Paragraph 2: Further evidence for the costs of begging comes from a study of differences in the begging calls of warbler species that nest on the ground versus those that nest in the relative safety of trees. The young of ground-nesting warblers produce begging cheeps of higher frequencies than do their tree-nesting relatives. These higher-frequency sounds do not travel as far, and so may better conceal the individuals producing them, who are especially vulnerable to predators in their ground nests. David Haskell created artificial nests with clay eggs and placed them on the ground beside a tape recorder that played the begging calls of either tree-nesting or of ground-nesting warblers. The eggs “advertised” by the tree-nesters' begging calls were found bitten significantly more often than the eggs associated with the ground-nesters' calls.
4. Paragraph 2 indicates that the begging calls of tree nesting warblers
○ put them at more risk than ground-nesting warblers experience
○ can be heard from a greater distance than those of ground-nesting warblers
○ are more likely to conceal the signaler than those of ground-nesting warblers
○ have higher frequencies than those of ground-nesting warblers