Barn owl health indices

We were able to establish baseline “norms” in the form of Shewhart charts for indicators that could be broadly categorised as indicators of change in:

(i) population demography because of altered recruitment, survival and mortality. Measures were:

proportion females;
proportion of first-year birds (having hatched in that calendar year or in the preceding calendar year);
proportion of deaths from starvation or disease.

(ii) nutritional status measures were:

body weight;
fat score;
condition index.

Descriptions of how these indices are calculated can be found in the supporting PBMS report.

Where the health index reports as a proportion of the population (e.g. percentage of females) then for statistical reasons the index is presented as the arcsine of the square root of the proportion. This value can theoretically range between 0 and 1.57, representing 0% and 100% of the population, respectively.

Proportions

The proportion of female and juvenile birds in each year's sample. Years where the value falls outside of the 95% prediction intervals may indicate change in the demographic structure of the species population.

Graph of year vs arcsin sqrt of proportion for female barn owls — ▲ Arcsin sqrt of proportion for female barn owls

Graph of year vs arcsin sqrt of proportion for male juvenile barn owls — ▲ Arcsin sqrt of proportion for male juvenile barn owls

Graph of year vs arcsin sqrt of proportion for female juvenile barn owls — ▲ Arcsin sqrt of proportion for female juvenile barn owls

Figure legend

Body Weight

Median postmortem body weight of birds each year for different sexes and ages of birds. The sample size in any single year was at least 5 birds to be included. Years where the value falls outside of the 95% prediction intervals may indicate change in the physical condition birds in that particular demographic which may indicate, in the case of years below the lower 95% prediction interval, that food availability may be restricted.

Graph of year vs median body weight for female barn owls — ▲ Median body weight for female barn owls

Figure legend

Putative cause of death

The proportion of birds that died of starvation or disease in each year. Years that fall below the lower 95% prediction interval indicates that there are fewer starved birds in the population (or demographic group), conversely an increase above the upper 95% prediction interval indicates there are more starved birds.

Graph of year vs proportion of female adult barn owls that were diseased or starved — ▲ Proportion of female adult barn owls that were diseased or starved

Graph of year vs proportion of female juvenile barn owls that were diseased or starved — ▲ Proportion of female juvenile barn owls that were diseased or starved

Graph of year vs proportion of male adult barn owls that were diseased or starved — ▲ Proportion of male adult barn owls that were diseased or starved

Graph of year vs proportion of male juvenile barn owls that were diseased or starved — ▲ Proportion of male juvenile barn owls that were diseased or starved

Fat score

The proportion of birds categorised as starved each year, based on their fat score. Years that fall below the lower 95% prediction interval indicates that there are fewer starved birds in the population (or demographic group), conversely an increase above the upper 95% prediction interval indicates there are more starved birds.

Graph of year vs arcsin sqrt of proportion of female adult barn owls that starved — ▲ Arcsin sqrt of proportion of female adult barn owls that starved

Graph of year vs arcsin sqrt of proportion of female juvenile barn owls that starved — ▲ Arcsin sqrt of proportion of female juvenile barn owls that starved

Graph of year vs arcsin sqrt of proportion of male adult barn owls that starved — ▲ Arcsin sqrt of proportion of male adult barn owls that starved

Graph of year vs arcsin sqrt of proportion of male juvenile barn owls that starved — ▲ Arcsin sqrt of proportion of male juvenile barn owls that starved

Figure legend

Condition

In the graphs below, years for which the average condition index (CI) is below the lower 95% prediction interval indicates that there are fewer birds with low fat reserves in the population (or demographic group), conversely an increase above the upper 95% prediction interval indicates there are more birds with low fat reserves. Low fat reserves means a bird is less likely to survive prolonged periods where they are unable to hunt successfully, or that the bird has starved. The condition index is used as a continuous measure of body condition, rather than categorical such as fat score.

The mean condition of birds per year was calculated for different ages and sexes. The Condition Index is calculated as follows:

CI = (BW - GW) / SD

where:

CI = Condition Index
BW = whole postmortem body weight (g)
GW = Wet weight of gizzard contents (g)
SD = Sternum diagonal, which is the distance between the posterior point of the sternum keel plate and the distal point of the clavical (mm)

Graph of year vs condition index of female adult barn owls — ▲ Condition index of female adult barn owls

Graph of year vs condition index of female juvenile barn owls — ▲ Condition index of female juvenile barn owls

Graph of year vs condition index of male adult barn owls — ▲ Condition index of male adult barn owls

Graph of year vs condition index of male juvenile barn owls — ▲ Condition index of male juvenile barn owls

Figure legend

Photo: Saxifraga: Martin Mollet