October 26, 2022
It was one of the more curious things I’ve observed in the ponderosa pine forest. At first the interaction seemed peaceful. A very active flock of pygmy nuthatches were flitting back and forth between a large orange ponderosa pine snag and a live ponderosa nearby. Lower on the snag, a male hairy woodpecker was foraging. The musical pedeep calls of the pygmy nuthatches alternated with the deep, rhythmic tapping of the woodpecker.
But then, the calls of the pygmy nuthatches intensified. Four or five of them were suddenly swarming around the much larger woodpecker. In determined, short, circular flights, they dove at him from the dead branches of the snag.
Within a minute of this apparently hostile encounter, the hairy woodpecker abandoned the snag and flew to a live ponderosa pine 30 yards away. The nuthatches followed. Photographer Rachel Ritacco, my field buddy this morning, traded raised eyebrows with me. “Did we really just see that?” we wondered.
Fall birds in the ponderosa forest
We were out in the open ponderosa pine (Pinus ponderosa) forest of the Scratchgravel Hills on this frosty morning, at the Head Lane Trailhead near Helena, Montana. Our goals for the morning were flexible. We were hoping to find some birds, learn something new about the natural world outside our doors, and perhaps get some good photos.
Until now, our bird sightings this morning had been more or less what I tend to expect in mature ponderosa pine forest in the fall. Some dark-eyed juncos had twittered at us from the mostly-leafless chokecherries in the draw. We had heard a Townsend’s solitaire making its heep calls from time to time, defending its precious winter supply of juniper berries.
We had gotten distant looks at some of the pine-forest specialists we hope to find in ponderosa stands like this one. A pygmy nuthatch flock had hopped from tree to tree with little concern for us, then faded rapidly into the distance. We had heard the raucous, nasal squawks of the Clark’s nutcrackers. And a small flock of red crossbills had flown past, making a crisp chorus of chips.
Pygmy nuthatches and more
It was only as we returned to the trailhead itself that we really got into some major bird activity. All of a sudden, the pygmy nuthatches were everywhere around us. From a smaller pine, a white-breasted nuthatch gave its laughing call. A male hairy woodpecker tapped emphatically, and a mountain chickadee wheezed in the background.
It was then that we witnessed the pygmy nuthatches diving at the hairy woodpecker, chasing him away. What was this about? I couldn’t remember ever noticing an interaction like this before.
I often see mixed-species flocks in the ponderosa forest during the quiet season. Nuthatches, chickadees, a woodpecker or two, and perhaps some crossbills will drift through together, foraging for pine seeds and insects.
These flocks rarely stay still for long. The birding experience is generally one of frenetic activity followed by periods of silence. The birds pass through in their groups, like schools of fish, leaving a quiet forest in their wake.
Why mixed-species flocks?
Mixed-species flocking is a well-documented phenomenon, and it’s common to see. But why join one of these flocks? The reasons depend a lot on the species of birds involved and the situation. But typically, it involves avoiding predators and finding food. More sets of eyes – and different eyes – reduce the danger of getting picked off by a merlin, a northern pygmy-owl, or a sharp-shinned hawk. Meanwhile, different foraging strategies keep competition low. Birds may also learn from other species in the flock about which trees are most food-rich, or they may take advantage of insects scared into movement by the passage of another bird. They may even steal food from another species, such as the gadwalls that often associate with flocks of coots on our lakes.
But here were these pygmy nuthatches, vigorously attacking their supposed flock collaborator, the hairy woodpecker. Why?
To try to answer this, let’s start by taking a look at the biology of these species.
Food and sociality
Pygmy nuthatches are habitat specialists in ponderosa pine forests and similar areas across western North America. They hunt the pines for weevils, leaf beetles, and other insects, which make up most of their diet. They also feed on pine seeds. During the winter, they may focus more heavily on these seeds. Pygmy nuthatches commonly cache food items in pine trees for later use. They’re strongly communal birds, traveling in family groups and joining up with other nuthatch families during the cold season.
Hairy woodpeckers tend to stay solitary or travel in pairs. When they join mixed-species flocks, they usually stay at the edges. They hunt the trunks of trees for bark-boring beetles and ants. To a smaller extent, they also feed on seeds and fruits.
So why were these pygmy nuthatches chasing the hairy woodpecker? From the context and the biology, here is what I suspect.
Pygmy nuthatches defending their food
The upper half of this ponderosa snag was riddled with round, marble-sized holes. While we watched them, the members of the nuthatch flock flew regularly between the snag and a nearby, living ponderosa. And over the 20 minutes that followed the departure of the woodpecker, we watched a handful of nuthatches return to the top of the snag several more times.
As mentioned earlier, pygmy nuthatches are food-cachers. I believe they were storing pine seeds, or perhaps insects, in the bark of this snag. As the hairy woodpecker inched his way up the tree, using his powerful bill to chip off pine bark, he threatened the nuthatches’ food supply. And in a rapid, well-coordinated defense, the nuthatches drove him away, preserving their carefully-cached food.
Is this actually why the nuthatches chased the woodpecker away? It’s the most believable explanation I’ve come up with so far, but by no means am I certain. As I researched this topic, I was impressed by the complexity of mixed-species flocking behavior. It’s a common and familiar phenomenon to most birders – but there’s so much going on in every one of these flocks. It’s actually really difficult to investigate what’s happening during mixed-flock interactions – let alone “why” birds are behaving how they are.
Whenever we’re out in nature, we’re surrounded by an infinite depth of stories. Every so often, there are moments like this. Pygmy nuthatches mobbing a hairy woodpecker: it was something that didn’t match with my simplistic understanding of mixed-species winter flocks. And in the search for an explanation, I came up with a clearer idea of how little I really know.
What are those nuthatches, hairy woodpeckers, and other flocking birds really doing? When and why do apparently peaceful interactions between species in a flock change to hostile ones? And are there different, equally plausible explanations for what we saw on this day?
Next time you run across a mixed-species flock, take a few minutes to really watch them. What are they doing? Think about what each species is finding to eat. How are they scanning for predators? How are they communicating?
Let me know what you notice! I also look forward to hearing any thoughts you have about the behavior of the pygmy nuthatches.
Harrison, N.M. & Whitehouse, M.J. (2011). Mixed-species flocks: an example of niche construction? Animal Behaviour 81(4):675-682. https://doi.org/10.1016/j.anbehav.2011.01.013
Jackson, J.A., Ouellet, H.R., & Jackson, B.J. (2020). Hairy woodpecker (Dryobates villosus). In Birds of the World Online (P.G. Rodewald & F.B. Gill editors). Ithaca, NY: Cornell Lab of Ornithology. https://birdsoftheworld.org/
Kingery, H.E. & Ghalambor, C.K. (2020). Pygmy nuthatch (Sitta pygmaea). In Birds of the World Online (P.G. Rodewald & F.B. Gill editors). Ithaca, NY: Cornell Lab of Ornithology. https://birdsoftheworld.org/