Refuge abandonment in a formerly harvested waterbird and the consequent formation of multi-species bird colonies

The intense human persecution of wildlife in the past forced many animal
populations to look for refuge in human-inaccessible habitats. With the
decline of direct persecution and the changes in the attitude of modern
urban societies towards wildlife during the last few decades, an
ecological process of abandonment of refuge habitats has become pervasive,
and many species make habitat choices deemed surprising presently, such as
colonizing sites that are more accessible to humans. We tested this
process in the Yellow-legged Gull (Larus michahellis) by analysing
long-term time series (1976-2022) of breeding pairs in 7 colonies in
southern Europe. Overall, Yellow-legged Gull numbers showed a strong
pattern of decrease during the study period, due to the recent closure of
open-air garbage dumps. However, while the number of pairs in colonies
located in human-inaccessible sites (islets with cliffs) declined rapidly,
new colonization, together with a monotonic linear population growth, was
detected in human-accessible sites, coinciding in time with the departure
from refuges. Moreover, we found that the new small Yellow-legged Gull
colonies acted as cores for the formation of multispecies waterbird
colonies, likely by heterospecific attraction, which provides an added
unexpected conservation value to Yellow-legged Gulls, traditionally
considered a pest species. To support this refuge abandonment idea, we
used GAMS to represent the evolution of breeding pairs of Yellow-legged
Gull colonies in inaccessible (Figure 1A), accessible (Figure 1B), and the
city of Vigo (Figure 1C) locations. To support the idea that Yellow-legged
Gulls act as attractors for other breeding sites, we have used GAMS to
represent the evolution of breeding pairs of three seabird species
(Haematopus ostralegus, Egretta garceta, and Larus marinus) and, in
addition, we have graphically represented the pattern observed in the
studied colonies.