Do exotic squirrels affect native birds in exotic forests of the Pampas?

MESSETTA, L.; ACOSTA, D.; ALMADA CHAVEZ, S.; MIRANDA, L.; MUFATO, N.; MILESI, F.A.; GUICHÓN, M.L.

Mendoza

Congreso; 10th International Mammalogical Congress; 2009

Biological invasions are a major threat to native species. The red bellied tree squirrel Callosciurus erythraeus was introduced in Luján, Argentina, in 1970 and it spread rapidly. Local inhabitants claim a negative effect on native birds; squirrels can affect birds through nest predation, interference and competition for food and nest sites. We aimed to compare bird richness and abundance in forested areas with and without squirrels. We selected 28 sites, 14 inside and 14 outside the distribution area (700 km2 in 2004) of the main population of these squirrels; half of these were paired sites of similar structure and tree composition. We did 4-5 point counts in each site during spring-summer 2008-09, recording all bird individuals identifiable to species level that were seen/heard during 8 min, <10-20 m from the observer. Points were at least 100 m apart and were visited twice, in the early morning. Species richness was rarefied to the smallest number of point counts per site. We did not find statistically significant differences in bird abundance (one-tail t-test n=14 and paired t-test n=7: t>-1.18 p>0.12). Species richness only differed in paired sites with (mean+SD=8.27+2.65) and without (9.88+3.54) squirrels (one-tail paired t-test: t=-2.09 p=0.04 n=7) (one-tail t-test: t=-1.02 p=0.16 n=14). To assess if squirrels do consume bird eggs and if they have any egg-size preference, we conducted cafeteria experiments in a 3x2x2m outdoor cage, offering fruit and three eggs (large, medium, and small) to locally-trapped, single squirrels (n=25). Squirrels consumed the yolk (n=2) or only broke the shell (n=4) of medium and small eggs. Our sampling effort prevents analyzing an effect on rare species, which could be detected with more intensive fieldwork. We have no conclusive evidence of a negative effect of squirrels on native birds, however, our results suggest that species richness may decline.