Glen A. Tyler

Reedbed management and breeding bitterns Botaurus stellaris in the UK

A survey of habitats at 17 reedbed sites in the UK which currently hold or have recently held breeding bitterns Botaurus stellaris has shown significant differences between sites with decreasing and sites with increasing numbers of birds over the period 1979 to 1994. Those sites where bitterns have declined had significantly higher values for variables describing scrub encroachment and the presence of wet herb species than those where the bitterns have increased. A model describing bittern numbers in 1990 at the 17 sites included reed area and a measure of scrub encroachment as the significant predictive variables. It is concluded that hydroseral succession has been a key factor in the recent decline in breeding bitterns in the UK.

Sex differences in the behaviour and measurements of Corncrakes Crex crex in Scotland

Radio‐tagged Corncrakes which incubated clutches and/or accompanied broods of chicks were not observed to sing. Birds which sang did not give parental care. Dissection of two birds which had been radio‐tracked indicated that the first group were females and the second group were males. This confirmed previous indications that, in the wild, only females care for the eggs and young and only males sing. The length of the head and bill and the maximum chord wing length were both greater in males than females. A linear discriminant function including these two measurements sexed 96% of birds correctly. Functions fitted separately to data from each of two study areas sexed 93% of birds correctly when applied to birds from the other area.

Estimating the age of Corncrake Crex crex chicks from body weight and the development of primary feathers

Two methods for ageing Corncrake Crex crex chicks are described. Data on the body weight of chicks of known age, from broods of radio‐tagged females, were used to produce a formula for determining the age of chicks of unknown age. This method was, however, prone to error for chicks older than about 22 days after hatching. An alternative method, based on the ratio of the length of the waxy sheath on a primary feather to the maximum chord wing length, was developed using measurements of captive‐bred chicks of known age. This method performed well for chicks aged 22–45 days old, but required testing on wild chicks before application because of possible effects of captive rearing. Estimates of the age of wild chicks from both weight and primary development indicate that the two methods gave similar results and, hence, that the latter method was useful for chicks older than 22 days.