Juvenile gray jays rely on their parents cached food: an isotope spiking experiment

Background/Question/Methods

Delayed dispersal can benefit juveniles by providing them access to resources that would otherwise be difficult to find. For species that cache food in harsh environments, it is possible that juveniles remaining on their natal territory rely on food stored by their parents for overwinter survival. Gray jays are scatter-hoarding birds that inhabit boreal forest throughout North America and dominant juveniles typically remain with their parents throughout the winter. To examined the hypothesis that juveniles rely on their parents cached food for overwinter survival, we supplemented adults in October with dried dog food that was spiked with heavy nitrogen (99% 15N, glycine) but prevented juveniles from accessing this food. Over the course of the experiment, we pulled and re-pulled the outer tail feathers of both adults and juveniles in all treatment and control (no food) groups. Because tail feathers regrow at a linear rate over 50 d, sub-samples taken along an individual feather shaft provided weekly measurements of stable-nitrogen isotopes. Thus, multiple regrown feathers provided measurements over the course of several months, which allowed us to infer when spiked caches were being retrieved over the winter and whether juveniles were consuming food cached by their parents. 

Results/Conclusions

Stable-nitrogen isotope values from the portion of the tail feather that was regrown before treatment birds were supplemented with spiked food were similar to values from the same part of the feather in control birds. For control birds, stable nitrogen isotope values in feathers were consistent over the entire winter (range 6-8 ppm). For adults that were supplemented with spiked food in October, stable nitrogen isotope values varied greater than tenfold, with values as high as 150 ppm. Based on the growth rate of feathers, supplemented adults were retrieving caches anywhere between 7 d and 3 months after they were fed. Consistent with the hypothesis that juveniles use their parents cached food, over half of the juveniles whose parents were supplemented with spiked food also had stable-nitrogen values that exceed that of control birds and had the same range of values as supplemented adults. We demonstrate a unique application of isotopic spiking for understanding behaviors that are difficult to observe in the wild. In doing so, our results also provide evidence for a novel mechanisms by which delayed dispersal can be advantageous for species that overwinter in harsh northern environments.