Young of the year- development of a burrowing owl
Breeding season is an exciting time for field researchers monitoring the Western Burrowing Owl (Athene cunicularia hypugaea) population in southern San Diego County, as a new cohort of chicks strives to survive and fledge to adulthood. Burrowing owl populations have been declining in many locations across the range of the species, and because of this, the owl is considered a species of conservation concern. In San Diego County, where owls historically occupied level grassland areas near the coast, urban development and habitat loss are the primary factors contributing to their decline.
In an effort to help breeding owls, artificial burrows have been installed on mitigation sites to increase the number of available burrows. Researchers at the Institute have modified existing artificial burrows to allow us to see inside the nest chamber. This has given us the ability to monitor all stages of nesting this season, from egg laying and incubation, to hatch and brooding, through emergence and dispersal.
Egg laying and incubation
Burrowing owls lay smooth, white eggs about the size of a ping pong ball in clutches of 2-12 eggs (Figure 1). Females lay one egg approximately every 36 hours until the clutch is complete. In this species, the female does all of the incubating, remaining in the burrow for most of the day and night except for a few emergences at dawn and dusk. She develops a brood patch for incubating and brooding. This is a featherless area on the breast with ample blood supply close to the skin surface that allows heat transfer from the female to the eggs or newly hatched chicks. The male provides all the food to the female during this time.
Hatch and brooding
The eggs are incubated for 28-30 days and then the chicks begin hatching asynchronously. At hatching, the chicks are helpless and mostly covered in a white down (Figure 2).
The chicks remain in the burrow being brooded by the adult female for approximately 14 days after hatching. The term brooding refers to the post-hatching behavior of the female, when she maintains close physical contact with the chicks to help keep them warm. During this time, the chicks open their eyes, become more mobile, and develop the ability to control their body temperature (Figure 3). During this part of development, the male is responsible for providing adequate food for the whole family.
After about 14 days, the chicks are big enough to emerge and spend some time outside (Figure 4). After emergence, the chicks spend more and more time outside. They begin learning how to pounce on prey and how to stretch their wings (Figure 5). Feathers continue to grow and develop and the chicks slowly lose the downy look and develop the light brown spotted pattern of an adult.
Older chicks will also begin to use satellite burrows next door to the main burrow. They will spend time helicoptering their wings as a strength-building exercise, and eventually they begin to fly. After 44 to 53 days post-hatch, chicks are considered fledged and can leave the nest, though many stay longer. By the end of the season, the fledglings are strong flyers and have adult feather patterns, making it very hard to distinguish the adults from the young of the year (Figure 6).
Our primary objective is to monitor population size and reproductive success each year. But foremost in our minds is the thought that each one of these fluffballs has an important role to play - to survive to adulthood and to join the resident breeding population. Having a better understanding of the risks chicks face at each nesting stage will enable us to make management decisions to support the continued success of this vulnerable population.