Functional explanation of extreme hatching asynchrony: Male Manipulation Hypothesis

Area of Science:

• Behavioral Ecology
• Evolutionary Biology
• Ornithology

Background:

• Hatching asynchrony in birds is traditionally viewed as a brood reduction strategy, allowing parents to prioritize feeding efforts towards stronger offspring during food scarcity.
• Extreme hatching asynchrony presents a paradox: last-hatched nestlings have low survival rates, and their prolonged survival challenges the adaptive explanation of brood reduction.

Purpose of the Study:

• To propose and experimentally test the "Male Manipulation Hypothesis" as an explanation for the brood reduction paradox in asynchronous hatching.
• To investigate whether females induce asynchronous hatching to increase male provisioning effort and potentially extend the period of parental resource allocation.

Main Methods:

• Experimental manipulation of hatching synchrony in common blackbirds (Turdus merula) to create asynchronous and synchronous broods.
• Quantification of parental provisioning effort by monitoring the number of food items delivered per hour by both male and female birds.

Main Results:

• Male common blackbirds significantly increased their provisioning effort (food items per hour) in experimental asynchronous broods compared to synchronous broods.
• Female common blackbirds reduced their provisioning effort in experimental asynchronous broods, aligning with the predictions of the Male Manipulation Hypothesis.

Conclusions:

• The study provides experimental support for the Male Manipulation Hypothesis, suggesting that hatching asynchrony can be an adaptive strategy for females to increase male provisioning.
• This strategy may involve females manipulating males into greater effort while potentially conserving their own energy by reducing direct feeding contributions.