My research program focuses on sexual selection, social organization and communication, and employs socially monogamous birds (especially zebra finches, Taeniopygia guttata) as experimental models. The study of birds has contributed much to the development of sexual selection theory , but historically most attention has been devoted to birds that are promiscuous or polygynous, while in fact most birds are socially monogamous. My early work established two mechanisms by which sexual selection can operate even among species that have substantial bi-parental care and that display mating fidelity to a single partner for extended periods of time: differential access (preferred individuals attract superior mates) and differential allocation (individuals allocate more parental care to offspring of superior mates)(Burley 1986a;1988a). Subsequent research on differential allocation has established it as a reproductive tactic found among a wide range of organisms and mating systems and a potentially very important sources of non-heritable, parental effects on developing phenotypes (reviewed in Burley 2010).
The evolutionary significance of mate preferences is a central question in social evolution, especially preferences for ornamental traits, such as the brightly colored plumage of many birds. Most contemporary behavioral ecologists emphasize the possibility that variation in such traits is maintained by the cost of developing and expressing them; thus, such traits are therefore hypothesized to be honest indicators of condition or quality. While this approach has proved extremely successful, it neglects the question of the origin and diversification of novel ornaments, as it is likely that indicator status must evolve. To probe this question, my lab engaged in a series of experiments that involved the addition of artificial (often plastic) traits to the legs and heads of several species of birds. As anticipated by Charles Darwin, experimental results from my lab indicate that they possess latent, “aesthetic” mate preferences for traits that have yet to evolve . Simply put, members of one sex of a given species tend to agree on which novel traits are attractive or unattractive, and vary their reproductive behavior accordingly; this phenomenon is also known as sensory bias. Birds have been found to respond to artificial traits that vary in attractiveness by altering sex allocation, parental effort, and by manipulating paternity via extra-pair copulations (eg., Burley 1985; 1986a & b; Burley et al. 1996). Thus, artificial traits impact reproductive success, and even survival, even though such traits are randomly assigned and thus fail to serve as indicators of quality. We have discovered latent aesthetic preferences among birds wherever we have looked for them (e.g., Burley 1988b; Johnson et al. 1993; Burley & Symanski 1998; Calkins & Burley 2003).
Over time, we have used phenotypic modifications involving artificial ornaments to probe additional questions, including how self-assessment of mate-getting ability influences mate selectivity (Burley & Foster 2006); and the potential for sexual imprinting to generate reproductive isolation when novel attractive traits arise within populations (Burley 2006 and several papers in preparation).
Other topics of recent study by members of my lab group include: functions of same-sex sexual behavior by male budgerigars (Abbassi & Burley 2012); mating system and functions of vocal imitation in budgerigars (Hile et al. 2005; Moravec et al. 2006, 2010); sex allocation in zebra finches (Burley & Foster 2004; Foster & Burley 2007); and evolution of avian parental care (Burley & Calkins 1999; Burley and Johnson 2002).
The lab’s current research focus is on how diet quality influences cognitive and social behavior in zebra finches, which are obligate graminivores (grass-seed eaters). While it is known that early protein deprivation has deleterious effects on cognitive development in humans and several rodent species, impact of diet on adult performance of birds is largely unstudied. If protein-limitation has consequences for development in relatively large-brained birds, such as passerines, then diet variation may have important influences on cognitive performance and social complexity.
Many, if not most, seed-eating birds supplement their diet with insects or other sources of protein/nutrients, but zebra finches do not. Nevertheless, the protein content of their diet naturally varies considerably: mature grass seed is low in protein and has a restricted range of amino acids, but half-ripe seed has more protein and a broader range of amino acids. Not surprisingly, zebra finches, which are capital breeders (females metabolize breast muscle to produce eggs), prefer to breed when half-ripe seed is available, although they do breed when only mature seed is available if it is sufficiently abundant (Burley et al. 1989; Zann et al. 1995). In the lab we supply boiled hen’s egg, which has an amino acid profile very similar to half-ripe seed.
Our first experimental paper on this topic has established that birds reared on an egg-supplemented diet perform significantly faster on an associative learning task than those reared on mature seed alone. Egg-supplemented birds also developed somewhat different body morphologies, with proportionately larger heads. Regardless of treatment, birds that invest more in head growth (vs body growth) during subadult development perform relatively well on this cognitive task as adults (Bonaparte et al. 2011).
Current research projects in the lab investigate how rearing diet influences reproductive competition in aviary breeding populations and how diet influences male song traits. Questions of interest include: Are birds reared on egg-supplemented diets preferred as mates? Does rearing diet influence mate-getting or mate-retention tactics, extra-pair behaviors, parental behaviors (including tendency to provision egg to offspring), nest site selection or nest building competence? What is the impact of rearing diet on other cognitive skills, such as predator avoidance?
