The Grey-backed Shrike parents adopt brood survival strategy in both the egg and nestling phases

Great diversity exists in the parenting pattern of altricial birds, which has long been considered as an adaptive response to specific environmental conditions but not to their life-history style.

We examined the egg-laying and nestling-raising pattern of the Grey-backed Shrike (Lanius tephronotus) that breeds only once a year on the Tibetan Plateau. We compared the dietary composition to that of its sympatric competitor, the Brown-cheeked Laughing Thrush (Trochalopteron henrici) that breeds twice a year.

Female Grey-backed Shrikes produced a fixed clutch size of five, with increasing egg size by their laying sequence. The last offspring in the brood is disadvantageous in the size hierarchy because it hatches later. However, they had the largest fledgling body mass. These findings indicate that Grey-backed Shrikes adopt the brood survival strategy in both the egg and nestling phases. Moreover, males and females exhibit no sexual division in providing parental care as they made an equal contribution to the total amount of food delivered to their brood. This parenting pattern of Grey-backed Shrikes, as well as their dietary items, differ significantly from those of the Brown-cheeked Laughing Thrush.

We suggest that the differentiation in life-history style between sympatric competitors, rather than a behavioral response to specific environmental conditions, plays a decisive role in driving avian parenting strategy diversification.