Thursday, February 18, 2010

Iron in the Beaks of Birds

Birders know well that birds migrate – the cycle of bird movements keeps birding interesting throughout the year. Many landbird migration routes are well-documented, and even some over-water routes are starting to be determined. What is less understood is how the birds know where they are going. Recent research has focused on how birds might sense, or even see, the Earth's magnetic field for orientation. At least one recent study proposed that some birds use an olfactory sense for guidance. Visual cues, such as the stars for nocturnal migrants or waterways for diurnal migrants, have also been proposed.

One new study finds iron-containing cells in the upper beaks of birds. Three years ago, the same research team reported that the upper beaks of homing pigeons contain iron minerals arranged in structures that could allow a pigeon to sense a magnetic field. This study found similar structures in the beaks of three other bird species: Garden Warbler (Sylvia borin), European Robin (Erithacus rubecula), and domestic chickens (Gallus gallus).

Prussian blue stained dendrites in the inner lining of the upper beak of (A) homing pigeon, (B) garden warbler, (C) European robin, and (D) domestic chicken. (E) General semi-schematic drawing of an iron containing dendrite. (F) Axon bundle with several iron containing dendrites.

In all four species, the iron-rich dendritic cells are located near the top of the upper beak and not in the lower beak or in any other tissue. The iron particles are arranged in narrow rod-like clusters, which appear to lie parallel to one another. The clusters are connected to the ophthalmic nerve, so they form part of a bird's sensory system. The iron minerals contained in the dendritic cells appear to be maghemite (Fe(III)2O3) rather than magnetite (Fe (II)Fe(III)2O4), but the sample was too small to determine that conclusively. Given that such similar clusters are present in several bird species that are not closely related, including a nonmigratory bird, they may be present in many more (or even all) bird species, and an important part of avian direction-finding abilities.

ResearchBlogging.orgFalkenberg, G., Fleissner, G., Schuchardt, K., Kuehbacher, M., Thalau, P., Mouritsen, H., Heyers, D., Wellenreuther, G., & Fleissner, G. (2010). Avian Magnetoreception: Elaborate Iron Mineral Containing Dendrites in the Upper Beak Seem to Be a Common Feature of Birds PLoS ONE, 5 (2) DOI: 10.1371/journal.pone.0009231