Large-scale variation in the temporal patterns of the frass fall of defoliating caterpillars in oak woodlands in Britain: implications for nesting woodland birds

Capsule Frass fall was later and of longer duration in woods to the north and west of Britain compared with those in the southeast. Aims Defoliating caterpillars are a major food resource for woodland breeding birds and our aims were to quantify large-scale patterns in the timing and duration of the spring peak in abundance of these caterpillars in oak woodlands in Britain. Methods We deployed traps to collect caterpillar frass at regular intervals through spring in 19 oak woods distributed through England, Wales and Scotland. Models of the temporal patterns of the rate of frass fall were used to explore relationships with geographic variables and average local temperature. Results The date of peak frass fall in 2010 ranged from 20 May to 18 June and was significantly related to altitude, latitude and local April-May temperature. The duration of the peak ranged from 20 to 53 days and was correlated with the date of the peak and April-May temperature. Limited data from 2008 and 2009 indicated considerable between-year variation in the date of the peak, which was consistent with the relationship with local temperature found in 2010. Conclusions The date of peak frass fall was later and the duration of the peak longer in the north and west of Britain compared with the southeast which will have considerable implications for nesting woodland birds such as Pied Flycatchers. The date of the peak was well modelled by local April-May temperature offering the prospect of good predictive models. However, the duration of the peak was less well modelled by local temperature and may be determined by other factors.