Blog written by Kayla Davis. Read the full paper here.
Imagine you are a newly fledged tern chick, bright-eyed and ready to take on the skies with your freshly grown feathers. There’s a problem though; catching fish is hard, and you always seem to be hungry because you are still growing! Luckily, your dad (or mom, if you were hatched second) is by your side at the breeding colony and during pre-migratory staging to make sure you stay healthy and strong by feeding you tasty sand eels. However, as with most growing youths, the relationship between parents and offspring changes as offspring grow and gain independence. Do you try to maximize parental care from your dad or mom by begging dishonestly? Or are you an honest tern chick that only begs when you need to be fed?
Honest signaling theory seeks to explain animal communication and interactions by describing signals as “honest”, meaning that the sender displays a reliable signal, or “dishonest”, meaning that the sender gives false information to the receiver of the signal. Theory states that costs imposed on creating signals should function to maintain honest signaling mechanisms. However, in the case of begging, especially for nearly adult-sized fledglings, this behavior may not be particularly costly. There is a long history of research on begging behavior as an honest signaling mechanism in birds, but most of this work has focused on nestlings. Parent-offspring communication is expected to change as offspring gain the functional independence needed to survive on their own, and the costs and benefits of behaving honestly or dishonestly are also likely to change during this time.
In our study, we conducted behavioral observations of roseate tern fledglings during the post-breeding, pre-migratory staging period at Cape Cod National Seashore, Massachusetts, USA to investigate the honesty of parent-offspring interactions during the postfledging period. Roseate terns are unique in that most of the northwest Atlantic breeding population departs the breeding colonies (Nova Scotia, CA to Connecticut, USA) after chicks have fledged to stage for several weeks on beaches and islands around Cape Cod, Massachusetts. While there, roseate tern fledglings continue to depend on their care-giving parent for food as they continue to grow and build fat reserves for fall migration to South America. This unique staging strategy gave us the opportunity to observe post-fledging parent-offspring interactions, including lots of begging behavior.
We located tern flocks at the Cape Cod staging grounds and conducted focal sampling of uniquely-marked roseate tern fledglings to quantify begging behavior as it related to date of the staging season and time of day. We expected tern fledglings to gain independence from their parents and improve their fishing skills over the course of the staging season, so we predicted that begging behavior would decrease with date of the staging season if begging was an honest signal of need. We also predicted that begging would increase with time of day if begging was an honest signal of need because roseate terns do not fish during the night, and dusk would be the last time fledgling terns could be fed before nightfall. Thus, we expected fledgling need, and therefore begging behavior, to be highest at the end of the day before a night-time fast.
We also were interested in identifying whether young terns begged at non-parents. In colonially breeding species like roseate terns, offspring may deceitfully beg at non-parents to try to receive extra-parental care. Based on previous work on parent-offspring recognition and discrimination, we expected that non-parents would be able to discriminate their offspring from the offspring of others and would therefore not be fooled by deceitful begging. Thus, we predicted that the lack of benefits to be gained from begging deceitfully at non-parents would result in honesty of parent-offspring interactions.
Our predictions about begging as an honest signaling mechanism were upheld. Roseate tern fledglings begged at their parents more than non-parents, but they did not always beg at true parents. Recent conceptual studies have shown that partial honesty, particularly if the signal is low cost to produce, may be an evolutionary stable strategy. It is likely that begging is a low-cost behavior for tern fledglings, so begging at non-parents may have more potential benefits than costs, even if non-parents are rarely fooled and begging at them often does not result in feeding. The relative lack of benefits to be gained from begging at non-parents has likely resulted in mostly honest communication between tern fledglings and adults; however, the low cost of this behavior may keep deceitful begging present at low frequency because the possible benefits of extra-parental care outweigh the low cost of deceitful begging.
Begging behavior increased with time of day as would be expected if fledgling needs were highest before nightfall and further supports our finding of honest communication between parents and offspring. We also found that begging behavior decreased throughout the staging period. However, tern fledglings continued to beg at their parents even at the end of the staging period, albeit at reduced frequency relative to the beginning of the staging season. This may be evidence to suggest that parental care continues past the staging period into migration and possibly the wintering period. If this is true, begging behavior may function as more than a signal to indicate need; it could also function to reinforce the parent-offspring bond prior to migratory departure from staging areas.