Insect discrimination in birds; Batesian trickery tricked

Hanna Jackson

Department of Biological Sciences, Simon Fraser University, Canada

*This report is a class project NOT peer-reviewed science


Introduction

Mimicry is when an organism, the mimic, takes advantage of the association a predator has with a model and it’s perception to gain a fitness advantage (de Jager and Anderson 2019). It occurs under three conditions; there must be a model, a receiver that interacts with models and mimics, and the receiver must exert selection on a mimic thereby selecting for and maintaining the mimic’s traits (de Jager and Anderson 2019). Under Mullerian mimicry, both model and mimic are unpalatable to the receiver, whereas in Batesian mimicry, there is a palatable mimic and an unpalatable model (Turner 1987). In Batesian mimicry, if the mimic and model cannot be distinguished from one another, they are predated at the same rate, so the mimic’s palatability causes the model and mimic to suffer from an increased predation rate. (Turner 1984, Lindstrom et al. 1997). Because of this, Batesian mimicry involves negative frequency dependent selection, wherein palatable mimics are more successful at low frequencies (Turner 1984). Considering this evidence, we hypothesize that there will be a difference between the amount of each form eaten that depends on the frequency of the mimics. Further, we predict that forms consisting of a higher percentage of mimics will be eaten more frequently than forms with less mimics and further that the form with no unpalatable mimics at all will be eaten the most frequently.

Methods

We set up a field assay during the fall consisting of 4 feeders baited with different types of model bird prey of varying palatability. We set them up on Burnaby Mountain, British Columbia, Canada in two separate courtyards. These feeders were suspended 1.3-m above the ground, covered, and the feeder platform was 1x1.5-m in dimension. Each individual prey was set in one grid space of a 20x5 grid on the platform surface.

We made bait prey that were 1.5 cm long, worm shaped, and made of flour, lard, and dye for coloured prey. To prey designated as unpalatable, we added quinine to deter the birds from eating them. We made 3 different species of prey by colouring some yellow, green, and blue. To replicate these different species having varying palatability, we made 75% of yellow prey unpalatable and 25% palatable, green prey 25% unpalatable and 75% palatable and blue prey all palatable.

We first added uncoloured palatable training prey to our feeders so the birds would learn to eat at our feeder. We then set the treatment prey onto the feeders in a random order in the correct palatability ratios. We set up, monitored, and cleared the feeders twice a day, once in the morning and again in the afternoon. We left prey on the feeder for 2.5-3 hours for each trial and which prey were eaten was recorded. We scored half eaten prey as eaten, and we did not count disturbances as prey eaten. After each trial we removed all prey. Juncos, sparrows, chickadees, and crows ate prey from our feeders and thus results pertain only to them.

We analysed our results in RStudio version 3.4.4 using repeated measures ANOVA, one way ANOVA, and linear regression models.

Results


The average percent of prey eaten for each treatment was as follows: 37.4% for yellow unpalatable, 45.0% for yellow palatable, 26.6% green unpalatable, 25.5% for green palatable and 25.6% for blue palatable.


Treatment significantly explained amount eaten (Figure 1, p=0.05, F1=3.17), with yellow prey being eaten significantly more than blue (p=0.018) and green treatments (p=0.0037). Days of the experiment elapsed did not explain amount eaten among all bait (p=0.33, F1,198=0.0.95), and did not significantly explain amount of yellow prey eaten (Figure 2; p=0.70, R2= 0.001, F1,78=0.15), green prey eaten (Figure 2; p=0.185, R2=0.02,F1,78=1.79), or blue prey eaten (Figure 2; p=0.95, R2=0.0001, F1,38=0.0047).



Figure 1. We gave birds the option between blue (B), green (G) and yellow (Y) model prey forms at varying proportions of palatability. Blue forms were 100% palatable, green forms were 25% unpalatable and 75% palatable, and yellow forms were 75% unpalatable and 25% palatable. Colour form treatment significantly explains amount eaten, with yellow prey being eaten significantly more than either blue or green prey. Blue and green prey were not eaten at significantly different rates.



Figure 2. Yellow, green, and blue prey of varying palatability were set out for birds to eat. Yellow forms were 75% unpalatable and 25% palatable, green forms were 25% unpalatable and 75% palatable and blue forms were 100% palatable. Birds did not change the amount of yellow, green or blue prey they ate from the beginning of the experiment to the end of the experiment, indicating a lack of learning to either avoid or have preference for any of our prey.

