Swallows

Swallows enter the account through an unexpected act by a nestling placed in a foreign nest: a newly hatched swallow repeatedly removed magpie eggs using a maneuver closely resembling the cuckoo’s. The observation did not settle what the behavior was for. Instead, it opened competing explanations—an inherited defense against brood parasites, a response to oversized or unwanted eggs, or a possible form of competition among siblings—and became a case study in the difference between observing a behavior and establishing its evolutionary history.

The Spanish Nest Observation

Researchers F. Alvarez, L. Arias de Reyna, and H. Segura were investigating whether potential foster parents could detect intruders, including cuckoo eggs and chicks. Their experiments involved introducing eggs and chicks of cuckoos and other birds, among them swallows, into magpie nests. The swallow’s appearance in the account is therefore experimental: it was placed in a magpie nest as part of a comparison, not encountered there as an ordinary feature of its life.

The day after a baby swallow had been introduced, the researchers found a magpie egg on the ground beneath the nest. Because it remained unbroken, they returned it to the nest and watched. The nestling pushed the egg onto its back, balanced it between its wing-stubs, and walked backward up the side of the nest until it toppled out. When the researchers put the egg back again, the swallow repeated the same action. The significant point was not merely that an egg disappeared, but that the young bird used a coordinated and recognizable method to remove it.

A Behavior Without an Established Function

The authors reported the action without claiming to have explained it. One possibility was an evolved defense against cuckoos: swallows might have inherited a form of counterattack that turns a brood parasite’s egg-ejection method against the intruder. The book notes that swallow nests are not normally parasitized by cuckoos and suggests that this might be connected, while keeping the proposal conjectural. In that interpretation, the magpie egg may have been ejected incidentally because it was larger than a swallow egg, rather than because the nestling recognized it as a magpie’s.

The hypothesis raises a difficulty of its own. If a swallow nestling can distinguish an unusually large egg from an ordinary swallow egg, the adult swallow might also be expected to do so—and could remove a cuckoo egg more easily. A second proposed function, the removal of addled eggs or debris, encounters a similar objection: adult swallows would seem better equipped to perform that task than weak, newly hatched young. The mismatch between the apparent complexity of the behavior and the nestling’s helplessness is what leads the book to entertain alternatives, not what proves any one of them.

The Fratricide Hypothesis

A more unsettling possibility is that egg-ejection has nothing to do with cuckoos. The behavior might occur among swallow nestlings themselves, with the first to hatch removing an unhatched sibling before it can compete for parental care. The evidence cited for this idea is indirect: the experimental swallow ejected magpie eggs, but the account does not describe an observed swallow removing the egg of a sibling in its own nest. The author explicitly acknowledges that the hypothesis is improbable and offers no convincing explanation for why such behavior, if it occurred, had not been observed.

The possibility is made somewhat less remote by a geographic qualification. Swallow populations differ: the Spanish race is described as differing in certain respects from the British one, and it had not received the same degree of intensive observation. The book therefore allows that fratricide might have been overlooked in the Spanish population, but does not present this as a discovered fact. The experimental episode remains a prompt for a testable question, not confirmation that swallow nestlings routinely eject one another.

Parental Investment and Sibling Conflict

The hypothetical benefit is framed through competition for parental investment. If a parent’s resources are distributed among five young, removing one potential competitor could increase a surviving nestling’s share from one-fifth to one-quarter; removing another could raise it to one-third. A swallow nestling would not need to eliminate every sibling, as a cuckoo nestling seeks to do with its foster competitors. Because full siblings share genes, the potential advantage of disadvantaging a sibling is constrained by their relatedness, but the book argues that this difference is one of degree rather than an absolute divide between harmless family life and the cuckoo’s more extreme competition.

That argument places the swallow case within a broader conflict between parents and offspring. A parent may have an optimal clutch size from the perspective of distributing care, while an individual young bird may benefit from a smaller number of competitors, provided it remains among those receiving care. The possible act of egg-ejection thus illustrates how one family can contain competing interests: the parent’s allocation of care, each offspring’s share, and the evolutionary consequences of behaviors that affect relatives as well as the actor.

Evidence and Interpretive Limits

The book cautions that chosen animal stories are illustrations, not serious evidence for general claims by themselves. The Spanish observation is unusually vivid, but its vividness cannot determine whether the behavior is an anti-cuckoo adaptation, a response to a large egg, a cleaning behavior, or a form of sibling competition. Each explanation requires a different account of how the behavior could operate in ordinary swallow life, and the available observation leaves those possibilities open.

That uncertainty serves the book’s larger argument about animal behavior and evolution. A behavior can look purposeful without its history or function being self-evident; proposed functions must be distinguished from what researchers directly observed. The swallow nestling’s repeated use of the backward egg-balancing maneuver supplies a concrete case of unexplained behavior, while the competing hypotheses show how questions about inheritance, parental care, and competition can be developed without treating speculation as established natural history.