Black-backed gulls at Wellington Airport: daily, seasonal and weather trends in numbers and activity, and relationship to bird strike risk

Abstract

Observations of black-backed gulls were made from a position overlooking Wellington Airport twice a week for 54 weeks. For each gull observation at the airport the following were recorded: number of gulls, time of the event, age of gulls, activity of gulls, whether the gulls entered into the airport's airspace (airside), altitude and, where appropriate, flight direction. In addition counts of black-backed gull numbers on playing fields and parks, adjacent to the airport, were made twice each observation session. Any other bird species observed at the airport or on fields were also recorded. This information was then summarised into hours and combined with the appropriate weather data collected from Wellington International Airport Limited (WIAL) for each hour. Black-backed gulls were the most frequently observed bird at the airport, accounting for 68% of all species seen. A multiple regression analysis found that the factors influencing the number of gulls at the airport were hours from noon, season, surface conditions and temperature. Those factors influencing black-backed gull numbers on fields were found to be time of day, season and surface conditions. Daily trends in black-backed gull activity at the airport were evident, with a peak in the early morning, a lull at midday and a further peak in the evening. Flight was the most frequently observed activity (70%), which suggests that the airport was used as a transitional zone. There was a peak in blacked-backed gull field loafing and numbers crossing the airside zone during the early morning. Higher percentages of juveniles engaged in airside crossing than adults. Seasonal trends were also apparent in both the number of black-backed gulls at the airport and on fields, with more gulls observed during autumn and winter. The lower numbers observed during spring and summer could be related to breeding activities. Weather trends in black-backed gull activity at the airport and on fields were also evident. More gulls were present at the airport during Beaufort force one and five winds, and few gulls were present in winds force six and over. Numbers loafing on fields were greatest at low and high wind strengths, indicating gulls were conserving energy in unfavourable winds. Surface conditions had a strong influence on gull behaviour, with fewer gulls present at the airport during damp and wet conditions, but greater proportions were observed to cross the airside zone during damp and wet conditions. More gulls were observed on fields during damp and wet conditions. Wind direction did not have a strong influence on gull numbers, but perhaps becomes increasingly important as wind force increases. Temperature appeared to affect gull numbers with fewer gulls flying and more loafing during warmer temperatures. Over the past five years there has been a decrease in the number of black-backed gulls involved in strike and near miss incidents at Wellington Airport, which is believed to be related to colony reduction programmes and better waste management. Lower numbers of black-backed gulls at the airport during spring and summer correlated with a lower risk of a bird strike incident. There was a correlation between high numbers of gulls during the morning peak and a high strike risk, but not during the evening peak. Instead black-backed gull strike risk appeared to be mostly highly correlated with the number of gulls crossing the airside zone. Weather conditions did not appear to affect the bird strike risk, however more data are needed to confirm this.