Bigger The Bird, Bigger The Punch

April 12, 2021

On the Cornell Lab’s Panama Fruit Feeder cam, there are dozens of charismatic bird species who visit a feeding table filled with fruit and rice. Staff at the Canopy Lodge, where the feeding table is located, have to restock it every couple hours because many times the birds eat through everything.

When these species come to the feeder, who gets to access this food? When battles ensue, who wins? Who loses? Starting in November 2020, the Bird Cams Lab community teamed up with Cornell Lab researcher Dr. Eliot Miller to find out!

After weeks of collecting and cleaning the data, the community started to explore the data they had collected–when were birds at the feeder? When were battles happening? Which species were initiating these battles?

Now, thanks to Dr. Miller, we have results from preliminary analyses that give us a window into the social dominance relationships of each species by way of a “dominance hierarchy.” He used the successful displacements (when one bird takes the perch or food of another birds) to rank the different species.

We invite you to explore the data with us. 

  • Read on to see what we found.
  • Share your interpretations or questions in the forum below.
  • Register for a live discussion about these findings with Dr. Miller and Bird Cams staff on Thursday, April 15, 3:00–4:00 P.M. ET. Save your spot today.

Larger species have higher rankings than smaller species

Dr. Miller took the successful displacements and ran a model that returned a dominance score for each species. The more positive the score, the higher the ranking; the more negative the score, the lower the ranking. Then, he plotted the dominance score against the mass of the species and found that the larger the species, the larger the dominance score (Figure 1).

How does dominance relate to mass

Figure 1. A scatterplot of the dominance score and mass of each species that initiated a successful displacement. The size of each point represents the number of successful displacements the species was in (initiated or received)–the more displacements the bigger the point. To the right of the scatterplot, species are listed according to their rankings, with highest ranking species at the top and lowest ranking species at the bottom. 

Some species punch above or below their weight class

To explore the strength of the relationship between mass and dominance score, we took the Log of mass and replotted it against dominance score (Figure 2). By taking the Log, we can estimate the magnitude of the linear relationship between the two variables and calculate a correlation coefficient (R). We found that R = 0.80, a very high value in the field of ecology that signals that the relationship is very strong – the bigger you are, the higher your dominance score.

However, there are species that didn’t fit this relationship perfectly (they didn’t fall right along the best fit line). For example, Crimson-backed Tanager and Flame-rumped Tanager appear above the line, which means that for their mass they have a higher ranking than expected.

How does dominance relate to Log(mass)
Figure 2. A scatterplot of the dominance score and the Log (mass) of each species that initiated a successful displacement. To the right of the scatterplot, species are listed according to their rankings, with highest ranking species at the top and lowest ranking species at the bottom. A best fit line is overlaid on the graph to illustrate the linear relationship between Log (mass) and dominance score.

Species interact at different rates

As we look at the dominance hierarchy and think about what it could mean in terms of how species relate to each other and access food, we need to take into account how we made the dominance hierarchy. For some species, like the Clay-colored Thrush, we had a lot of information– the Clay-colored Thrush displaced several species and several species displaced it. But for other species, like the Silver-throated Tanager, we only documented one interaction and its place in the hierarchy is less certain.

Who’s battling who?
Figure 3. A horizontal stacked bar chart in which the number of successful displacements is shown on the horizontal axis and the initiating species along the vertical axis. The initiating species are arranged from highest ranking to lowest ranking, top to bottom. The colors of each bar refer to the target species in each successful displacement and is listed to the right of the bar chart. Note that not all species in the dominance hierarchy initiated displacements and are not on the vertical axis.

Share your thoughts in the forum below and tune in for a live discussion about the data in a free webinar on Thursday, April 15 @ 3:00–4:00 P.M. ET. Register to save your spot today.