Population dynamics of a colony of little egrets Egretta garzetta at an estuary in northern Spain

Doi: http://dx.doi.org/10.13157/arla.61.2.2014.285

Authors: Aitor GALARZA and Juan ARIZAGA

E-mail: jarizaga@aranzadi-zientziak.org

Published: Volume 61.2, December 2014. Pages 285-296.

Language: English

Keywords: herons, Iberia, population size, survival, Urdaibai and waterbirds


The population of little egrets Egretta garzettain Spain is over 20,000 adult breeding birds, making it one of the largest in Europe. Apart from its population size and population trends, the parameters associated with the dynamics (e.g., survival) of the species in Spain are virtually unknown. Our aims were to develop models to assess (1) the colony growth rate, and (2) apparent survival rate of a colony of little egrets breeding in northern Iberia. We used capture-recapture data of little egrets ringed as chicks within the colony, of normally < 30 adult breeding pairs, over a 14-year period starting in 1999. Colony size was observed to be increasing in a linear tendency broken by specific catastrophic events: a very strong hailstorm in 2004 and a pair of peregrines Falco peregrinusthat killed several adults in 2005. By 2012, the colony had still not reached the size that it was before the decrease, so it can be concluded that sporadic catastrophic events can have a significant effect on colony size and subsequently population size, especially in small colonies. Annual apparent survival (±SE) was constant and differed between age classes (first-years: 0.15 ± 0.05; adults: 0.78 ± 0.06). Our survival estimate was relatively high compared with other little egret populations, especially for adults. This result, however, may not necessarily apply to other colonies given our small sample size and the lack of data on other factors that also affect the dynamics of the study population.

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