Figure 3. Yellow, green, and blue prey of varying palatability were set out for birds to eat. Yellow forms were 75% unpalatable (YU) and 25% palatable (YP), green forms were 25% unpalatable (GU) and 75% palatable (GP) and blue forms were 100% palatable (BP). Birds did not respond differently to palatable and unpalatable versions of the same colour form, and yellow forms were more attractive than green and blue forms.




Discussion

We tested whether birds would learn to avoid prey forms with a higher percentage of models, unpalatable quinine dipped bait, present. We predicted that birds would eat blue forms the most because 100% of this form was palatable. We also predicted that green form prey would be eaten the more frequently than yellow, because the green form prey had 25% unpalatable model prey and 75% palatable mimics, and the yellow form prey were 75% unpalatable mimics and 25% palatable models. Contrary to these predictions, we found that blue forms were not eaten more than yellow or green forms, they were eaten the same as green forms and significantly less than yellow forms. Further, yellow forms were eaten significantly more than green forms.

Previous studies have shown that both mimics and models of a given form are eaten less when mimics are less frequent (O’Donald and Pilecki 1970, Lindstrom et al. 1997), which would lead us to expect that our yellow form would be eaten the least. Our results run contrary to these predictions. Birds did not learn to eat more blue prey, which were 100% palatable, over the one week of our experiment and similarly did not learn to eat less of the prey forms with unpalatable prey mixed into palatable prey, yellow and green.


We found that birds did not learn to avoid or favor any of the prey form over the week of our experiment. The birds, our predators, will only avoid the mimic if the model is present (Pfennig et al. 2001), as this is not an innate behavioural avoidance by birds. Our one-week time frame may have been insufficiently long to have the birds recognise the models, and not long enough for the birds to learn. All of which lead to other factors being the dominant determinants of which prey are eaten.

It has previously been shown that if models are only slightly distasteful, a rare mimic has an advantage over a common mimic, in frequency dependent selection (O’Donald and Pilecki 1970). However, if models are highly distasteful to predators, then this frequency dependent selection is no longer present, as predators are unwilling to risk eating the highly unpalatable prey (O’Donald and Pilecki 1970). In our results, forms with 100% palatable prey and 25% palatable prey were eaten at the same rate. This perhaps indicates a low unpalatability of our quinine solution. Furthermore, it appears that birds in our study area had a pre-existing preference for yellow bait. This preference for yellow prey appears to have been strong enough to overcome the deterrent of presenting unpalatable prey.


In our design, we assume that birds have no pre-existing colour preference, that birds cannot tell the difference between palatable and unpalatable prey and that absent prey represents predation by birds. A previous similar study showed that birds could not tell the difference between quinine treated bait and palatable bait except by taste, leading to mimics and models with the same level of fitness (O’Donald and Pilecki 1970). Our data conforms this, showing no significant difference between the unpalatable and palatable versions of the same forms (Figure 3). It has previously been shown that birds can remember and retain preferences of the palatability of prey from season to season (Pilecki and O’Donald 1971). Our experiment was done in preliminary trials in the previous year, but interestingly this effect seen by Pilecki and O’Donald seems to have not been particularly strong in our trials, as birds did not eat the palatable blue prey more often than yellow or green.


Our study provides evidence for a lack of selection on the palatability of prey in the assay we presented to the birds. It appears that the birds tested have a significant pre-existing colour preference for yellow prey, possibly due to the makeup of locally available prey, that cannot be overruled by unpalatable mimic prey present for the duration of a one-week experiment.

References

de Jager, M. L., and B. Anderson. 2019. When is resemblance mimicry? Functional Ecology 33:1586–1596.


Lindstrom, L., R. V. Alatalo, and J. Mappes. 1997. Imperfect Batesian mimicry - The effects of the frequency and the distastefulness of the model. Proceedings of the Royal Society B: Biological Sciences 264:149–153.


O’Donald, P., and C. Pilecki. 1970. Polymorphic Mimicry and Natural Selection 24:395–410.


Pfennig, D. W., W. R. Harcombe, and K. S. Pfennig. 2001. Frequency-dependent Batesian mimicry. Nature 410:323.


Pilecki, C., and P. O’Donald. 1971. The Effects of Predation on Artificial Mimetic Polymorphisms with Perfect and Imperfect Mimics at Varying Frequencies. Society for the Study of Evolution. http://www.jstor.org/ 25:365–370.


Turner, J. R. G. 1984. Mimicry: the palatability spectrum and its consequences. The Biology of Butterflies 14:141–161.


Turner, J. R. G. 1987. The evolutionary dynamics of batesian and muellerian mimicry: similarities and differences. Ecological Entomology 12:81–95.