Beschreibung:
<jats:p> Deer mice (Peromyscus maniculatus (Wagner, 1845)) in the Kananaskis Valley were monitored from 1985 to 2003 by livetrapping, and first parturition dates were compared among years and examined in relation to spring weather. On average, first litters were conceived on 2 May, well after the winter snowpack melted (19 March) and just before average temperatures reached 0 °C (8 May). First parturitions took place on 26 May, when average temperatures were above freezing. The average temperature at the time of conceptions (late April – early May) declined by approximately 2 °C, and the date that the average temperatures reached 0 °C was 11 days later, between 1985 and 2003, with potential effects for summer phenology. Spring temperatures, but not snowfall, were related to the El Niño Southern Oscillation index. The initiation of breeding by deer mice was variable among years, but was not related to snowfall or temperature and did not change, on average, between 1985 and 2003. The decrease in spring temperatures had no noticeable effects on breeding success. We conclude that photoperiod may be a primary cue for the initiation of spring breeding and that food resources over winter may explain the among-year variation in the initiation of breeding. </jats:p>