Why Do Kestrels Soar?

Abstract

Individuals allocate considerable amounts of energy to movement, which ultimately affects their ability to survive and reproduce. Birds fly by flapping their wings, which is dependent on the chemical energy produced by muscle work, or use soaring-gliding flight, in which chemical energy is replaced with energy harvested from moving air masses, such as thermals. Flapping flight requires more energy than soaring-gliding flight, and this difference in the use of energy increases with body mass. However, soaring-gliding results in lower speeds than flapping, especially for small species. Birds therefore face a trade-off between energy and time costs when deciding which flight strategy to use. Raptors are a group of large birds that typically soar. As relatively light weight raptors, falcons can either soar on weak thermals or fly by flapping with low energy costs. In this paper, we study the flight behavior of the insectivorous lesser kestrel (Falco naumanni) during foraging trips and the influence of solar radiation, which we have adopted as a proxy for thermal formation, on kestrel flight variables. We tracked 35 individuals from two colonies using high frequency GPS-dataloggers over four consecutive breeding seasons. Contrary to expectations, kestrels relied heavily on thermal soaring when foraging, especially during periods of high solar radiation. This produced a circadian pattern in the kestrel flight strategy that led to a spatial segregation of foraging areas. Kestrels flapped towards foraging areas close to the colony when thermals were not available. However, as soon as thermals were formed, they soared on them towards foraging areas far from the colony, especially when they were surrounded by poor foraging habitats. This reduced the chick provisioning rate at the colony. Given that lesser kestrels have a preference for feeding on large insects, and considering the average distance they cover to capture them during foraging trips, to commute using flapping flight would result in a negative energy balance for the family group. Our results show that lesser kestrels prioritize saving energy when foraging, suggesting that kestrels are more energy than time-constrained during the breeding season.

Wind turbines cause functional habitat loss for migratory soaring birds

“…We must emphasize that our models include some level of error (see Table 1), likely because that were other environmental variables influencing the movement of the birds that were not included as predictors. However, that amount of error is comparable to that found in previous studies linking bird soaring behaviour to uplift proxies (Bohrer et al, 2012;Dodge et al, 2014;Hernandez-Pliego, Rodriguez, & Bustamante, 2015;Santos et al, 2017;Sapir et al, 2011). The fact that uplift predictors were estimated for a single generic circumstance in time may also have added inaccuracy to our models.…”

The gateway to Africa: What determines sea crossing performance of a migratory soaring bird at the Strait of Gibraltar?

“…It seems obvious that the success of the sea crossing depends largely on how far birds can reach with their initial gliding, which directly depends on how high the gliding starts. It is surprising, however, that there was no influence of solar irradiance, as this variable is directly related with thermal convection (Garratt, 1994), which in turn influences soaring performance of birds (Hernandez‐Pliego et al, 2015; Santos et al, 2017). We believe that thermal convection was not a limiting factor for the birds we tracked since most sampling days were bright and the edge of the Strait of Gibraltar is mostly bare soil, which favors the formation of thermal uplift (Santos et al, 2017).…”

“…This variable was calculated as described for the tailwind component using the same wind dataset. Solar irradiance (min)—solar irradiance at the time when bird started the over sea flight, measured by a sunshine recorder located at Ceuta weather station. This variable can be used as a proxy for thermal uplift (Hernandez‐Pliego, Rodriguez, & Bustamante, 2015). …”

Seasonal and circadian biases in bird tracking with solar GPS-tags

“…Both basic ecology and conservation studies use location fixes obtained from animal with tags carrying GPS receivers. Among others, there are studies on habitat-selection [ 2 , 3 ], animal behavior [ 4 , 5 ] or human-wildlife conflict [ 6 , 7 ]. However, obtaining a GPS fix, and how reliable this location is, depends on many factors that are not always taken into account in these studies.…”