Fledging the young

Back in high school I ran an obstacle course race at Camp Pendleton, a marine corps base outside San Diego. I entered the team division with some of my high school cross country teammates, but unlike other XC races where you score the top 5 by place, the entire team had to cross the finish line linking arms. You truly were only as strong as the weakest link (Ben, I’m looking at you).

And in many ways fledging young is much the same way. In birds, the burden of rearing young is largely shared – some 81% of bird species share the responsibility of fledgling nestlings (source). It takes smaller species 2-3 weeks to take the hatched young to independence, larger species closer to 3-5 weeks (some birds, like ducks and shorebirds, are precocial, born ready to leave the nest).. During that time, the parents are tied to the young. Where one goes, so go the others. Sure, sexual reproduction may be the road to genetic immortality, but it is an arduous road full of obstacles, traps, and terrifying predators. The family unit races races towards that finish line, frantically stuffing food into the young, fattening them up and teaching them how to ultimately forage on their own.

Jon Young says “Nature abhors adolescence” as though everything was conspiring to kill teenagers (and teenagers are doing as much as possible to stay entirely oblivious to the threats of the world around them). Mortality rates vary widely amongst fledglings (birds that have left the nest). The number’s around 10% in chickadees but closer to 70% in juncos (source). In order to rush their young through that period of intense vulnerability, parents (or parent as is more often the case) must act as a constant conveyor belt, bringing food from the surrounding environment in a direct line back to their young’s mouths.

Babies are anything but helpless in the process. At the approach of a parents, each nestling shrieks and opens its mouth wide trying to corner the market on their parent’s attention and get first dibs on that sweet wriggly worm. As with the cardinal nestlings above, the mouths of nestlings are usually brightly colored targets to guide their parents right to the bullseye. I imagine that like with humans, the begging calls of nestlings trigger an immediate and unconscious emotional reaction in the parent’s brain (source).

Fortunately adolescence isn’t all that long in most birds. In mourning doves, for example, the time between hatching and independence is between 18 and 28 days (source). For the first 2+ weeks of that, the juveniles are entirely dependent on their parents for food (male and female mourning doves are both involved from nest building to rearing young). The timeline is similar for black-capped chickadees: free of the nest after 10 days, fully independent 18 days after that (source). Initially, the male chickadees is primarily responsible for foraging for the young while the female broods the young.

Male and female house sparrows both feed the young. Female feeding rate was positively associated with bill depth – the bigger the bill, the more she fed her young. Interestingly, the larger the badge of the male (the black throat patch), the more time he spent feeding the young – the badge seems to be an honest signal of future parental investment (source). And females pick up on this as males with larger badges are selected by females earlier than those with smaller badges (source).

In American robins, the female will initiate a second nest while her first brood has yet to reach full independence. At that point the male takes over primary responsibility for feeding the young and so has less ability to guard his mate and ensure that he sires the next brood. The female may use that time to gauge the male’s quality and ability to care for the young. Despite the lack of certainty around parentage, there’s no difference in time/energy investment of the male from brood to brood, indicating that the male often succeeds in siring the second brood because, as the researchers wrote: “confidence of paternity ensures male parental care at the second nest” (source).

I often think of fall as a time of great fattening. All the animals seem to be gorging on everything in sight in preparation for long migrations or a cold season of hardship. Summer’s sort of the same, but instead of putting all that weight on their own body, parents are stuffing food as quickly down their offspring’s throats to fatten them up as quickly as possible.

The larger the brood size, typically the more foraging trips a parent makes (though the fewer feedings each bird receives). In mockingbirds, a single nestling in the brood gets fed about 5 times an hour. With 4 young in the nest, the number drops to about 2 time an hour (source). There seems to be a balancing act as parents weigh the risk of higher predator detection rates from increased traffic to/from the nest, the energetic cost to the parent of foraging for their young (and not eating themselves).