Elias Awad ENVS340 Project Ideas 4/4/07

Begging Signals of Nestling Birds: How Costs of Begging (probably) Keep Signals Honest [There is nothing in this title about modeling!] [Start with a paragraph that very briefly states the problem and then indicates how you intend to solve this problem using a model. Then the background info is fine] Background: Behavioral ecologists have long applied a synthesis of optimality theory and game theory to the investigation of parent-offspring signaling systems. No group of organisms in this regard has been better studied than the avian clades. We intend to construct a model to investigate the how potential fitness costs associated with nestling begging can render begging signals into “honest” statements of the offspring’s fitness, as well as determine how parent birds divide resources among a brood. It is standard in behavioral ecology to assume that there must be some cost, physiological, predatory, parasitic, etc., associated with begging; this model will additionally allow inquiry into situations where begging signals have no associated fitness costs. Will there still be “honesty” in signals and need- based resource allocation among offspring?

The theory surrounding the evolution and maintenance of signaling requires the signal to have a cost to keep it honest. Without a cost, any individual can signal at top intensity, leaving open the possibility of deceit [I’m not clear on precisely what deceit means in this context. Presumably, they are begging because they will increase their fitness by receiving additional food. How is that deceitful?]. If every signaler is deceitful, then the signal receivers cannot accurately respond to need. In the case of nestling birds signaling food need to their parents the interests of the nestlings and the parents largely overlap. The nestling will have the greatest fitness if it gets as much food as possible, and the parents will also have the greatest fitness (through the reproductive value of the nestlings) if all of its offspring get all of the food they need. There is a conflict of interest because the parents have a limit to the amount of food they can provide, and it does not benefit the parents to provide food to a nestling that doesn’t need it if there are other nestlings that do need it. Thus it pays the parents to respond to a signal that the nestling needs food, but only if that signal is honest. [This makes sense]

The problem is that evidence suggests that costs to nestling signaling, while significant, are very low [significant, but very low? Not clear what you mean by significant, which usually has statistical meaning in a scientific context]. Energetic costs in songbird species are on the order of standing around [? Your meaning is not clear here], and predation costs are only significant for species that nest close to the ground. There is some question as to whether these costs are large enough to keep signaling honest.

Objectives: Current literature conflicts in regard to costs of begging [be more specific, it is not clear how it conflicts]. We intend to construct a model that simulates a cost-of- begging situation that renders the intensity of begging signals “honest.” We will then restructure the model to find out if signal honesty can be attained with no costs, or if there is a given magnitude of fitness costs that render the signals honest. The resulting model has the potential to be used in Prof. Keith Tarvin’s Behavioral Ecology class.

Key Variables:

-Stocks: 1) Fitness of each Nestling Fitness will be defined as the product of the probability of survival of a nestling and its expected lifetime reproductive success. [I understand that this can be thought of as a stock, but I am not clear on how one models it as such. A stock grows and experiences losses via flows, but what you describe here is a product of two probabilities – this does not sound stock like to me. See my summary comment on this.] 2) Resources provisioned by parents This will be the amount of resources (number of food items, trips made by parents, etc.; to be defined after a more thorough search of the literature) provided to a nestling from birth until fledging. [this makes sense to me as a stock] 3) Parent fitness Defined as the product of the probability of survival of the parent and its expected lifetime reproductive success (including current brood).

-Flows: 1) Fitness increase or decrease of nestlings (mediated by energetic costs of begging, parental feeding, level of predation and disease) 2) Fitness increase or decrease of parents (mediated by energetic costs of resource provisioning, levels of predation and disease) 3) Parental food provisioning (affecting parents) The amount of work the parents put into providing resources to the nestlings. It will affect the parent fitness stock. 4) Parental food provisioning (affecting nestlings) The amount of resources provided to the nestlings, whatever resource units are chosen.

