Social Selection, Family Life & Brood Parasitism

I began field research on American coots (Fulica americana) in 2005 as a graduate student working with Dr. Bruce Lyon at UC Santa Cruz. Bruce has studied these birds since he was a graduate student, and together we have learned a lot about the ecology and evolution of family life using this study system. The research I have been involved in primarily involve data collected in field studies that took place in British Columbia in 2005-2008.

Gina Peters looking for coot nests
Parental control through favoritism and aggression
Parent tousling one chick while the other chick eggs them on.

Raising a family is challenging. Conflicts of interest arise between parents, who seek to optimize family size each breeding season, and offspring, who seek to maximize the amount of care they receive (to a point). How parents manage their investment in the face of these conflicts has been a focus in life history theory.

American coots (Fulica americana) have a decidedly active — and aggressive — strategy of parental intervention.  My research shows that coots use a mixed strategy for controlling brood size: they use both brood reduction and parental control, but at different points in the parental care stage. After an initial period of high mortality for late-hatching chicks (brood reduction), Parental behaviors such as parental aggression, brood division and favoritism act as mechanisms for parental control that allow adults to optimally invest in their offspring (Shizuka & Lyon 2013). In the process, parents create a “favoritism hierarchy”, that is distinct from the competitive hierarchy created by hatching asynchrony. Changes in parental care strategies across time might be common, but it requires intensive observations throughout the parental care period (both in the nest & outside the nest), which is tractable in subprecocial birds like coots, but may be more difficult in altricial birds.

Learning, recognition and coevolution between brood parasites and hosts

Coot nest with two parasitic eggs: can you find them?
Family life in coots is complicated by another factor–they are facultative conspecific brood parasites, meaning that neighbors opportunistically lay eggs in each other’s nest. This is a challenge similar to those faced by hosts of more famous brood parasites like cowbirds and cuckoos: hosts gain no fitness by raising foreign offspring to which they are not related. However, American coots have an ability to do what most birds seem unable to do: recognize and reject brood parasitic chicks (Shizuka and Lyon 2010). More shockingly, they do this despite the fact that parasitism occurs within species, and thus parasitic chicks look much like their own. Through a series of cross-fostering experiments we showed that coot have a reliable learning rule: learn the attributes of the first-hatched chicks in a brood as a template for discriminating between the later-hatched chicks. This works in coots because parasitic chicks rarely hatch on the first day. Moreover, they manipulate incubation patterns to ensure that suspected parasite eggs hatch later than expected (Shizuka & Lyon 2011). Put together, our study shows that when the risk of learning errors can be mitigated, chick recognition can evolve — confirming a key prediction made by the ‘cost of misimprinting’ model (Lotem 1993).
Ok, which one of you is not my chick?

This work has led me to further ponder the ubiquitous role of learning mechanisms in ecological and evolutionary processes. Equally important is the ecology of information in which organisms evolve, which ultimately influences the evolution of learning. For example, we have now modeled the costs and benefits of parasitic chick recognition under different learning systems (i.e., ‘template acquisition strategies’: Shizuka & Lyon 2020). For example, what is the best way to learn your chicks–to learn all the chicks from your first nest, learn the first-hatched chicks in your first nest, or re-learn the first-hatched chicks each year? This modeling exercise shows us that, while some aspects of the learning mechanisms are neatly explained by the natural history of information (e.g., hatching patterns) other aspects may be constrained by factors such as genetic mechanisms and mating systems. For example, eggs are maternal phenotypes, while chicks contain genes from both mom and dad. Thus, using cues learned from past breeding attempts may be useful for eggs (same mom), while the utility of past information for chicks depends on the mating system (e.g., mate fidelity).

Parental Choice for Ornamented Chicks
Adult coots are drab, but baby coots are bright red and orange

Adult coots are drab gray birds, but coot chicks are born as bright red and orange puff balls. It’s one of the most extreme cases of juvenile ornamentation, i.e., when babies are adorned with conspicuous traits that seem to have some function as signals. The ornamental traits of baby coots–bright red beak and papillae (feathers around their beaks), and the bright red/orange/yellow tips of feathers around their necks–are reminiscent of the bright plumage of many adult (primarily male) birds. But unlike the male birds, juvenile ornaments are not the result of sexual selection because they lose these ornaments well before they reach sexual maturity. Rather, they represent a clear case of non-sexual social selection operating through parental choice. Lyon et al. (1994) showed that, if you trim the brightly colored tips off of the feathers of half the chicks in the brood, the parents will preferentially feed the chicks that retain the bright feathers. In our more recent study (Lyon & Shizuka 2019), we test whether the juvenile ornaments were driven to extremes in American coots because of the coevolutionary dynamics between brood parasites and hosts within species. That is, did the ornaments evolve to extremes because it benefitted parasitic chicks by inducing host parents to favor them? We measured color from nearly 1,500 coot chicks and observed patterns of parental care in many of them to see how this worked.

We actually did not find support for the hypothesis that parasites benefit by being more red. Rather, we find a strong correlation between chick color and the order in which eggs are laid–chicks that come from later eggs are redder. This is a tell-tale sign the mothers are calling the shots by making their youngest babies redder. And we also show that redder chicks are more likely to be favored when they are the youngest chicks. So it seems that chick color has more to do with the internal dynamics of families than coevolution of brood parasites and hosts–at least currently. The big question still stands though: why are coot babies SO red? Many animals seem to manage the allocation of food within families with less gaudy cues (e.g., begging), so why are coots so flashy?

Relevant Papers:

  • Lyon, B.E., Eadie, J.M. and Hamilton, L.D. (1994) Parental choice selects for ornamental plumage in American coot chicks. Nature371(6494), p.240. <https://www.nature.com/articles/371240a0&gt;
  • Shizuka, D. and Lyon, B.E. (2010) Coots use hatch order to learn to recognize and reject brood parasitic chicks. Nature 463: 223-226. [open access pdf] [link to original]
  • Shizuka, D. and Lyon, B.E. (2011) Hosts improve the reliability of chick recognition by delaying the hatching of brood parasitic eggs. Current Biology. 21:515-519.
  • Shizuka, D. and Lyon, B.E. (2013) Family dynamics through time: Brood reduction followed by parental compensation with aggression and favoritism. Ecology Letters 16: 315-322
  • Shizuka, D. & B.E. Lyon. (In Press). How to learn to recognize conspecific brood parasitic chicks. Philsophical Transactions of the Royal Society, B. 
  • Lyon, B.E. and Shizuka, D. (In Press) Extreme offspring ornamentation in American coots is favored by selection within families, not benefits to conspecific brood parasites. PNAS. 

This work was funded in part by an NSF Doctoral Dissertation Improvement Grant (NSF: please bring this program back!!)

 

Media coverage of this work:

Nature — Quirks and Quarks (CBC Radio) — Science News — Science Daily — Vancouver Sun