Abstract Variation in traits expressed during social interactions can be attributed to direct individual effects (DIEs) of the focal individual’s identity and indirect individual effects (IIEs) of social partner identity. When of genetic origin and covarying with direct effects, indirect effects affect the expressed variation upon which selection can act; this can explain why evolution is slower or faster than predicted by classic theory. Little is known about how DIEs and IIEs covary across traits, even though such relationships should affect micro-evolutionary trajectories. We also do not know whether IIEs change over time or contexts. Here we tested game theoretical predictions of producer-scrounger tactic use during social foraging games in wild house sparrows (Passer domesticus). We used automated high-throughput phenotyping, where we assayed individuals repeatedly against different social partners. We provide evidence for IIEs and DIEs in producer-scrounger behaviour, and high cross-year repeatability. Both IEEs and DIEs were correlated among traits: producers depressed producing—but elicited increased scrounging—in others, and vice versa. This structure likely strongly constrains behavioural evolution. Indirect effects decreased the phenotypic variation in both behaviours. IEE-DIE correlations among and within traits may thus explain the long-term maintenance of stable social foraging strategies.
Groot et al. (2026) studied this question.