Kicked out or moving out? Fledging behaviors of grassland songbirds

 

Leaving one’s family home is a momentous occasion for all children, human or otherwise, but for birds it’s not well-known how much of this event is up to the parents’ influence or the nestlings’ choice. This is especially true in the grasslands, where birds build well-camouflaged nests down among the grasses and low shrubs, and where sightlines can stretch for miles in all directions, making undetected observation by researchers a logistical challenge.

To answer this question, we set up cameras on nests of songbird species in the grasslands of Alberta, Canada and Wisconsin, USA to monitor the behavior of parents and soon-to-be fledglings. We examined parental behavior prior to fledging of 390 nestlings at 127 nests. Our focal species were Chestnut-collared Longspur, Eastern Meadowlark, Bobolink, Grasshopper Sparrow, Henslow’s Sparrow, Savannah Sparrow, Common Yellowthroat, and Song Sparrow.

Unfortunately for bird families, what is best for the parents may not be what is best for the nestlings, and vice versa. Parents may want to evict their current nestlings so they can start raising a new brood while nestlings may rather postpone leaving so they can continue to reap the benefits of parental care. It is thought that fledging a brood earlier raises the chances of at least one member of the brood surviving, while survival of each individual increases with later fledging. Assuming parents call the shots, we might expect to see the classic parenting tactic of bribing their children – in the case of birds, standing just outside the nest holding out food. We might also see parents hanging about nearby to encourage and confirm that nestlings are leaving the nest. However, if it is the nestlings’ choice, parents would presumably behave normally, maintaining their regular feeding schedule, while nestlings decide whether they are ready to leave. In this case, a parent may not necessarily be around at fledging time.

Eastern Meadowlark chicks run out of nest to get fed and do not return. Credit: USGS—Wisconsin Cooperative Wildlife Research Unit.

What we found was that parents were rarely at the nest when their offspring fledged, with some variation across species. When we could determine parental behavior prior to fledging, we found that in two-thirds of fledging events the nestling had been fed in the minutes before it left the nest. Similarly, about two-thirds of fledging events with parents present involved feeding. These feeding-fledging events took one of two forms: either nestlings saw their parents approaching with food and rushed out, mouths wide open, to accost them, or the nestlings ate in the nest and left while the parent was present or followed their parents out the door. The rushing out behavior is likely a strategy only possible for ground-nesting birds and has not been previously documented in most of our study species.

Overall, the choice to fledge appears to be made by nestlings themselves. It seems plausible that among grassland songbirds, parents would be more likely to control the timing of independence (i.e. when they stop feeding their fledglings) than fledging itself. Additionally, we noticed that nestlings took many exploratory trips out of the nest before leaving for good, sometimes beginning the day before final departure. We suggest that it may be better to think of fledging as a transitional period between nestling and fledgling for these grassland songbirds rather than a big single event.

 

This research was recently published in The Journal of Field Ornithology:

Ribic, C. A., D. J. Rugg, N. Koper, K. Ellison, and C. S. Ng. 2019. Behavior of adult and young grassland songbirds at fledging. Journal of Field Ornithology https://doi.org/10.1111/jofo.12289

 

Guest post by:

Christine A. Ribic
U.S. Geological Survey, Wisconsin Cooperative Wildlife Research Unit
University of Wisconsin
Department of Forest and Wildlife Ecology