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This research aims to analyse the biodiversity of small birds (passerines) and its trends in a Western Mediterranean semi-arid wetland by means of captures for ringing during fourteenyears (1991-2014). The palustrine species outnumbered non-palustrine in individuals but not in species. The diversity of passerines is about two-fifth parts of the total diversity of birds of the wetland. Fourteen-palustrine species outweigh as dominants in number and biomass, belonging to the family Acrocephalidae or Reed warblers. The heavyweight of non-palustrine species is exerted by a Leaf warbler, the Common Chiffchaff Phylloscopus collybita. The biodiversity falls significantly over time with losses of 0.072 nats per year. Similar trends identified for the richness, which leaves losses of 1 species per year, ranging from 9 to 10 species gains and losses for the overall passerine's assemblage. Diversity and richness correlate weakly and negatively with rainfall and positively with temperature, accounting from 15.1 to 20.4% of the variation in diversity of passerines, respectively. The loss of diversification of invertebrate preys due to global warming is considered a probable effect of the species decline.

One of the most important direct effects of climate change is the continuous increasing of droughts, particularly in arid and semiarid regions. By the use of temporal climatic datasets of fourteen years (1991-2004) and information arisen from ringing captures of small reed passerines it is demonstrated that drought cycles in a wetland of SE Spain, immerse in a semiarid landscape, had not effect on the two small passerines that conform the gross of the reed-bed bird's assemblage, in terms of abundance and biomass. Weak breeding seasons caused by drastic environmental events may have a synergic effect on the different flexibility of the moult strategies and morphological adaptations of well adapted individual body-sizes and hence not affecting them. Finally, novel studies focused on genomic sequence and environmental change in non-model species like birds could offer future prospects to find drought-resistant genes in small individual birds associated to terrestrial aquatic bodies in especially sensitive areas as the semiarid landscapes.

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