Highly aroused or scared animals may produce a variety of sounds that sound harsh and are somewhat unpredictable. These sounds frequently contain nonlinear acoustic phenomena, and these nonlinearities may elicit arousal or alarm responses in humans and many animals. We designed a playback experiment to elucidate whether specific nonlinear phenomena can elicit increased responsiveness in great‐tailed grackles (Quiscalus mexicanus). We broadcast two control sounds (a 0.5‐s, 3‐kHz pure tone and the song of tropical kingbirds (Tyrannus melancholicus) and three test sounds that all began with a 0.4‐s, 3‐kHz pure tone and ended with 0.1 s of either a 1‐ to 5‐kHz band of white noise, an abrupt frequency jump to 1 kHz, or an abrupt frequency jump to 5 kHz. In response to these three nonlinear phenomena, grackles decreased their relaxed behavior (walking, foraging, and preening) and increased looking. A second experiment looked at the rapidity of the time course of frequency change and found that the abrupt frequency jump from 3 to 1 kHz, as opposed to a gradual downward frequency modulation over the same bandwidth, was uniquely arousing. These results suggest that while nonlinear phenomena may be generally evocative, frequency jumps may be the most evocative in great‐tailed grackles. Future studies in other systems can evaluate this general hypothesis.

Cyanobacteria, an increasingly important epiphyte on macroalgae and seagrass, have been shown to have strong effects on its hosts; this association has been identified as a driving mechanism that maintains algal blooms on coral reefs. We examined both the costs and benefits of epiphytism on 2 algal congeners of Halimeda (H. tuna and H. opuntia), both of which are abundant members of tropical reef communities in the Caribbean. To evaluate potential benefits of an associational defense as well as costs to growth, we manipulated herbivore access to (uncaged/caged) and cyanobacteria presence on (epiphytized/cleaned) 2 species of Halimeda on shallow patch reefs in Belize and measured change in branch length and segment number after 10 (H. tuna) and 5 (H. opuntia) days. Cyanobacterial epiphytes did not serve as an associational defense from herbivory as there were no differences between caged and uncaged treatments for either response variable. The presence of cyanobacterial epiphytes did not affect the growth of branches or net generation of new segments, demonstrating there was also no cost to growth. The robustness of both species of Halimeda to epiphytism contrasts strongly with recent research that found strong effects of epiphytes on several other species of tropical algae. Our results may be attributed to the unique characteristics of Halimeda, a heavily physically and chemically defended algal genus, and the shallow nature of the patch reefs reducing the potential for significant light limitation. These findings suggest that close interactions such as epiphytism may not be as generalizable as originally assumed; studies must consider differences among host species, as this may lead to a better understanding of community-wide effects.