BODY-SIZE VARIATION IN AUDOUINS GULL LARUS-AUDOUINII - A DENSITY-DEPENDENT EFFECT

A significant decrease in the body size of Audouin's Gulls Larus audou inii breeding at the Chafarinas Islands is reported. The decrease in l inear measurements in the current breeding population ranged from 2.5% to 5.6% in males and from 0.61% to 4.4% in females. This was detected when assessing the reliability of a sex-discriminating function deriv ed for the same colony 13 years earlier. When applied to the current p opulation, this function failed to predict the sex of a large proporti on of males (44%). The relative decrease in mean size was significantl y greater in males than in females for culmen, nalospi and tarsus leng ths, while for bill depth at culmen, wing length and body mass the rel ative decrease was similar in both sexes, Since the extent of differen ces depended on sex, these differences cannot be attributed to a syste matic between-observer bias. Mean body size reduction might be either the result of a greater proportion of small breeding birds in the curr ent population, because of increased availability of nesting sites (co mpetition relaxation hypothesis), or an outcome of environmental facto rs affecting growth parameters (environmental constraint hypothesis). According to the first hypothesis, the changes observed would be assoc iated with higher variability values, Conversely, if the second is tru e, the degree of variability should be similar. Since there are no sig nificant differences in the degree of variability shown in the two dat a sets, our results support the second hypothesis. The environmental c onstraint acting via growth parameters is probably related to the incr ease in the number of Audouin's Gull breeding pairs while food availab ility was depleted. Our data suggest that changes in the duration of t he growth period, rather than in the growth rates themselves, are invo lved in the body size differences found.