State-dependent behaviour in breeding barn swallows (Hirundo rustica): consequences for reproductive effort

Area of Science:

• Behavioral Ecology
• Life-History Theory
• Physiological Ecology

Background:

• Intraspecific variation in reproductive effort is explained by life-history theory, with trade-offs between current and residual reproductive success.
• However, unexplained variation in reproductive effort persists, with underlying causes often unclear.
• Body state, specifically energy reserves, is investigated as a potential moderator of reproductive effort.

Purpose of the Study:

• To examine how energy reserves influence reproductive effort in female swallows.
• To determine if experimental manipulation of energy reserves affects reproductive behaviors.

Main Methods:

• Overnight heating and cooling treatments were used to alter dawn energy reserves in nesting female swallows.
• Reproductive effort was quantified by daytime energy expenditure (using the doubly labeled water technique) and the number of feeding visits.
• Changes in reproductive effort were measured in terms of energy expenditure.

Main Results:

• Female swallows experimentally manipulated to have higher energy reserves increased their daytime energy expenditure and feeding visits.
• Conversely, swallows with experimentally reduced energy reserves decreased these measures of reproductive effort.
• These responses demonstrate a direct link between energy reserves, life history, and behavior.

Conclusions:

• Energy supply and expenditure play a regulatory role in reproductive effort.
• Experimental manipulation of energy reserves directly influences reproductive behaviors, providing evidence for this regulatory role.
• Energy expenditure units likely carry context-dependent fitness costs and benefits, linking physiological state to life-history outcomes.