A brood parasite selects for its own egg traits.

Area of Science:

• Behavioral Ecology
• Evolutionary Biology
• Ornithology

Background:

• Brood parasitic birds often lay eggs mimicking their hosts' eggs.
• This mimicry is typically attributed to host rejection of non-conforming eggs.
• However, parasitic females may also face selection to remove rival parasitic eggs from nests.

Purpose of the Study:

• To experimentally investigate the source of selection on egg size and shape in the greater honeyguide (Indicator indicator).
• To determine if selection pressures originate from host defenses or intraspecific competition among parasites.

Main Methods:

• Experimental manipulation of egg size and shape in greater honeyguide nests.
• Observation of host discrimination against experimental eggs.
• Assessment of parasitic female responses to experimental and control eggs.

Main Results:

• The commonest host species did not discriminate against experimental eggs differing in size and shape.
• Laying female greater honeyguides preferentially punctured experimental parasitic eggs over host or control eggs.
• This behavior was observed in nests where multiple parasitism is common.

Conclusions:

• Selection on greater honeyguide egg morphology is primarily imposed by interference competition among parasitic females.
• This competitive pressure, rather than host defenses, likely drives egg trait evolution in this brood parasite.
• The findings highlight the role of intraspecific competition in shaping brood parasite adaptations.