To Your Best Health,
Dr. Brian Paris
In North America at least, barn swallows frequently engage in a mutualist relationship with ospreys. Barn swallows will build their nest below an osprey nest, receiving protection from other birds of prey that are repelled by the exclusively fish-eating ospreys. The ospreys are alerted to the presence of these predators by the alarm calls of the swallows.[33] There are normally two broods, with the original nest being reused for the second brood and being repaired and reused in subsequent years. The female lays two to seven, but typically four or five, reddish-spotted white eggs.[3] The clutch size is influenced by latitude, with clutch sizes of northern populations being higher on average than southern populations.[64] The eggs are 20 mm × 14 mm (3⁄4 in × 1⁄2 in) in size, and weigh 1.9 grams (29 grains), of which 5% is shell. In Europe, the female does almost all the incubation, but in North America the male may incubate up to 25% of the time. The incubation period is normally 14–19 days, with another 18–23 days before the altricial chicks fledge. The fledged young stay with, and are fed by, the parents for about a week after leaving the nest. Occasionally, first-year birds from the first brood will assist in feeding the second brood.[3] Compared to those from early broods, juvenile barn swallows from late broods have been found to migrate at a younger age, fuel less efficiently during migration and have lower return rates the following year.[65] The barn swallow will mob intruders such as cats or accipiters that venture too close to their nest, often flying very close to the threat.[55] Adult barn swallows have few predators, but some are taken by accipiters, falcons, and owls. Brood parasitism by cowbirds in North America or cuckoos in Eurasia is rare.[2][33] Hatching success is 90% and the fledging survival rate isAverage mortality is 70–80% in the first year and 40–70% for the adult. Although the record age is more than 11 years, most survive less than four years