-Converters: 1) Begging costs (this will be a function of nestling fitness, and will effect outflows from nestling fitness) [strikes me as a loss] 2) Signal intensity (This will be the “honesty” converter, it will be a function of both nestling fitness and begging costs. It will help investigate whether costs, or what magnitude of costs, are required to render signals honest .) 3) Predation risk to nestlings (This is a function of signal intensity) 4) Predation risk to parents (This is a function of resource provisioning) 5) Disease risk to nestlings (A function of parental disease risk) 6) Disease risk to parents (A function of resource provisioning) 7) Lifetime reproductive success (Forcing fuction) 8) Resource provisioning costs (A function of parental fitness, predation and disease risk to parents)

Responsibilities and timeline: We will all meet with Prof. Keith Tarvin to go over the basics of this model and the possibilities for use in his Behavioral Ecology class. This will hopefully occur within the week, depending on Prof. Tarvin’s schedule. [The challenge here is that Keith is an expert in the topic, but not in what STELLA can and can’t do. You need to help him to understand that STELLA models in stocks and flows – perhaps very different from other models that he might be familiar with.] We will find all of the literature resources we will use, also within the week, and determine forcing functions, units, and equations. Most work will be done together, because it is important that we all understand and know what is being done and why.

Bibliography Brilot, B.O., and R.A. Johnstone. 2003. The limits to cost-free signaling of need between relatives. Proceedings of the Royal Society of London Series B-Biological Sciences 270 (1519): 1055-1060. - This article, though not specific to the nestling-parent interactions we am focusing on, delves into the reasoning behind current theories regarding the need of costs in signaling systems to render the system an “honest” one.

Halupka, K. 1998. Vocal begging by nestlings and vulnerability to nest predation in meadow pipits Anthus pratensis; to what extent do predation costs of begging exist? Ibis. 140 (1): 144-149 - This paper found that predation costs of begging do not exist in this species.

Haskell, D. 1994. Experimental evidence that nestling begging behavior incurs a cost due to nest predation. Proceedings of the Royal Society of London. 257(1349): 161- 164 - This paper found that costs of begging are significant in species that nest close to the ground, but not in species that nest in trees.

Kilner, R. M. 2001. A growth cost of begging in captive canary chicks. Proceedings of the National Academy of Sciences. 98(20): 11394-11398 - This paper reports that nestling begging impairs the growth of nestlings, and that previous studies (that showed that begging is not very costly) do not actually measure the costs of begging.

Krakauer, D. C. and Johnstone, R. A. 1995. The evolution of exploitation and honesty in animal communication – A model using artificial neural networks. Philosophical transactions of the Royal Society of London. 348(1325): 355-361 - This research used neural network simulation of signal evolution to demonstrate that signal cost maintains honesty in the system, and that costs are necessary to maintain honesty. Price, K., H. Helene, R. Ydenberg. 1996. Begging tactics of nestling yellow-headed blackbirds, Xanthocephalus xanthocephalus, in relation to need. Animal Behavior 51 (2) 421-435. - This paper nicely summarizes a lot of the theory regarding the honesty of begging signals.

Rodriguez-Girones, M., A. M. Enquist, and P.A. Cotton. 1998. Instability of signaling resolution models of parent-offspring conflict. Proceedings of the National Academy of Sciences. 95(8): 4453-4457 - This paper describes a computer model of begging and parental response. This model suggests that a system in which signaling is supposedly stabilized by the cost of the signal is unstable – populations that start at signaling equilibrium depart from it quickly.

Searcy, W. A. and S. Nowicki. 2005. The Evolution of Animal Communication: Reliability and Deception in Signaling Systems. Pages 1-77. Princeton University Press. - This is relevant because it describes the basics of animal signaling and the problems this model would address.

Welham, C.V.J., and G. Beauchamp. 1997. Parental provisioning in a variable environment: Evaluation of three foraging currencies and a state variable model. Evolutionary Ecology 11 (4): 399-417. - - The model used in this paper will provide a scaffold for investigating parental foraging as affected by nestling begging.

