Challenges of Feeding Juvenile Elegant Terns
By Manuel Stevos
Project Overview
Feeding offspring is a mundane task. A table stakes item for which parents don’t receive credit for doing well. However, for elegant tern parents raising juveniles after their northern migration, this isn’t a matter of simply delivering meals, but a journey of overcoming underappreciated obstacles. Avoiding predators, outsmarting thieves and managing temper tantrums are daily challenges for these birds. This project shares three easily seen but often overlooked obstacles elegant tern parents encounter when trying to feed post-fledgling juveniles.
Background
About the Colony
The colony I observed inhabits an expansive beach within a protected bay. The beach extends for approximately 1.5 miles. Sandstone cliffs back the beach, protecting its inhabitants from strong westerly winds and providing calmer waters. The area is prone to fog in the early morning and late in the afternoon, which reduces visibility and shortens the time the birds can dedicate to fishing.
Starting in late July, the birds migrated from Mexico and southern California, arriving in small groups that eventually brought the total population to roughly 300 individuals. The elegant terns integrated with a large group of Heermann’s gulls, numbering between 500 and 750 individuals. A living arrangement that also occurs on their breeding grounds in Mexico.
While the colony was large, the birds often separated themselves into subgroups. Two to three subgroups were common, but there could be up to seven subgroups.
The terns remained at this location through early November, after which they started their migration to South America.
Feeding Requirements
The juvenile terns are three to four months old when they migrate to northern California and still depend on their parents for food. Parental dependency likely decreases during their stay, as there aren’t any documented observations of parents feeding juveniles in South America.
I estimated juveniles ate 3-5 times per day. However, that could vary depending upon the juvenile’s ability to fish for itself and a parent’s decision to withhold food to force the juvenile to fish. The diet of this colony was almost exclusively northern anchovies.
Most of my observations of parents feeding juveniles were in the early morning or late afternoon, but midday feedings occurred. The feeding schedule seemed to change in response to the tides and, potentially, the movement of the anchovies.
Fishing took place over the swells, beyond the breaking waves. The birds primarily fished in the ocean, but occasionally fished in the bay immediately in front of the beach.
Locating Juveniles: A Larger Than Expected Challenge
Elegant tern parents often struggled to find their offspring when returning to the beach with prey. There were two main reasons for their struggles.
Juveniles Are Mobile
Juveniles frequently changed locations while the parents were away fishing. The largest displacements resulted from disturbances of the colony. If a gull or tern spotted a predator such as an eagle or coyote, the birds would take to the air simultaneously. These disturbances broke the colony into subgroups that frequently formed and reformed throughout the day. As a result, a juvenile that was in one subgroup may have moved to another subgroup during the parents’ absence. The new subgroup could be 40 to 300 feet from the original location.
I tracked 96 events in which a parent approached the colony with prey. Only 64 of those events (67%) ended with a feeding attempt in the subgroup the parent first approached. Meaning, the parents often had to search a second area of the beach to find their offspring.
Juveniles also moved in response to events within the colony. As birds return to the colony from fishing, they jostle for standing room within the cluster, forcing members to move positions. In addition, fights and play within the colony are common and displace birds from their previous positions. I also found that parents circled or passed over the cluster an average of 3.5 times prior to locating their juvenile. Clearly, parents found it difficult to locate their young.
Elegant Tern Parents Can Misidentify Juveniles
Juveniles will request food from non-parents and, occasionally, those juveniles will, at least initially, fool the adult birds.
I recorded nine events in which an elegant tern parent was about to feed a juvenile and pulled the fish back at the last moment. The parent subsequently found and fed another juvenile in each of those cases, so I’m comfortable stating that these impostor juveniles initially fooled the parent.
I couldn’t find any documentation of this behavior in peer-reviewed literature regarding elegant terns. However, one study of roseate terns found juveniles directed 14% of their begging interactions at non-parents, so this isn’t an unprecedented behavior. Unfortunately, the study did not mention the outcome of those begging interactions. While it is accepted that elegant terns can identify the unique calls of their offspring, these events prove that the method isn’t foolproof.
Managing Begging Requests From Juveniles
Juveniles often begged for food immediately after eating. That forced parents to make a tough decision: spend more energy to fish again and aid the juvenile’s development or save their energy and negate the request. Interestingly, parents denied the begging request most of the time but needed to spend some energy to do so.