I expect the study of diet effects on zebra finch cognition to make significant contributions to, and provide linkages between, the topics of comparative cognitive ethology, sexual selection, and social evolution. This is a very promising system in which to bridge mechanistic and functional approaches to a range of important questions.
Papers cited and representative publications:
Abbassi, P. & N.T. Burley. 2012. Nice guys finish last: same-sex sexual behavior and pairing success in budgerigars, Melopsittacus undulatus. Behavioral Ecology (in press).
Bonaparte, K., C. Riffley-Yokoi, & N.T.Burley. 2011. Getting a head start: diet, sub-adult development, and associative learning in a seed-eating passerine. PLoS ONE 6: e23775.
Moravec, M. L., G.F, Striedter, & N.T. Burley. 2010. 'Virtual parrots' confirm mating preferences of female budgerigars. Ethology 116: 961-971.
Burley, N.T. 2010. Differential allocation. Pp. 520 – 525 in Encyclopedia of Animal Behavior, M.D. Breed and J. Moore (eds). Academic Press, Oxford, U.K.
Foster, V.S. & N.T. Burley. 2007. Sex allocation in response to maternal condition: different tactics of care-giving in male and female zebra finches. Ethology 116: 961 -971.
Burley, N.T. & V.S. Foster. 2006. Variation in female choice of mates: condition influences selectivity. Animal Behaviour 72: 713 – 719.
Burley, N.T. 2006. An eye for detail: selective sexual imprinting in zebra finches. Evolution 60: 1076 – 1085.
Moravec, M.L., G.F. Striedter, & N.T. Burley. 2006. Assortative pairing based on call contact similarity in budgerigars, Melospsittacus undulatus. Ethology 112: 1108 – 1116.
Hile, A.G., N.T. Burley, C.B. Coopersmith, V.S. Foster, & G.F. Striedter. 2005. Effects of male vocal learning on female behavior in the budgerigar, Melopsittacus undulatus. Ethology 111: 901 – 923.
Burley, N.T. & V.S. Foster. 2004. Digit ratio varies with sex, egg order, and strength of mate preference in zebra finches. Proc. R. Soc. London B 271: 239 – 244.
Calkins, J.D. & N.T. Burley. 2003. Mate choice for multiple ornaments in the California quail, Callipepla californica. Animal Behaviour 65: 69 – 81.
Burley, N.T. & K. Johnson. 2002. Evolution of avian parental care. Philosophical Transactions of the Royal Society of London B 357: 241 – 250.
Burley, N. 2002. Zebra finches. Pp. 152 – 154 in Encyclopedia of Evolution, Marc Pagel, ed. Oxford University Press, Oxford, U.K .
Burley, N.T. & J.D. Calkins. 1999. Sex ratios and sexual selection in socially monogamous zebra finches. Behavioral Ecology 10: 626 - 635.
Burley, N.T. & R. Symanski. 1998. "A taste for the beautiful": latent aesthetic mate preferences for white crests in two species of Australian grassfinches. The American Naturalist 152: 792 - 802.
Burley, N.T., P.O. Parker & K.J. Lundy. 1996. Sexual selection and extra-pair fertilization in socially monogamous passerine, the zebra finch (Taeniopygia guttata). Behavioral Ecology 7:218-226.
Zann, R.A., S.R. Morton, K.R. Jones, & N.T. Burley. 1995. The timing and breeding of zebra finches in relation to rainfall in central Australia. Emu 95: 208-222.
Price, D.K. & N.T. Burley. 1994. Constraints on the evolution of attractive traits: selection in male and female zebra finches. The American Naturalist 144: 908 - 934.
Johnson, K., R. Dalton & N.T. Burley. 1993. Preferences of female American goldfinches (Carduelis tristis) for natural and artificial male traits. Behavioral Ecology 4:138-143.
Burley, N., R.A. Zann, S.C. Tidemann & E.B. Male. 1989. Sex ratios of zebra finches. Emu 89: 83 - 92.
Burley, N. 1988a. The differential allocation hypothesis: an experimental test. The American Naturalist 132: 611 - 628.
Burley, N. 1988b. Wild zebra finches have band-colour preferences. Animal Behaviour 36: 1235-1237.
Burley, N. & C.B. Coopersmith. 1987. Bill color preferences of zebra finches. Ethology 16: 133-151.
Burley, N. 1986a. Sexual selection for aesthetic traits in species with biparental care. The American Naturalist 127: 415 - 445.
Burley, N. 1986b. Sex-ratio manipulation in color-banded populations of zebra finches. Evolution 40: 1191 - 1206.
Burley, N. 1985. The organization of behavior and the evolution of sexually selected traits. Pp. 22 - 44 In Avian Monogamy, P.A. Gowaty and D.W. Mock, eds. Ornithological Monographs # 37, American Ornithologists' Union, Washington, D.C.