“Two swallows perching on a wooden beam, a male and female lightly dancing…” – this is an enchanting verse from “A Song for the Swallows”, composed by Tang dynasty poet Bai Juyi. This poem celebrates the greatness of parental nurturing, and even elementary school pupils easily recite this catchy phrase. The poem vividly depicts the challenges of parenthood through the portrayal of the swallows nurturing their young. Although the poet uses the imagery of two swallows raising their young to depict the selfless dedication of parents to their children, he displays a keen understanding of how swallows raise their offspring in his description. Let us now delve deeper into the poem’s contents from a biological perspective.
Night and day four nestlings grow, persistent begging calls for food
Research indicates that as the number of nestlings in a swallow's nest increases, the average body weight of the nestlings decreases. This highlights the inherent need for competition among the nestlings within the same nest to secure their share of food resources. The most direct way for them to compete for parental feeding is through their persistent begging calls for food. Interestingly, researchers have discovered that young swallows adapt their begging calls to the intensity of their hunger. They exhibit remarkable changes such as emitting higher-pitched calls and prolonging their vocalizations.
Due to the physical limitations of the nestlings, the ability to produce extended high-pitched calls serves as an indication of their robust growth. Swallow-parents instinctively allocate their precious food resources to the healthier offspring, so nestlings with higher-pitched calls tend to attract parental feeding. It's no wonder there are “persistent begging calls for food”!
Fresh worms are hard to catch, hungry mouths are left unsatiated
The “mouth” of the young swallow is its beak. One remarkable feature of the beak is the vibrant soft tissue that surrounds the edges of the gape flange. This visual enhancement makes it more conspicuous, enticing parental feeding. It proves especially effective in dimly lit environments.
Further research indicates that swallow-parents have a tendency to allocate food to nestlings with brighter gape flanges. Researchers speculate that during the early stages of nestling growth, the brightness of the gape flange increases in proportion to the intake of carotenoids. Higher levels of carotenoids reflect stronger immunity of the nestlings. As a result, swallow-parents prefer feeding offspring with enhanced immune capabilities to increase survival rate. So, opening their “hungry mouths” wide is the nestlings’ way of showing their parents how healthy they are!
In a flash flying in and out, in fear of hunger within the nest
How arduous is the parental task of raising young swallows? In a study conducted with a swallow nest housing four nestlings, researchers meticulously documented the feeding frequency of the parents throughout a day. The results revealed a remarkable dedication. The swallow-parents tirelessly shuttled back and forth from dawn until dusk, continuously providing sustenance to their offspring. Their relentless efforts covered more than 14 hours each day.
The nestlings need more and more energy as they grow, which means the parents need to feed them more frequently. On sunny days, the peak feeding time for swallow-parents occurs between 11 am and noon. This is likely because their primary prey is most active during this period. During this time, the feeding frequency can surpass 40 times per hour, which means that on average, swallow-parents have to complete a hunting and feeding cycle every minute and a half. The poet was not exaggerating in his depiction of “in a flash flying in and out”. The tireless efforts of swallow-parents truly showcase their remarkable dedication!