<b>Egg recognition in barn swallows (</b><b><i>Hirundo rustica</i></b><b>) is independent of brain size</b>

Active nests were inspected (at approximately three-day intervals) to monitor breeding progress. Upon detection of eggs, nests were monitored daily and designated as experimental nests. One to two days after clutch completion, we placed a blue model egg (n = 49) representing a parasitic egg into each experimental nest.

Model eggs were made of clay and were comparable in size to barn swallow eggs (Figure 1) but slightly heavier (Yang et al. 2015b). Because the model eggs were placed in the same way in all nests and at roughly the same time of day, egg scent, time of placement, other factors were not considered relevant in the study design. As barn swallows are grasp ejectors, minor differences in egg mass have been demonstrated not to affect rejection behavior (Yan and Liang 2024, Yan et al. 2025).

Experimental nests were checked on days 2 and 3 after model egg placement. If the model egg remained in the nest on day 6 with no signs of pecking and the host had not abandoned the nest, the outcome was recorded as “acceptance”. If the model egg showed pecking damage or disappeared while the host had not abandoned the nest, the outcome was recorded as “rejection” (Yan and Liang 2024).

Nests that were predated, destroyed, or abandoned within 6 days were excluded from the analysis (Liang et al. 2016).</p><p><br></p><p dir="ltr"><b>Capture and measurement</b></p><p dir="ltr">After egg recognition experiments were completed, we captured female barn swallows from experimental nests using mist nets once nestlings reached 6–8 days of age. Phenotypic measurements of female barn swallows were taken in a standardized manner following (Møller 2010), including body length, wing length, and tail feather length (to the nearest 0.1 cm), tarsus length measured with a digital caliper (to the nearest 0.01 cm), and body mass measured with a Pesola electronic scale (to the nearest 0.1 g).</p><p dir="ltr">Prior research on barn swallows has demonstrated a high correlation between female head size and brain volume and has shown that body size is a poor predictor of head size (Møller 2010).

Hence, we estimated head size by measuring head length (L), head width (W), and head height (H). Head length (L) was defined as the straight-line distance from the base of the bill to the posterior margin of the head; W as the maximum width of the head; and H as the maximum height from the top of the head to the underside of the jaw at the point of maximum width (Møller 2010). All measurements were taken by the same observer (KL).</p><p dir="ltr">Head size was calculated using two methods to facilitate comparability with existing research.

The first, head size = L × W × H (mm³) (Møller 2010), predicts 99.5% of the variance in barn swallow brain mass. The second, head size = W × H (mm²) (Liu et al. 2025), represents the optimal cross-species predictor of brain mass.</p><p dir="ltr">Barn swallows (Hirundo rustica) offer an ideal model system to resolve this controversy, given the validated, exceptionally strong correlation between head size and brain volume in this species (r² = 0.995).

We conducted egg rejection experiments in 49 barn swallow nests (by placing a blue, non-mimetic model egg in each nest), and measured head size of female swallows (using two calculation methods) along with body morphology metrics. Egg acceptors (n = 27) and rejecters (n = 22) did not differ significantly in terms of head size and showed a negligible effect (Cohen’s d < 0.13). The two groups also showed no significant differences in body morphology, ruling out body size as a confounder.

Leveraging the validated relationship between head size and brain volume in barn swallows, these results provide strong evidence that egg recognition and egg rejection in this species is unrelated to overall brain volume. This finding addresses the limitation of unvalidated proxies in prior studies and offers reliable evidence that this specific cognitive task operates independently of brain size. Our results suggest that egg recognition may rely on simple visual contrast mechanisms or functional specialization of particular brain regions rather than on the whole brain expansion, deepening our understanding of the evolution of avian cognitive abilities.</p>