud plastering behaviors. Second, our experiments showed that nest predators (e.g. red squirrels) and nest site competitors (e.g. house wrens) had a strong preference for nestboxes without sap applied to the entrance. Our study provides support for an adaptive function of sap use in red-breasted nuthatches and highlights another fascinating example of the behavioral co-option of plant secondary compounds by animal species.
🐿️ About the research
Our field site was just shy of 8000ft in elevation in central Arizona, USA, right at the edge of the Mogollon Rim – a massive cliff marking the beginning of the Colorado Plateau. The forest there consists of a mixture of ponderosa pine and Gambel’s oak with a lush collection of firs, maples, and aspen becoming dominant at the bottom of winding snowmelt drainages. Tom Martin ran a long-term research project there from 1988-2017 where hundreds of aspiring ecologists got their start. The field crew camped out for the breeding season from April to July and used large canvas tents as working spaces and a kitchen/dining room that served as the heart of the community. The field crew captured and searched for the nests of all the birds species breeding at our site and graduate students (like me) ran around to collect data for their dissertations and side projects.
Red-breasted nuthatch tend to nest in cavities very high in trees making them hard to access. So instead of studying sap use on real nuthatch nests, we took advantage of the fact that other birds will nest in artificial nest boxes that we could place low to the ground. Red squirrels, which are a key predator of nuthatches also spend time down low and will enter nest boxes to eat the eggs and nestlings. So our plan was to set up a bunch of nest boxes in pairs: one box would have sap smeared around the outside of the entrance hole and the other would have no sap. The boxes were all the same and they were right next to each other, so any preferences we found in potential nest predators or nest site competitors were likely caused only by the sap and not something else. If nest site competitors like house wrens avoided nesting in the boxes with sap, then we would conclude that the sap would help deter wrens at real nuthatch nests as well. In another group of boxes, we also put a mixture of cat food and peanut butter into each box to tempt hungry squirrels. If squirrels ate the bait in the box without sap first, or never ate the bait in the box with sap, we would conclude that the sap was deterring nest predators like squirrels.
We set up the boxes and came back each day to check all the boxes to see if nest material had been brought by breeding birds or if any of the bait had been chewed. Once checking started, it became clear right away that house wrens and squirrels were avoiding the sap covered boxes and preferentially entered the control nest boxes without sap. Sometimes only a portion of the bait would be removed, but you could see the teeth marks in the bait balls clearly. Red squirrels will also chew the wood around the entrance hole to make it easier for them to fit in. Nest entrances on boxes without sap were also chewed before the adjacent box with sap had been touched. Once we had all our data in hand, our analyses confirmed our impression from the field: sap seemed to deter potential nest site competitors and nest predators!
The biggest challenge we faced in our research (besides a bear destroying a box or two…) was other research getting in the way of finishing the paper. Cameron Ghalambor first conducted this experiment in 1998 (when I was about to enter the eighth grade). Sixteen years later, in 2014, Riccardo Ton and I got excited about the idea and replicated it at the same field site. The data continued to get passed over by other projects we were all working on for another 9 years before we finally got together our results and wrote the paper. I don’t know that we won any races for this one, but slow and steady got us to the finish line eventually.
This was a fun experiment to conduct because it was so simple. However, one limitation of our design was that our boxes were not real nuthatch nests. It would be interesting to study active nuthatch nests directly to see if the amount of sap around the nest is to predation of the eggs or nestlings. Moreover, our study took place in central Arizona, but nuthatches use sap on their nests across their range. Could it be that nuthatches don’t use as much sap in places where the risk of nest predation and competition are reduced?
Sap use in nuthatches also highlights an interesting and widespread phenomenon where species use chemical compounds produced by other species for their own gain. Humans are great at this, but an increasingly large number of other species have been observed engaging in this behavior. Trying to fully understand the diversity and evolution of this sort of behavior across a wide range of animal species would be fascinating!
👋 About the author
I (James) am now an assistant professor at The Ohio State University, but it took me a long time to figure out I wanted a job where I could conduct ecological research. I have always loved to spend time outdoors (e.g. camp, fish, hunt) and had an excellent biology teacher (Thanks Mr. Nik!) in high school. When I started college I really had no idea what I wanted to do, but started out as a biology major because that was the only thing I could think of. I quickly discovered that I did not want to be a medical doctor, and more slowly figured out that I liked classes about wildlife. By the time I graduated, I had a path in front of me, but I still wasn’t sure if it was for me. So instead of pursuing grad school or another job working in ecology, I made a hard break and spent 4 years as a Marine Officer. By the time I reentered the civilian world, I was confident I wanted to be in the field studying wildlife and started working toward that goal. I love my job because I get to think creatively about my science, go outside into nature to collect data, and see the impact my research and teaching has for conservation and students. I also get to work with a gang of talented, hard-working, and generally hilarious people who are passionate about science and what they study to no end!
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