Fitting the bill: do different winter food resources influence juniper titmouse (Baeolophus ridgwayi) bill morphology?

Abstract

Divergent adaptive selection is a prominent mechanism influencing patterns of morphological diversity. We used the juniper titmouse [Baeolophus ridgwayi (Richmond, 1902)], a nonmigratory passerine that inhabits woodlands throughout western North America, to investigate variation in bill morphology in relation to diet and geography. We gathered data from museum specimens and used morphometric techniques to determine the relative strength of support for competing hypotheses using information theoretics: (1) differences in bill morphology are predicted by a key winter food resource that each regional population consumes (seeds of different juniper tree species); or (2) bill morphology scales with body size, and both increase along a latitudinal gradient. Juniper species emerged as the variable with the most support explaining variation in bill size, supporting the hypothesis that seed sizes influence bill size, independently of body size. The shape analysis revealed no distinct patterns in bill shape variation, but employed a powerful method for evaluating the strength of support for numerous candidate models. The differences in bill size of juniper titmice across their range are likely to reflect adaptive variation, because bill morphology is highly heritable in birds, juniper titmouse gene flow appears to be relatively low, and there is a clear mechanistic explanation for why bill sizes may differ among the ranges of the three juniper species.

Song characteristics track bill morphology along a gradient of urbanization in house finches (Haemorhous mexicanus)

“…Bill shape in birds is also strongly associated with diet (e.g. short and thin in insectivores, deep and hooked in granivores and carnivores; [ 32 ]), such that foraging pressures can work either with or against directional selection on bill size/shape for song production [ 23 , 24 ]. Urban environments offer novel foraging opportunities that may shift bill morphology, and in fact city effects on bill morphology have been documented in house finches, Haemorhous mexicanus [ 33 ], whereby bill size increased in urban birds perhaps as a function of the availability of larger, harder-to-husk seeds at backyard bird feeders.…”

“…For the first time in studies examining urban impacts on animal signals, we quantified a series of different metrics of urbanization, including human population density and seven measurements describing land-use patterns within the 1-km radius around each of our trapping sites [ 35 ], to assess how these factors may be associated with bill shape/size and song characteristics. Based on previous studies, we predicted that minimum song frequency would be associated with urban background noise [ 13 , 18 , 19 ] and that bill size (length, width, and height) would increase in urban habitats [ 32 ]. In addition, given that the angle between the upper and lower mandibles should decrease with an increase in bill length (considering a similar aperture at the bill tip), urban birds with longer bills should have a proportionally longer orotracheal cavity with a reduced high resonating frequency compared to rural individuals with shorter bills [ 22 ].…”

Do foraging methods in winter affect morphology during growth in juvenile snow geese?

“…As adults show segregation into habitats by body size, we would expect the same in juveniles, given that structural size has a genetic component. Bill size and body size also have a genetic component and are moderately to highly heritable in birds (Abzhanov, Protas, Grant, Grant, & Tabin, ; Francis & Guralnick, ; Husby, Hille, & Visser, ). Body size variation has an environmental component as well; for example, body size in several birds, including lesser snow geese, has declined in recent decades, possibly in response to climate change (Aubry et al., ; Cooch, Lank, Rockwell, & Cooke, ; Husby et al., ; Van Buskirk, Mulvihill, & Leberman, ), climatic variability, or primary productivity (Goodman, Lebuhn, Seavy, Gardali, & Bluso‐demers, ).…”

A test of a corollary of Allen's rule suggests a role for population density

“…Selection outside of the breeding season can also be a dominant force in bill evolution (Schluter and Smith 1986, Grant and Grant 1993, Francis and Guralnick 2010. Selection outside of the breeding season can also be a dominant force in bill evolution (Schluter and Smith 1986, Grant and Grant 1993, Francis and Guralnick 2010.…”

“…It is important to note that bill size is not subject to the selective forces of temperature during just the summer months. Selection outside of the breeding season can also be a dominant force in bill evolution (Schluter and Smith 1986, Grant and Grant 1993, Francis and Guralnick 2010. Multiple bird species show correlations between bill size and minimum winter, not maximum summer, temperatures Tattersall 2010, Danner andGreenberg 2015).…”