Abstract
The black-billed gull (Lams bulleri) is a small gull that nests in dense colonies on gravel-bedded rivers in southern New Zealand. This dissertation describes research undertaken in Southland in 2004-2006 that investigates black-billed gull population decline and its causes.
Historical ground counts of nests were calibrated with aerial photography of colonies and population trends analysed. The decline in breeding birds on Southland's four main rivers was equivalent to 6.0%/year (SE 1.8) or 83.6% in 30 years justifying the species' present listing as Endangered. Aerial monitoring is a poor index of the breeding population and major temporal variation was identified in counts.
All re-sighted second-year birds exhibited natal dispersal and -70% left the natal catchment. Forty one percent of birds banded as adults also dispersed to other catchments. Breeding dispersal is apparently unrelated to previous breeding success and the availability of the previous year's colony site, and dispersing birds did not move as groups. Southland's black-billed gulls constitute a single inter-mixing population.
Black-billed gulls selected sites on islands and banks according to availability, and selected rivers consistent with the number of gravel patches. Site vulnerability to floods varied significantly. Re-use of colony sites was positively influenced by use in previous years, the extraction of gravel, site stability and low weed cover. Widespread introduced weeds on colony sites preventing nesting and may have increased flooding risk. Colony size was related to colony area, which was related to gravel patch size.
Investigation of historical changes in breeding habitat availability in Southland indicated a major reduction in gravel habitat on the Lower Oreti River between 1976 and 2002 due to river works including gravel extraction. On the Waiau River, Manapouri Dam construction in 1970 initially increased gravel areas, since reversed in the mid Waiau, and caused a 75% decline in the number of islands. Gravel patch sizes are still declining on the Lower Oreti and possibly the Mid Waiau. Hundreds of gravel patches remain on Southland rivers.
Over 5000 nests in 21 colonies were monitored during incubation. Colony nest success was most influenced by colony location, averaging 90.1% (SE 2.1) on islands within rivers, and 66.8% (SE 2.2) on riverbanks, indicating that terrestrial predators exert the greatest influence on productivity. Breeding success, the mean number of fledglings produced per nest by colony, varied between 0 and 0.88 fledglings (mean 0.32, SE 0.08). Both parameters were positively related to colony size. The three smallest colonies failed to breed successfully, suggesting the presence of an Allee effect. Deterministic matrix models were used to investigate population trends using survival and productivity estimates for the closely related Kaikoura red-billed gull (L. novaehollandiae scopulinus). Adult survival, followed by breeding success and survival of first year birds, had the greatest influence on population projections. Improvements in most parameters are probably required to reverse the decline of Southland's black-billed gulls.
Predation and disturbance by introduced mammals and the native black-backed gull (L. dominicanus) had the greatest impact on black-billed gull productivity. Most human disturbance is insignificant compared to predator disturbance, but illegal shooting of large numbers of adult gulls has major impacts in smaller sub-populations. Damming and excessive water abstraction reduces island habitat due to lowering of flows. Major climatic events such as droughts, and chemical ingestion through a diet dominated by agricultural invertebrates, are potential major threats about which little is known. Management actions including the collation and analysis of all unpublished black-billed gull count datasets, a thorough test of the accuracy of aerial monitoring, trialling of decoys to attract colonies to nest on islands, predator control at bank colonies, targeted weed control on high quality sites, advocacy and education and further research are recommended.