As the songs of spring morph into chips from hungry young birds to their diligent parents, we are seeing many fledgling birds in the nets. These fledglings are taking the next harrowing step towards adulthood as they leave the nest and finish growing. While all our songbirds start out helpless and require constant care from their parents, not all birds do.
Consider for a moment how human babies are born entirely helpless, then think about other mammals like deer, whose fawns totter to their tiny hooves almost immediately after birth. This is the difference between altricial and precocial species.
Songbirds are altricial and therefore hatch with their eyes closed, mostly bald, and are incapable of doing much beyond eating, pooping, and growing. There are also semi-altricial species such as hawks and herons, who are similarly helpless, but born downy with their eyes open.1
Above: Eared Grebes are more in the middle of the precocial-altricial spectrum. Their babies are born fuzzy and able to move around, but can often been seen on one parent’s back as the other dives to deliver food to their offspring. Photo taken on Kimiwan Bird Walk by Robyn Perkins.
On the other hand, precocial birds are born downy with open eyes and often leave the nest after only a couple days. These include shorebirds and ducklings that can find their own food quickly. Gamebirds such as ruffed grouse are similarly independent after first being shown how to find food by their parents.1
Understandably, brain size differs between altricial and precocial species upon hatching. Precocial hatchlings have larger brains relative to their body size, allowing for their quick independence. This also means mothers of precocial chicks must consume more nutrients before laying energy-rich eggs. Alternatively, the bulk of nutrient gathering for altricial species occurs after hatching, with both parents often bearing this burden. This gives altricial chicks greater brain growth post-hatching, and a larger adult brain relative to body size.1,2
Above: This American Redstart captured this week and its siblings are altricial and leave the nest before they are fully able to care for themselves.
These strategies are in response to food availability and predation risk. One of the advantages of an altricial species is a shorter incubation period. The American Redstart, a fan favourite at the bird observatory, incubates their eggs for 10 to 13 days, followed by a nestling period of another 7 to 13 days.3 Should the brood fail for any reason, there are more opportunities for American Redstarts to start another clutch of eggs before the summer is out, whereas the incubation period for a precocial Canada Goose is twice as long and leaves less time for another attempt.3,4
In defense of precocity, it obviously helps to be able to move towards food and away from danger. Precocial species, such as Common Goldeneyes, may choose habitat that is safe for nesting but lacks the food resources for a brood of hungry chicks, so they herd ducklings between safe locations and food-rich, but riskier ones.5
While there are more strategies employed by different species, all are an evolutionary response to solve the balancing act of growth and risk. After the evolutionary feat of hatching and growing is complete, they begin their next monumental task of migrating south. Fall Migration Monitoring started on July 12 and already we have banded 173 songbirds of 27 species. Stop by during scheduled drop ins most Wednesdays and Saturdays until September.
By Bronwyn Robinson, LSLBO Assistant Bander
References
- Ehrlich, P. R. (1988a). The Birder’s Handbook (2nd ed.). Simon & Schuster. p581-587. http://web.stanford.edu/group/stanfordbirds/text/essays/Precocial_and_Altricial.html Accessed on [7 July 2024].
- Ehrlich, P. R. (1988a). The Birder’s Handbook (2nd ed.). Simon & Schuster. p555-557. http://web.stanford.edu/group/stanfordbirds/text/essays/Parental_Care.html Accessed on [7 July 2024].
- Cornell Lab of Ornithology. 2019. All About Birds. Cornell Lab of Ornithology, Ithaca, New York. https://www.allaboutbirds.org Accessed on [3 July 2024].
- Naef‐Daenzer, B., & Grüebler, M. U. (2016). Post‐fledging survival of altricial birds: ecological determinants and adaptation. Journal of Field Ornithology, 87(3), 227–250. https://doi.org/10.1111/jofo.12157
- Belda, E. J., & Clark, R. G. (2008, January 30). Space use, habitat selection and reproductive output of breeding common goldeneye (Bucephala clangula). OuluREPO. https://urn.fi/URN:ISBN:9789514287152
