The Colonization of the Western Yellow-Legged Gull (Larus michahellis) in an Italian City: Evolution and Management of the Phenomenon

Abstract

The phenomenon of gull urbanization affects several species of Laridae and many cities around the world. In European cities, there are several species that have colonized urban areas. The yellow-legged gull Larus michahellis and the herring gull L. argentatus, however, are species that have been more successful in this process. The phenomenon, which started in the early twentieth century, has grown rapidly and gull urban populations have now reached high numbers and densities. This is to the point that it not only represents an interesting ecological phenomenon to study but a management problem for municipal administrations. Gulls represent a discomfort to city dwellers, especially during their breeding season. In Italy, the phenomenon has been monitored for some years, giving way to interesting studies in Trieste (north-east) of population control methods. To inform municipal administrators and the public about the biology of the species, their urban colonization phenomenon, and subsequent strategies for the prevention of this phenomenon, a consortium of stakeholders that included municipalities and ornithologists, joined to create a technical informative document.

Appendix: Experiments for the Management of the Species in the City of Trieste

The species in the urban area of Trieste has been breeding since 1987 and is annually monitored using standardized census methods.

The number of pairs has progressively increased and in 2018 the breeding pairs counted totalled 571 (estimated 600–620), with an average annual increase of 14.6% recorded in the period 1988–2018 (Fig. 7.16), levelling off at 4.3% in the period 2000–2018 (Fig. 7.17).

Fig. 7.16

Trend of the breeding population in Trieste

Fig. 7.17

Percentage trend of the growth of the breeding population in Trieste

Since the end of the 1990s, the municipal administration has undertaken an experimental campaign aimed at limiting the breeding population, carried out on sample pairs, through birth control (1999–2004) (Benussi 2005) and removal from reproductive sites (2014–2018).

Recorded below is the information obtained in a total of 11 years of experimentation using different types of intervention:

• (a) Drilling of eggs (1999, 2000, 2001, 2004) (Fig. 7.18). A single intervention carried out on 60 sample pairs: 29 (48.3%) continued incubation for a greater number of days and then definitively abandoned the site and moved elsewhere; 26 (43.3%) laid again on a new nest built in the immediate vicinity, successfully hatching their eggs; and 5 (8.3%) immediately abandoned the clutch and moved elsewhere from the site; the data obtained shows that, often but not always, pairs leave the nest to build another in a new site, often nearby, and thus breed successfully. The experimentation was considered ineffective for the purposes intended.

• (b) Removal of eggs and destruction of the nest (2000, 2001, 2004, 2014–2018). Out of 210 sample pairs 132 (62.8%) laid again within 15 days at the same site in a new nest but generally fewer eggs (63% one egg, 31% two eggs, 6% three eggs), demonstrating, however, a high percentage of clutch replacement (up to three times) and strong attachment to the site; 78 pairs (37.1%) permanently abandoned the site, probably moving elsewhere. This experimentation was to be considered effective if carried out with interventions which have to be repeated periodically until the pair leaves the roof. The problem at the site in question is resolved for one season (aggressiveness, disturbance, damage) but the gulls presumably move elsewhere and it is quite possible that they will reoccupy the site the following season. Such interventions do not favour a decrease and a numerical control of the species.

• (c) Administration of bait treated with the contraceptive substance ‘Glisol-T’ (1999). This partial experimentation revealed difficulties in the preparation and dosage of bait, difficulties in the distribution and control of its balanced consumption and high costs. Out of six ringed pairs, four (66.6%) did not breed successfully (infertile eggs), two (33.4%) reproduced normally but four out of six eggs in the nest were infertile (probably as a result of insufficient intake of the drug). This experimentation is not to be considered widely applicable on a large scale.

From the results obtained, it can be stated that the interventions implemented with the aim of containing the urban population have proven to be ineffective. It is therefore objectively impossible to prevent urbanization and nesting by gulls using cruelty-free methods.

Any containment through the reduction of hatching, if carried out on a substantial part of the urban population, may represent a way to slow down (but not stop) the process of population growth. This avoids drastic high impact measures such as euthanizing juveniles and adults, which targeted management would recommend.

The management of a population characterized by a marked dynamism such as that of the yellow-legged gull must plan for continuous action over a period of time (5–8 years). Occasional interventions are ineffective and nullify the results obtained and constitute a waste of public resources.

Combined actions are essential for the drastic reduction of available trophic sources from anthropic sources such as landfills, rubbish bins and food provided directly by humans (‘adopted’ gulls). Therefore, civic education projects are as important as public awareness campaigns.

It is also difficult to predict the evolution of the city colonies due to the difficulty of quantifying the potential capacity of the urban ecosystem. A further limitation is caused by the impossibility of predicting the immigration of other gulls from surrounding areas.

Fig. 7.18

Drilling eggs did not provide the expected results in limiting nesting owing to the difficulties in reaching all urban nests (E. Benussi)