The occurrence of fault bars in the plumage of nestling Ospreys.

We document the occurrence of fault bars in a population of nestling Ospreys Pandion haliaetus under natural conditions. Ospreys had an average of 9.9 fault bars on their rectrices, however variation was large. Fault bar formation declined linearly with age and increased symmetrically from outer to inner rectrices. Fault bar incidence is consistent in all plumage groups and those groups most essential for flight are least affected. We also examine fault bar occurrence in relation to two hypotheses; food shortage and handling effect. The former is investigated indirectly by comparing the number and intensity of rectrix fault bars in nestlings to their feeding rank and to their brood size. Neither variable has a significant effect on fault bar severity. The possibility of weather-mediated food shortage is examined, but no strong effect of weather is detected. The role of handling is assessed by comparing the number and intensity of fault bars in nestlings experiencing different numbers of nest visits. Nestlings visited repeatedly had more fault bars providing support for the handling hypothesis. Analysis of the timing of fault bar formation with respect to nest visits is suggestive as further evidence for the role of handling.

Machmer M.M., Esselink H., Steeger C. & Ydenberg R.C., ARDEA 80 (2): 261-272.