Parents Adjust Food Allocation, Not Foraging Strategy, in Storm-Petrels
A new study using a bespoke burrow-scale monitor reveals how Leach’s Storm-Petrels adjust food delivery during a chick’s first month of life.
Understanding how storm-petrels raise a single chick
Leach’s Storm-Petrels Hydrobates leucorhous lead extraordinary lives, travelling hundreds or even thousands of kilometres across the open ocean before returning at night to their burrows to feed a single chick. These long-lived seabirds must carefully divide the energy they bring back from the sea between their own needs and those of their growing young.
For decades, researchers could measure the outcome of feeding by tracking changes in chick weight, but they lacked a way to see how much food an adult actually carried into and out of the burrow. That gap has now been filled by the development of a new tool – the Burrow Scale Monitor – allowing scientists to weigh each parent as it enters and exits the burrow during nightly feeds.
The result is the clearest picture yet of how storm-petrel parents balance their competing priorities.
A new way to weigh visiting parents
The Burrow Scale Monitor (BSM) sits inside the burrow entrance, recording the mass of an adult storm-petrel each time it arrives and departs. By comparing the two measurements, researchers can calculate precisely how much food was transferred to the chick during each visit.
The system was deployed on 16 burrows on Kent Island, New Brunswick, home to around 25,000 pairs of Leach’s Storm-Petrel. Over the first 30 days of chick development, it recorded more than 240 feeding events, providing an unprecedented dataset on parental allocation.
The researchers also used radio-frequency identification tags to track individual birds and combined these data with known chick ages, allowing them to test how parental behaviour shifts as young develop.
Parents deliver more food as chicks grow
One clear trend emerged: older chicks receive larger meals. The mass delivered during a visit increased steadily through the first month of development, reflecting the young chick’s rising capacity to accept and process food.
However, the mechanism behind that increase was not what many might expect. Parents were not returning heavier from sea as chicks aged. Instead, they adjusted how much of their own arrival mass they were willing to give away.
In essence, adults did not forage harder – they simply gave up more of what they had.
Parents do not change their foraging effort with chick age
Despite the growing demands of their chicks, adults returned to the burrow at broadly similar masses throughout the study period. The variation in arrival weights reflected the natural unpredictability of pelagic foraging, not a developmental shift in strategy.
This finding runs counter to the idea that storm-petrel parents “plan ahead” by working harder at sea as their chick grows. Instead, adults appear to forage consistently, regardless of chick age or hunger level.
This supports the hypothesis that decisions about how much to feed are made at the nest, not offshore.
Feeding decisions happen at the burrow
The strongest shifts occurred in the proportion of body mass allocated to the chick. Younger chicks received a smaller fraction of the adult’s arrival mass, likely because they were limited in how much food they could physically accept. As the chicks matured, parents delivered a steadily rising proportion, resulting in larger meals even without heavier arrival weights.
The study found no evidence that parents adjust their mass-gain strategies at sea in response to chick age. Instead, adults alter the balance between self-maintenance and offspring provisioning only once they return to the burrow.
This flexible allocation – rather than a change in foraging – is the key driver behind increasing food delivery in the early weeks of chick rearing.
Why not forage harder for older chicks?
Storm-petrels are extremely long-lived birds, with some individuals breeding for decades. In such species, natural selection tends to favour caution: maintaining adult survival is paramount for lifetime reproductive success.
Venture too much energy at sea, or return too depleted, and a parent risks its own survival. The study’s findings suggest that storm-petrels avoid altering their foraging strategy based on short-term chick needs, instead using allocation at the burrow as a safer, more controlled mechanism.
This strategy reflects the reality of pelagic life: food availability is uncertain, long trips are costly, and adult condition must remain stable for future breeding seasons.
A window into parental trade-offs
By providing direct measures of both parental intake and delivery, the Burrow Scale Monitor gives researchers a new tool for exploring how seabirds navigate the classic trade-off between self-care and offspring provisioning.
The approach could be extended to other burrow-nesting species, or those with predictable entry routes, opening new avenues for studying foraging efficiency, energetic costs and individual differences in parental strategy.
For Leach’s Storm-Petrels, the findings offer a nuanced view of how parents meet the demands of early chick growth – not by flying further or feeding more frequently, but by giving up a larger share of their own reserves as the chick matures.
November 2025
Read the full paper here
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