Rapid morphological divergence of introduced Red‐whiskered BulbulsPycnonotus jocosusin contrasting environments

Abstract

New environmental conditions may impact on behaviour and morphology, and consequently affect population dynamics. Rapid response to new conditions is likely to be a key factor in species introduction success. The Red‐whiskered Bulbul Pycnonotus jocosus is an Asian species which, following accidental introduction only 30 years ago, has colonized the two climatically and ecologically contrasting sides of the island of Réunion in the Indian Ocean. We assessed the degree of morphological divergence of mature birds in 11 non‐seasonal and three seasonal characters, in windward and leeward sites. Our study showed (1) that sexual dimorphism existed, (2) that when sexual dimorphism is controlled, nine non‐seasonal characters differed significantly between windward and leeward sites, (3) that these non‐seasonal characters define groups of geographically proximal sites, especially in males, and (4) that classification according to the most indicative character, the bill, also clearly separated birds from windward and leeward sites. These results indicate very rapid morphological divergence, particularly in bill size, in an introduced bird species in fewer than ten generations. We suggest that differences in diet could partly explain this variability.

Stable isotope analysis to examine possible diet-morphology relationships among Red-whiskered Bulbuls introduced on Réunion Island

“…Traps with a captive decoy bird were used with fruit as bait, and cages were placed at a height of 2 m, mainly within orchards, on both windward (51 birds from Wa, Wb and Wc sites) and leeward (25 birds from La, Lb, Lc and Ld sites; Figure 1) sides of the island. The five morphological traits that best discriminate birds between leeward and windward sites were measured following Amiot et al (2007), i.e. wing length (WL, of the folded wing), tail length (TaiL, from the distal end of the uropygial gland to the tip of the largest rectrix), bill length (BL, from the feather base above the nostrils to the tip of the bill), bill depth (BD, width at the bill base) and bill height (BH, height at the bill base).…”

“…For some passerine birds, variation in morphological measurements has been demonstrated at the population level within only a few generations (Przybylo et al 2000, Kanuscak et al 2004. Amiot et al (2007) recently reported rapid morphological divergence among Red-whiskered Bulbuls Pycnonotus jocosus that spread from a single introduction point on Réunion Island. This bird, native to southern Asia, was introduced via escaped pet birds in the south-east (Saint-Philippe) of the island in 1972.…”

“…Notably, significant differences were found for the males' bill measurements. Variation in diet linked to contrasting environments on opposing sides of the island was suggested as a possible cause of the observed geographical divergence in morphology (Amiot et al 2007).…”

Body size changes in passerine birds introduced to New Zealand from the UK

“…Baker and Moeed 1979, Baker 1980, Yom-Tov et al 1986; but see Baker 1992) and elsewhere (e.g. Johnston and Selander 1964, 1971, 1973, Amiot et al 2007). On the other, most of the observed size changes observed in New Zealand are decreases.…”

“…Mathys and Lockwood (2011) also showed that five out of six exotic passerine bird species they examined on the Hawaiian islands showed morphological divergence across islands 80 to 140 years after introduction, although they do not present comparisons with the source population. Amiot et al (2007) demonstrated morphological divergence within an island by exotic red-whiskered bulbuls (Pycnonotus jocosus) over a period of around 30 years. Clines in body size have also been documented for exotic house sparrow and common myna (Acridotheres tristis) populations introduced to New Zealand in 1870, with a trend for larger body size in the northern parts of New Zealand (Baker andMoeed 1979, Baker 1980).…”