I recorded 36 instances of juveniles begging for food immediately after eating (56% of feeding events). Perhaps like human teenagers, juveniles are perpetually hungry. Begging was far more likely to occur when the juvenile was on the outskirts of the cluster versus within the cluster or in the water. Begging events were also more prevalent in the late afternoon (after 3:30 PM).
I witnessed four unique parental responses to juvenile begging. Flying a long distance away was the most common response, occurring 44% of the time. It was impossible to determine whether flying a long distance meant that parents intended to fish again to feed the juvenile or if the parents wanted to relocate some distance away to mitigate the juvenile’s begging.
Flying a short distance was the second most common response and seemed to make a point about the parent wanting to cease any interaction with the beggar. This behavior was only witnessed when feeding occurred on the outskirts of the cluster, and often in response to more extreme beggars. Below is an example of a juvenile begging aggressively. The juvenile’s actions forced the parent to jerk its head back, after which the parent flew a short distance away (10-15 feet) to rejoin the colony
The walking-away response, much like flying a short distance away, seemed to be associated with extreme begging. Parents were never physically aggressive when employing negative responses. However, the only time I saw a parent use a partially raised wing display toward a juvenile was right before the parent employed the walking away response. The partially raised wing display is a behavior that is often thought of as a threat display.
Read a narrative example of a parent walking away from extreme begging/.
Parents appeared to prefer using a prolonged ignorance response when they were within the cluster or near other adult birds. It’s possible that the lack of space was a factor in choosing this response, but it is interesting that parents never presented this response to extreme beggars.
Defending Against Steal Attempts from Gulls
Heermann’s gulls frequently sought to steal from elegant terns. Gull attacks occurred on 13 of the 96 occasions I recorded a parent approaching the colony with a fish. All the attacks were from younger gulls (first and second cycle). Interestingly, the larger western gulls did not attack the terns.
Roughly half of the gull attacks (7) were against a parent prior to locating its offspring. The attacks resulted in a chase with an average chase duration of just over one minute (70 seconds). However, one chase lasted for two minutes and 39 seconds, so they can be long events.
The terns were often successful in defending themselves when chased. Parents escaped the gull and fed their offspring after five of the seven attacks.
The remaining 6 attacks occurred while the parent was trying to transfer a meal to a juvenile. None of those attacks succeeded.
In summary, Heermann’s gulls interrupted 13.5% of feeding attempts, and the gulls were successful 15.3% of the time.
Strategies to Mitigate Attacks from Heermann’s Gulls
While Heermann’s gulls were a problem, elegant terns had ways to reduce the likelihood of an attack.
Having juveniles move to the outer ring of the subgroup helped parents identify their offspring faster and reduced gull attacks. Gulls primarily chased adult terns with prey who had trouble locating their offspring.
However, attacks seemed to be more likely if the juveniles on the ground separated too much from the subgroup. All 6 gull attacks against parents trying to execute a food exchange with a juvenile occurred when there was a significant separation from the group (> 10 feet).
Parents that successfully fed juveniles away from the subgroup often placed their back to a gull and raised their wings, shielding the food transfer from the gull.
Completing the food transfer on the open water was another successful strategy to mitigate gull attacks. Parents often led juveniles that were in the middle of the subgroup or located close to a gull, to the open water. Gulls did not interrupt the food transfers between elegant terns on the water. That was a surprise because Heermann’s gulls often harassed pelicans on the water.
Postscript
When I started scouting and photographing elegant terns, I did not know it would evolve into a project. My original intent was simply to get artistic pictures of the birds fishing and feeding. However, the more I observed, the more I learned about their challenges.
I spent 28 hours observing and photographing this colony but only took detailed notes over the last 12 hours. That means I missed the early part of their stay (August and September). I’m curious to see if there are behavioral differences in the early days of the colony versus the October-November period I documented here.
I’ll return to monitor the elegant terns next year and will certainly update this project. Here are some questions I hope to learn more about in the future:
- Does the subgroup size impact the time required for parents to find their offspring?
- Is subgroup size a factor in the frequency with which a parent attempts to feed the wrong juvenile?
- Is the fly a long distance away response employed more often to beggars that placed themselves in safe locations (e.g. outer ring of the subgroup)? If so, one could view that as a reward or an attempt to shape behavior.
- Are there behavioral differences between the first half of the post-summer migration (August-September) and the second half (October-November)?
- Why don’t parents feed juveniles in the open water more frequently? Feeding in the water was associated with fewer gull attacks and fewer post-meal begging events from juveniles. Seems like the best outcome for both parent and juvenile.
I do welcome feedback and questions about this project. Please feel free to contact me or add to the comments below.
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