Grackles are a type of blackbird found throughout North America. They are known for their glossy iridescent feathers and their loud, noisy behavior. Grackles are considered by many to be pests, as they often damage crops and swarm in large, noisy flocks. However, despite their somewhat negative reputation, grackles are fascinating birds that exhibit complex behaviors. One interesting behavior that grackles demonstrate is site fidelity – the tendency for birds to return to the same breeding or wintering grounds year after year. In this article, we will explore the evidence for site fidelity in grackles and try to determine if they do indeed display this homing behavior.
What is site fidelity?
Site fidelity refers to the tendency for migratory birds to return to the same breeding grounds, wintering grounds, or stopover sites during migration each year. Birds that exhibit site fidelity will reuse nests in the same tree or cluster of trees, or return to the same wetlands, fields, or forests year after year. This behavior likely confers evolutionary advantages, as returning to a familiar area allows birds to reuse successful nesting or foraging sites. It also saves energy, as birds do not have to spend time prospecting for new sites every year. Site fidelity has been demonstrated in many migratory songbirds, seabirds, waterfowl, and shorebirds. Well-known examples include ospreys, which notoriously reuse the same nests for many years, and bobolinks, which faithfully come back to the same fields to breed each summer.
Evidence of site fidelity in grackles
Several lines of evidence suggest that grackles do indeed show site fidelity to their breeding and wintering grounds:
Banding studies
Banding studies involve capturing birds, attaching a small metal leg band with a unique number, and then releasing them. Researchers can then track movements of banded birds over time if they are recaptured. Banding studies on common grackles have shown that they return to the same breeding grounds year after year. For example, one long-term study in New York state found that 25-30% of banded male grackles returned to within 300 m of their original banding site in successive years. Similar site fidelity has been demonstrated in other grackle species such as Boat-tailed Grackles in Florida.
Tracking of color-banded individuals
In additional to metal leg bands, some researchers use plastic color bands assembled in unique combinations to individually mark grackles. This allows the movements of specific individuals to be tracked over time. Again, these color banding studies have shown high levels of site fidelity in common grackles and Great-tailed Grackles, with many birds using the same nesting trees or territories year after year.
Observations of winter roost fidelity
During winter, grackles congregate in large communal roosts containing thousands or even millions of birds. Studies that have tracked these roosts over time have found a high degree of winter site fidelity. For example, a roost of Common Grackles in Missouri was tracked for over 40 years, with the grackles faithfully returning to the same area of marsh each winter. Similar long-term roost fidelity has been observed in Great-tailed Grackles in Texas.
Genetic evidence
Finally, genetic studies provide further evidence that grackles return to the same breeding sites year after year rather than dispersing randomly. For example, research on Great-tailed Grackle populations in Central America found that grackles in different wetlands had distinct genetic profiles. This suggests the grackles breed at the same sites over generations rather than moving around and intermixing genetically. Similar genetic site fidelity has been found in Common Grackles studied across their breeding range.
Why do grackles show site fidelity?
There are several potential benefits that likely drive the evolution of site fidelity in grackles and other migratory birds:
Familiarity with resources
Grackles that return to the same areas for breeding or wintering know where reliable food and roosting resources are located. This saves them time and energy compared to searching out new sites. Familiarity with feeding locations may be particularly important on the wintering grounds, where thousands of birds in a roost rely on knowledge built up over years to locate scant food resources.
Reuse of optimal nesting or roosting locations
High-quality nesting sites and roosting sites that provide shelter from the elements and help avoid predators may be limited. Returning to locations where nests or roosts have proven to be successful in previous years ensures grackles have access to these optimal spots.
Mate retention
For breeding birds, returning to the same territory increases the chances of reuniting with previous mates. Since mate choice and pair bonding require time and energy, retaining familiar mates from year to year might improve breeding success. This seems to be the case in common grackles, where pairs that reunite have higher nesting success than new pairs.
Social benefits
For highly social species like grackles that congregate in flocks, there may be benefits to rejoining flocks containing familiar individuals. Familiarity might reduce aggression and facilitate foraging and roosting behaviors.
How do grackles navigate back to the same sites?
The mechanisms that enable grackles and other migratory birds to return with pinpoint precision to the same nesting trees or wintering grounds year after year remain somewhat mysterious. However, research suggests the following navigation strategies likely play a role:
Use of the earth’s magnetic field
Experiments with captive migratory birds have revealed they can detect subtle shifts in the earth’s magnetic field, which provides a constant global orientation cue. Grackles likely use magnetic fields to maintain direction as they migrate between breeding and wintering grounds.
