Invasive ants can disrupt ecosystems by altering arthropod populations, directly and indirectly affecting plant communities, and harming vertebrate species.  Some species of invasive ants can have negative and sometimes catastrophic effects on colonial nesting seabirds.  The tropical fire ant (Solenopsis geminata) and the big-headed ant (Pheidole megacephala) are among 45 invasive ant species introduced to the Hawaiian Islands.  Both species have been observed attacking nesting seabirds, but there has never been an attempt to determine the frequency of these attacks or how they affect seabird growth and survival.  We used the experimental control of ant numbers to investigate the influence of invasive ants on seabird hatching success, growth-rate and nesting success.  Our study was conducted on two pairs of small (<5 ha) islets similar in topography, geology, vegetation composition, bird species composition and dominant ant species.  Following one year of baseline data collection, P. megacephala was eradicated and S. geminata densities were reduced on one of each pair of islets using the pesticide hydramethylnon.  There was no difference in hatching success, growth, or fledging success of shearwaters on the pair of islets dominated by P. megacephala.  On islets dominated by S. geminata, about half as many injuries (37% vs. 100%) to seabird chicks were observed on the treated vs. the untreated islet following ant control.  An increase in hatching and fledging success coincided with reduced ant numbers on the treated islet.  Just over five percent of shearwater chicks sustained severe injuries from fire ants.  These individuals weighed substantially less than uninjured chicks and, although four of the six survived until the last sampling period, they remained in their burrows after other chicks of the same age had already fledged.  Colonial nesting seabirds drive ecosystem processes and losses or reductions in nesting birds can have large-scale effects on ecosystems.