Acta Univ Latv (2009) 753: 81–87

Onset of breeding in Tawny Owl Strix aluco in eastern Latvia

Abstract

Factors influencing onset of breeding in Tawny Owl in eastern Latvia were determined by examining time of egg laying in 344 nests during 1991 - 2008. Data on owls breeding in natural cavities and nestboxes in six districts (Aizkraukle, Balvi, Gulbene, Ludza, Madona, Rēzekne) were used. Capture and ringing of females was performed at 132 nests and morphological parameters and age of the female were recorded. Data on small mammal abundance at the Teiču Nature Reserve (Madona District) and open-access information on weather conditions at the Zīlāni weather station (56° 31' N; 25° 55' E) were used for analyses. Onset of breeding between years varied substantially: the mean date of the laying of the first egg ranged between March 13 (2002) and April 14 (1996). Range of the recorded extremes is even greater: February 20 (2002) and April 30 (1998 and 2001). There was no trend of the timing of the laying of the first egg during the study period of 18 years. Female characteristics (age, weight, wing-length) had no effect on the onset of breeding. Also small mammal abundance had no effect neither in previous autumn, nor in summer of the breeding season. The only two factors having a statistically significant impact were mean air temperature in February (positive effect; r2 = 0.38; p = 0.008) and March (positive effect; r2 = 0.55; p = 0.0005), as well as depth of snow cover in February (negative effect; r2 = 0.53; p = 0.0009) and March (negative effect; r2 = 0.51; p = 0.0013). Mean air temperature and snow depth in January had no effect. We conclude that Tawny Owls benefits by warm springs with little snow cover. These factors may explain the range expansion of the species in the past and, probably, future.