Thursday, 24 January 2013

Tawny Frogmouth breeding success

An average brood size of two Tawny Frogmouth chicks
I have been monitoring the breeding success of Tawny Frogmouths Podargus strigoides in Canberra, Australia, for several years and have found that the pattern is fairly constant. About half the breeding pairs rear two young to fledging; a few rear three in any year and about a quarter rear one chick. The proportion of pairs which fail to rear any chicks is about 24% on average over the years, ranging from 17 – 34% (Figure 1). I do not disturb the birds to record clutch size, but it is known to range from 1-3.

Figure 1: The number of young reared per breeding pair of Tawny Frogmouths in 2006 - 2012.

In recent years I had thought that more birds were failing to rear young, but this is just the impression gained as I have added more pairs to the study in the past few years. Although I have been recording more failures, this has been in proportion with larger sample sizes, there is no statistical difference (χ2 = 3.8, df = 6, P = 0.43). The main causes of failure, which are usually the loss of whole clutches or broods, are predation by unknown species, but likely Brush-tailed Possum or Brown Goshawk. One male was taken off the nest by a feral cat. A few nests have been blown out of their trees by strong winds.

Figure 2: The number of Tawny Frogmouth pairs which failed to rear any young in any year is proportional  c24%, to the number studied (r = 0.899, P = 0.003).
Further study will aim to determine whether there are any differences between the breeding success of frogmouths in grassy woodland, dry sclerophyll forest or suburban remnant woodland; or if there is any difference between years of drought and high rainfall. Fortunately the study has already covered these criteria.

A male Tawny Frogmouth protects his chick

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