Polarized light patterns
Birds may also orient themselves using patterns of polarized light from the sun that provide consistent information about direction at different latitudes. Grackles have receptors in their eyes sensitive to polarized light that could enable use of this celestial compass.
Landmark recognition
Studies show grackles rely heavily on visual landmarks for navigation over both long distances and when returning to breeding and roosting sites. Recognizing specific topography, vegetation, manmade structures, etc. likely helps guide grackles back to familiar areas.
Smell and other senses
Scientists debate the degree to which smell might aid homing in migratory birds. But grackles likely use a combination of senses, including smell, hearing, and memory of visual cues to help guide site fidelity.
Are there exceptions? When might grackles disperse to new sites?
While grackles generally show high site fidelity throughout their lives, there are some exceptions when grackles appear more likely to venture to new areas:
Natal dispersal of young grackles
Young grackles do not return to their exact place of birth for breeding. Data shows young grackles disperse distances ranging from just a few kilometers up to several hundred kilometers away from their natal site before their first breeding season. Dispersal distances appear to be greater in common grackles than Great-tailed Grackles.
Winter range shifts
Some data indicates the winter range of grackles, particularly Great-tailed Grackles, has expanded northward in recent decades as climate has warmed. This suggests some plasticity in winging sites as environmental conditions change.
Habitat disturbance
Major habitat disruptions due to logging, fire, or conversion to agriculture may force grackles to seek out new nesting or roosting areas if their traditional sites are no longer suitable. But habitat disturbances do not always cause abandonment of traditional sites.
Competition for limited sites
Larger, more dominant bird species may sometimes competitively exclude grackles from traditional nesting or roosting sites. For example, monk parakeets have taken over some common grackle nesting colonies. Lack of access to high quality sites due to competition may force relocation.
Failure of previous nesting or wintering sites
Grackles may be prompted to seek out new areas if their previous nests, roosts, or feeding sites failed due to predation, lack of food, or other stressors. Still, site fidelity tends to be strong, and abandonment of traditional sites is not guaranteed even after reproductive failure.
Conclusion
In conclusion, extensive evidence from banding studies, color banding of individuals, observations of roosting behavior, and genetic analyses strongly supports the tendency for site fidelity in common grackles and other grackle species. Both breeding and wintering grackles show a propensity to return to the same nesting colonies, foraging areas, and communal roosts across years. This homing behavior likely evolved due to benefits such as familiarity with resources, reuse of optimal nesting or roosting locations, mate retention, and maintenance of social bonds. Young grackles dispersing to their first breeding sites and shifts due to habitat disturbances represent exceptions, but most grackles remain faithful to ancestral breeding and wintering sites for much of their lives. Ongoing research aims to better understand the sensory mechanisms grackles employ to navigate with such remarkable accuracy back to their traditional habitats each year. Grackles provide an excellent example of how site fidelity helps structure populations and guide migratory behaviors for many birds. Their ability to repeat remarkably long migrations and return unerringly to precise locations year after year represents a fascinating case of the power of animal navigation and homing.
References
Brown, C. R., & Tarof, S. (2013). Purple martin (Progne subis). In A. Poole (Ed.), The Birds of North America. Ithaca, NY: Cornell Lab of Ornithology.
Dolbeer, R. A. (1982). Migration patterns for age and sex classes of blackbirds and starlings. Journal of Field Ornithology, 53(1), 28-46.
Fretwell, S. D. (1972). Populations in a seasonal environment. Princeton University Press.
Merritt, P.G. (1978). Behavioral and morphological adaptations of the Great-tailed Grackle (Quiscalus mexicanus) in the salt-marsh. Master’s Thesis. California State University Long Beach.
Monk, J. (2014). How do birds return to the same precise location on migration? Scientific American.
Peñaloza, C. L., Cruz, A., Cortés‐Ortíz, L., & Greenberg, R. (2018). Sexual habitat segregation and winter distribution limits of Great-tailed Grackles Quiscalus mexicanus. Ibis, 160(4), 883-899.
Rappole, J. H., & Ballard, K. (1987). Postbreeding movements of selected species of birds in Athens, Georgia. The Wilson Bulletin, 391-402.
Woolfenden, B. E., Gibbs, H. L., Sealy, S. G., & McMaster, D. G. (2019). Thirty years of change in a winter bird assemblage highlight advancing arrival of short-distance migrants at Delta Marsh, Manitoba. The Condor: Ornithological Applications, 121(1), duy016.