SEASONAL PATTERN IN AGE, SEX AND BODY CONDITION OF BARN OWLS TYTO-ALBA KILLED ON MOTORWAYS

The Barn Owl Tyto alba was the most common owl killed on motorways in northeastern France. The possible causes of this mortality and the age , sex and body condition of the road-killed birds in 1991-1994 have be en investigated. The number of birds killed on roads was highest in th e period from early autumn to late winter, i.e. during the nonbreeding period, and showed a pattern similar to that of the temporal differen ce between sunset, which varies with day length, and peak of traffic, the occurrence of which is constant throughout the year. An autumnal m ortality peak, concomitant with the postfledging dispersal, was mainly of immature birds, especially females. A second mortality peak in lat e winter was composed mainly of mature birds, with an equal proportion of males and females, From autumn to winter, there was no significant change in body mass in the different age and sex categories of birds killed on roads, except for mature males which had a significantly low er body mass in winter, From early autumn to late winter, the mean bod y mass of immature owls killed on motorways did not differ significant ly from that of captive immatures fed ad libitum. This suggests that t he immature birds were in good body condition, In contrast, the body m ass of road-killed mature females was significantly lower than that of captive mature females over the same time periods. In mature males in late winter, a drop in body mass in both road-killed and captive bird s suggests an endogenous seasonal phenomenon. Except for mature female s, Barn Owls killed on roads in 1991-1994 were in good body condition, This does not support the idea that only birds in poor body condition were killed, We conclude that the mortality of Barn Owls on motorways in autumn and winter was probably related to the concomitance between the peak of traffic and the onset of hunting activity and the large n umber and dispersal of immature individuals during the same period.