Selection on beak size in birds

The rapid evolution in the size and shape of bird beaks provide some of the best demonstrations of evolution in action. A classic example is the work of Rosemary and Peter Grant in the 1980's on Darwin's finches in the Galapagos. They showed that both competition and climatic conditions influenced beak size through changes in the availability of different sized seeds.

Gould's famous drawing of four of the fifteen species of Darwin's finches ([1] the large ground finch, [2] the medium ground finch, [3] the small tree finch and [4] the green warbler-finch). The Grants work primarily on the medium ground finch population on the island of Daphne Major, which they've visited every year since 1973.

Larger seeds are more difficult to crack. In drought years when they become proportionally more common, the finches with larger beaks did better because they had access to food that other birds didn't. Similarly, when a large seed eating competitor arrived, the birds with smaller beaks did better because smaller seeds became proportionally more common as competition for them was weaker.

The medium ground finch, Geospiza fortis. The Grants showed that its beak rapidly evolved to larger sizes in drought years and smaller sizes when faced with greater competition from the bigger beaked large ground finch, G. magnirostris (photo Wikipedia).

Although bird's beaks are clearly strongly shaped by diet, they are not just used for getting and eating food. The size and shape of beaks has also been linked to their use in preening and song production. More recently, studies have found that beaks may also play an important role in thermoregulation. Bigger beaks radiate more heat and could also help birds to conserve water in drier environments.

My pet kakariki (Cyanoramphus novaezelandiae). His beak is perfectly adapted for causing trouble.

In 2010, Symonds and Tattersall found that bird beaks conformed with Allan's rule, which posits that the relative size of body extremities should be smaller in colder environments to reduce heat loss. Bird beaks are highly vascularised and uninsulated extremities that can exchange a substantial amount of heat with the environment. Symonds and Tattersall found that beaks were relatively smaller in birds living at higher latitude and elevation.

In a recently published paper, Greenberg et al. develop this idea further. They argue that in drier environments birds could dissipate heat through their beaks rather than using evaporative cooling, thus conserving water. They test the hypothesis by examining the beaks of two subspecies of song sparrow, Melospiza melodia, that occupy habitats that contrast markedly in summer temperatures.

The song sparrow, Melospiza melodia (photo Wikipedia)

The Atlantic song sparrow lives in dune scrub and salt marsh edges along the mid-Atlantic coast, while the eastern song sparrow is widespread in gardens and wild scrublands. The coastal habitat of Atlantic song sparrows has higher summer temperatures and the availability of freshwater is limited relative to the habitat of the eastern song sparrow. And, consistent with the hypothesis, Atlantic song sparrows have beaks that are ~13% larger than eastern song sparrows.

To further evaluate the hypothesis, Greenberg et al. brought song sparrows into the laboratory where they could measure heat loss by thermally imaging the birds at constant ambient temperatures. Both subspecies maintained their beaks at higher than ambient temperature and higher than their body temperature. Heat lost through the beak was 5.6 - 10% of total heat loss, despite it making up less than 2.5% of total surface area.

A thermal image of an Atlantic song sparrow at an ambient temperature of 29°C (image Greenberg et al. 2012)

--> The beaks of the Atlantic song sparrows dissipated 33% more heat than the eastern song sparrow. Most of this difference can be explained by beak size, but Atlantic song sparrows may also maintain their beaks at higher temperatures than eastern song sparrows. Greenberg et al. estimate that the greater "dry" heat loss means an Atlantic song sparrow would conserve 7.7% more water than an eastern song sparrow of similar size.

In another study, Greenberg and Danner surveyed the beak size of song sparrows across California. They found that differences in beak size were strongly explained by the climatic conditions in which the birds lived. As summers became hotter and drier, song sparrow beaks became larger. Contrary to Allen's rule, winter temperatures poorly explained beak size differences, suggesting that heat dissipation is under stronger selection than heat conservation in the song sparrow.

Taken together these three studies support the hypothesis that climate is a significant selective pressure on the evolution of bird beaks. They highlight a little appreciated fact of evolution, that traits are often under multiple selection pressures and phenotypes are likely to reflect comprises between them. However, unlike the work on Darwin's finches, it has not been established that climate related beak size differences influence fitness variation. So, the work is strongly suggestive, but it's not a closed case.

References:

Symonds, MRE, & Tattersall, GJ (2010). Geographical Variation in Bill Size across Bird Species Provides Evidence for Allen’s Rule The American Naturalist, 176 (2), 188-197 DOI: 10.1086/653666  

Greenberg R, Cadena V, Danner RM, & Tattersall G (2012). Heat Loss May Explain Bill Size Differences between Birds Occupying Different Habitats. PloS one, 7 (7) DOI: 10.1371/journal.pone.0040933  

Greenberg R, & Danner RM (2012). The influence of the California marine layer on bill size in a generalist songbird Evolution DOI: 10.1111/j.1558-5646.2012.01726.x