I’m trying to get my head wrapped around the concept of fitness, as you describe it, as a stock and I can’t quite do it. Knowing far less about the phenomenon than you do, my tentative suggestion is that you focus on energetic resources as the stock and then consider fitness to be a function of this stock – the more energetic resources an offspring has, the greater their probability of surviving and reproducing. But perhaps there is some threshold at which acquisition of more energy is no longer beneficial and may actually cost the individual. From a parental perspective, might there be a “never cry wolf” type reaction of parents, where the parent habituates a constantly crying offspring, but responds to offspring that periodically cry? Or might a parent conclude that an offspring who cries all the time is somehow defective, and therefore not worth the investment of additional resources? It seems to me that an offspring might have some sort of a crying curve as a function of energetic needs – with lithe energy, there is little energy available for crying, more energy increased crying, more energy (satiation) less crying.

I’m still very unclear on your overall conceptualization here. Are you proposing to model multiple individual offspring, each with its own fitness (or energetic) stocks, but perhaps with different coefficient values that represent different strategies? [this sounds doable] Are parents also represented by stocks, but perhaps by only one stock? I’m a bit concerned that the problem described above may be framed in a way that does not fully match to the tool at hand (STELLA) and also that the situation you describe is very complex. I suggest that you look hard for models that might serve as analogs for the question you are trying to address and perhaps consider adjusting your questions to match the tool. Lots of thinking needs to go into this to appropriately match the model you develop with the questions you hope to address. Let’s talk about this next class – please think through the range of possibilities beforehand.

Reviews:

#1 The bulleted points are very clearly spelled out and easy to follow. The Background shows that the group is fairly knowledgeable on the subject, and, in Objectives, I get a clear understanding of what it is precisely that they want to model. The Bibliography is incredibly extensive and ambitious. The idea seems to follow closely what is taught in Prof. Tarvin’s class, and so I imagine it could be very useful in helping to explain these concepts for students of that class.

I had a little trouble understanding how exactly values like “begging costs”, “signal intensity”, “disease risk”, “predation risk”, etc. could be quantified so that they all could be taken into account in the same terms (i.e. units), but I assume the papers on this topic have described methods by which this can be done. So I’m sure it would be feasible, and seems to be a good application for STELLA. Otherwise, I can’t think of any ways to improve the project -- it sounds really interesting and I am excited to hear the presentation and all that they find. [Thoughtful review]

#2

Modeling the begging signals of nestling birds sounds extremely difficult and complex [yup!]. However, this proposed project also sounds interesting and unique in exploring a topic that is contested in scientific literature. The fact that there is not an agreement on whether or not the costs of begging keeps signals honest might be difficult to actually model [yes], but I think this group’s plan works. Initially assuming that it does keep signaling honest and then modifying the model to have no costs associated with begging seems like an appropriate question to explore with a model.

Will the group get equations/forcing functions from actual data/literature? [I strongly recommend against this – the model will be hard enough to get together in a theoretical mode and data are not actually crucial to answering the question of whether it is possible] This might be difficult to keep consistent if the issue is not agreed upon in the literature. This model sounds like it will be useful for the intended audience, especially after consulting Professor Tarvin while building it.

The other difficult task with this proposal seems to be keeping the model simple [yes]. There are lots of forcing functions/variables to keep straight but it sounds like the group has a strong background in the subject. I think it will be feasible for them so long as they do not try to get too complicated. I don’t have any further suggestions because I do not know enough about signaling to know if they seem to be thinking about everything--it seems like they have a good grasp on everything they need to include. [The point about avoiding too much complexity is the critical challenge of this project]

#3 Sounds like an interesting problem to work on—evolutionary theory is neat. It looks like you guys have thought out the details pretty well. The state variables seem to be on the right track, but some questions: Are there going to be differences between male and female birds? Are nestlings raised by both parents? Also, for the resources, it may be easier to have a stock of resources in the environment flowing into a stock of resources provisioned by the parents (this flow would be mediated by the things you described). [yes, stocks of resources make sense]

PS- Have you read The Selfish Gene, by Richard Dawkins?