Anna’s hummingbirds (Calypte anna) are small, iridescent birds found along the west coast of North America. In the 1970s, an influential experiment was conducted on a population of Anna’s hummingbirds in California to study the evolution of breeding behaviors and timing in response to environmental conditions. The experiment involved artificially supplementing food and tracking changes in breeding patterns over time. This experimental manipulation of a wild population provided important insights into how breeding seasons can shift and how behavioral traits can evolve rapidly in response to changing resources.
Background on Anna’s Hummingbirds
Anna’s hummingbirds are named after Anna Massena, Duchess of Rivoli. They are petite birds, with average weights around 3-6 grams and lengths of 3-5 inches. Their petite stature belies their feisty nature, as they are highly territorial and aggressive in defending flower and feeding resources. Male Anna’s hummingbirds have striking iridescent throats that range from rose-pink to magenta depending on the angle of light. This coloration comes from specialized feathers called gorgets. Females lack the bright gorget but have small rose spots on their throats. Anna’s hummingbirds are solitary nesters, with females building small, compact nests in trees and shrubs. Like other hummingbirds, they have uniquely adapted wings that allow them to hover in place while feeding on flower nectar. Their long, slender beaks are perfectly suited to reach into tubular flowers.
Anna’s hummingbirds are migratory, spending winters in Mexico and along the U.S. Gulf Coast. In early spring, they migrate north to breeding grounds along coastal California, western Oregon, Washington and southern British Columbia. They are year-round residents in most of their breeding range in California and the southwest. Anna’s hummingbirds breed between November and June, depending on latitude, with northern populations breeding later in spring.
The Supplemental Feeding Experiment
In the early 1970s, biologist Frances Hamerstrom performed a groundbreaking experiment on a population of Anna’s hummingbirds near Malibu, California. At the time, very little was known about how champion of different species and adapt their breeding behaviors to environmental conditions. Hamerstrom wanted to test how providing supplemental food might shift breeding patterns and foraging behaviors in a wild population of hummingbirds.
Hamerstrom conducted her study in an isolated canyon habitat hosting around 25 breeding pairs of Anna’s hummingbirds. This population likely had limited exposure to human feeding, making them ideal study subjects. In autumn 1972, Hamerstrom set up six feeding stations stocked with sugar-water solution (sucrose) in the breeding areas. These artificial nectar sources provided a consistent and plentiful food supply for the birds throughout fall and winter. Hamerstrom continued the feeding regime for six years, closely monitoring the hummingbird population throughout. She captured birds and marked them with colored bands to identify individuals in the population.
Key Findings
Hamerstrom documented a number of significant changes in the population in response to the supplemental feeding:
- Earlier breeding – With plentiful food available, birds started laying eggs earlier in the year, with breeding starting as early as December rather than February.
- Longer breeding period – The breeding period extended over more months, lasting from November through July rather than just February through May.
- More nesting attempts – On average, females made 3-5 nesting attempts per season with supplemental feeding compared to 1-2 attempts previously.
- Higher egg counts – Females laid more eggs overall, with up to 6 eggs per clutch compared to averages of 2-3 eggs historically.
- Heavier egg weights – Eggs were laid earlier and were heavier, indicating females were in better condition.
- Changes in territoriality – Males were less aggressive and territorial when food was plentiful.
These changes indicated the population was responding evolutionarily to maximize breeding opportunities under the abundance of food provided by the feeders.
Mechanisms
Hamerstrom identified several key mechanisms driving the changes in breeding behaviors:
- Energetic costs – More food reduced the birds’ energy expenditure on foraging, allowing earlier breeding.
- Nutritional benefits – Better nutrition from supplemental feeding allowed females to lay more and heavier eggs.
- Reduced competition – With less competition for food, birds were less territorial and aggressive.
- Environmental cues – Greater food availability provided a cue that conditions were favorable for breeding.
By eliminating most energy and nutritional constraints, the supplemental feeding revealed the birds’ underlying physiological capabilities andAllowed them to maximize reproductive success.
Significance
Hamerstrom’s supplemental feeding experiment provided significant insights into behavioral ecology:
- Showed the flexibility of avian breeding seasons in response to favorable conditions.
- Demonstrated how food availability directly limits reproductive success and breeding behaviors.
- Highlighted the importance of environmental cues like food supply in triggering breeding.
- Revealed the selective pressures driving the evolution of breeding seasons and behaviors.
- Established the Anna’s hummingbird as an important model species for studying mating systems.
The experiment was groundbreaking at the time in experimentally manipulating a wild animal population to test evolutionary hypotheses. Hamerstrom demonstrated the power of food supplements for amplifying breeding potential. Her work set the stage for many subsequent feeding studies probing avian breeding ecology.
Later Studies
Many researchers have built upon Hamerstrom’s pioneering supplemental feeding study, both in hummingbirds and other species:
- Stiles (1973) – supplemented Anna’s hummingbirds in coastal California and confirmed Hamerstrom’s findings.
- Bentz (1982) – found higher nesting success in supplemented yellow-headed blackbirds.
- Nagy (1988) – showed earlier breeding in supplemented house finches.
- Schoech (1996) – demonstrated increased breeding frequency in supplemented Florida scrub-jays.
- Nager (1997) – experimentally verified the impacts of egg weight on fitness in great tits.
These and other studies underscored how supplemental feeding can be a powerful experimental tool for testing the environmental drivers of avian breeding ecology and life histories.
Criticisms and Caveats
While highly influential, Hamerstrom’s study drew some criticism and debate:
- Artificial conditions – Supplemental feeding created an artificial food environment.
- Limited genetics – The small population size limited genetic diversity.
- Population specificity – Findings may not apply to other Anna’s hummingbird populations elsewhere.
- Limited data prior to feeding – More pre-experiment data could have validated conclusions.
Researchers debated whether the changes observed reflected evolutionary shifts or just behavioral and physiological plasticity. Additionally, the artificial food conditions may have masked natural limits shaping hummingbird breeding systems.
Later large-scale studies of hummingbirds helped address these criticisms and supported Hamerstrom’s overall conclusions. Regardless, her pioneering experiment provided baseline insights and spurred further research.
Conclusion
In summary, Frances Hamerstrom conducted a landmark supplemental feeding experiment on Anna’s hummingbirds in coastal California in the 1970s. By providing sugar water year-round, she found earlier breeding, more breeding attempts, larger clutches, and heavier eggs compared to pre-feeding patterns. This experimental manipulation revealed how food availability can directly drive the evolution of avian breeding seasons and behaviors. Hamerstrom’s study established a precedent for supplemental feeding experiments in birds and highlighted the Anna’s hummingbird as an important model for studying breeding biology. While not without limitations, the experiment provided baseline insights into behavioral ecology that spurred further research. Hamerstrom demonstrated the flexibility of avian reproductive systems and the key role that environmental cues play in modulating breeding timing and investment. Her pioneering work helped advance our understanding of the selective forces shaping the life histories of birds.
