Table 1 Summary of tested hypotheses on the evolution of growth patterns in passerine birds

 Nest predation

High risk of nest depredation selects for fast growth and early fledging.

Faster growth rate under high nest predation. Growth rate of wings might be especially prioritized to enable escaping from predators.

Early fledging, potentially with less developed locomotor traits, under high nest predation.

 No of caregivers

Higher energy availability enables fast growth and better trait development.

Growth rate faster with more care-givers (Cooperative breeding > pairs > female-only care).

Early fledging and/or better development of locomotor traits with more care-givers.

 Clutch size

Energy requirements increase with both chick number and growth rate.

Slower growth in large broods due to energetic constraint.

Delayed fledging or less developed locomotor traits in large broods.

 Nest height

Ground nesting enables early fledging and thus escaping whole-brood depredation (for a given nest predation rate).

Fast growth in ground-nesters; slower growth in species nesting higher up.

Less developed locomotor traits in ground-nesters; better trait development and delayed fledging in species nesting higher up.

 Foraging substrate

Ground foraging enables early fledging and thus exploitation of resources by parents; vegetation and air foraging requires high-quality locomotor traits, esp. wings.

Fast growth in ground-foragers; slower growth in species foraging on vegetation and especially in the air (wings).

Less developed locomotor traits in ground-foragers; better trait development and delayed fledging in species foraging on vegetation and especially in the air (wings).

 Latitude

Tropical species have overall slow life history and invest into fewer, higher-quality young.

Slow growth in tropical species.

Better developed locomotor traits and delayed fledging in tropical species.