Feeding behavior Crossbill

red crossbill skull , jaw anatomy william yarrell s history of british birds (1843)

the different species specialise in feeding on different conifer species, bill shape optimised opening species of conifer. achieved inserting bill between conifer cone scales , twisting lower mandible towards side crosses, enabling bird extract seed @ bottom of scale tongue.

the mechanism bill-crossing (which usually, not always, occurs in 1:1 frequency of left-crossing or right-crossing morphs) developed, , determines direction, has hitherto withstood attempts resolve it.

it probable there genetic basis underlying phenomenon (young birds bills still straight give cone-opening behavior if bills gently pressed, , crossing develops before birds fledged , feeding independently), @ least in red crossbill (the species has been thoroughly researched regarding question) there no straightforward mechanism of heritability.

while direction of crossing seems result of @ least 3 genetic factors working in case of epistasis , autosomal, not clear whether 1:1 frequency of both morphs in cases result of genetics, or environmental selection. populations feed on cones without removing or twisting them show 1:1 morph distribution no matter genetic basis may be: fitness of each morph inversely proportional frequency in population. such birds can access cone lower mandible tip pointing towards extract seeds, , high number of birds of 1 morph result in food availability each bird of morph decreasing.

they can utilise other conifers preferred, , need when preferred species has crop failure, less efficient in feeding (not enough prevent survival, enough reduce breeding success).