Last night on the way back from town, I observed and drove along with a flock of ~250-300 sandhill cranes returning to the refuge from foraging north of here. (Couldn’t do an accurate flight count and drive at the same time.)
Observing the flight dynamics of this group was really quite interesting. Unlike Canada geese that typically form several dynamic ‘V’ patterns, this flock of cranes did not. Except for the front ‘V’ with a very truncated side ‘arm’ of five individual cranes, the rest of the flock was one long linear flock stretching for a few miles.
Typically with geese the front leading bird of the ‘V’ formation frequently changes, trading places with nearby individuals. This seems logical because energy is then conserved among the leading birds. Not so with this flock. The leading crane never left its position as lead and never ‘coasted’ in flight, aka never altering or ceasing wing-flap flight. I was impressed, but also suspected it was very energy consumptive. Additionally, the five cranes forming the truncated arm behind the leader never changed positions and also never faltered in consistent wing activity.
The cranes following in line behind the leader, on the other hand, often coasted with short periods of folding wings alongside their bodies. Whether this was to rest (energy conservation) or to retain their position in the long line, I cannot be sure. I did notice that some individuals did change positions when some birds slowed or fell slightly out of line. Consequently, the line was truly maintained as one long linear fight pattern! I also wondered how many of these birds were immature cranes who aren’t yet as strong as the older adults, or if they haven’t yet mastered the ‘protocol’ of flying in large flocks. I watched one crane fly under and ahead of five other cranes in front of its former position and fill in a gap in the line. It reminded me of how blackbirds and other flock songbirds change positions while roosting on utility lines, with some hop-scotching to keep a tight and consistent line formation.
I was fascinated by the flock and flying dynamics of these several hundred sandhill cranes! Very different from the geese. Although my original intent was just to monitor where this flock landed, I was treated to an entirely different perspective of flock flying dynamics.
Because questions of daily roosting versus feeding movements and populations of sandhill cranes arose during a meeting between state and federal managers this week, I thought watching where this flock was destined might be interesting. Only a small percentage of the total flock (~24) landed on the small ‘crane ponds’ (small shallow water impoundments just north of the main refuge where cranes often roost). I expected a higher sub-set of this flock to land there. I briefly watched these birds try to locate the members of their family units for roosting overnight, then I drove ahead to monitor the remainder of the flock.
As the larger sub-set of birds approached the center of the refuge where two large impoundments are shallowly flooded for roosting and feeding, the linearity of the flock dissolved as smaller flocks formed and dispersed. Approximately 50% of the flock settled on these impoundments, and the other 50% flew east to roost on the sandbars of the Rio Grande River.
New Mexico state biologists survey populations of waterfowl on the river via aerial surveillance once a month. This refuge conducts feeding surveys every week. We are discussing now whether to incorporate ground roosting surveys for geese and cranes once or twice/month to add to the total data on use of the resources and land in the mid-Rio Grande Valley, especially since most of the cranes now present here are flying north to feed during the day at state-managed refuges (~38 and ~60 miles north of here).
Estimating the light geese roosting population would be easier than that of the cranes because the former typically take flight in one large massive flock. Cranes, on the other hand, usually depart from their overnight roosting places in small family units of 2-4. However, cranes congregate in overnight roosting flocks at only a few locations on the refuge. Counters posted at each of these locations can easily count the family units and obtain total numbers as they take flight. It might take longer, but it might also be more enjoyable.