MORE EGGS THE BETTER: EGG FORMATION IN CAPTIVE BARN OWLS (TYTO ALBA)

Auk, The, Jan 2004 by Durant, Joel M, Massemin, Sylvie, Handrich, Yves

ABSTRACT.-We studied rapid yolk deposition (RYD) in Barn Owls (Tyto alba) on the basis of the analysis of 26 eggs laid by "dye-dosed" captive female Barn Owls in five different broods. Pictures of yolks were examined to assess daily rates of yolk deposition. We used those data in combination with data from the dissection of ovaries of another five breeding females. We found that the total duration between initiation of RYD and laying of the corresponding egg was only 13.6 days, with an interval between yolk completion and oviposition of 2.4 days. The total number of follicles that may have given eggs was found to be 25. That high number of follicles and the short RYD period explain the particularly high reproductive potential of this nocturnal raptor species. Received 22 January 2003, accepted 15 September 2003.

RESUME.-Nous avons etudie le depot rapide du vitellus (DRV) chez Tyto alba. L'etude est basee sur l'analyse de 26 oeufs repartis en cinq couvees provenant de femelles de Tyto alba captives ayant ingere des pigments lipophiles. Les photographies de vitellus ont ete examinees pour estimer les taux journaliers de depot en vitellus. Nous avons exploite ces donnees combinees a d'autres issues de la dissection des ovaires de cinq autres femelles reproductrices. Nous avons trouve que la duree totale entre l'initiation du DRV et l'eclosion des oeufs correspondant etait seulement de 13,6 jours, avec un intervalle de 2,4 jours entre l'achevement de la formation du vitellus et l'oviposition. Un total de 25 follicules se seraient developpes pour donner des oeufs. Ce grand nombre de follicules et la courte duree de la periode de DRV expliquent le potentiel reproductif important de ce rapace nocturne.

REPRODUCTION EFFORT PER breeding attempt is one of the main parameters defining life-history strategies (Stearns 1992). In birds, the yolk constitutes the major energy content of an egg (Sutherland and Rahn 1987). An estimate of the daily amount of yolk deposited on developing ova and the sequence of laying are important when evaluating the total cost of egg production. In birds, the vast majority of the yolk mass is deposited in a relatively short period of time compared to other oviparous vertebrates; in some passerines, rapid yolk deposition (RYD) may last only three days per yolk (Ricklefs 1974, Grau 1984). During RYD, growing follicles take up from the blood the yolk precursors that are produced and secreted by the liver (Griffin and Hermier 1988, Shen et al. 1993); follicle growth occurs in a hierarchy with at most one follicle reaching maturity and being ovulated each day (Christians and Williams 2001).

Clearly, the energy cost of egg production is determined by clutch size and the variation in rate of growth of follicles recruited, duration of the follicles' growth, or both. Even though the energetic demands during clutch formation were modelled 30 years ago (King 1973), and a technique is available to assess daily energy-nutrient demands during yolk formation (Warren and Conrad 1939, Gilbert 1972, Grau 1976), few studies have attempted to empirically examine such demands in birds (Astheimer and Grau 1985).

In birds that lay big clutches, total number of eggs in formation (Haywood 1993) and laying interval (Astheimer 1985) must be taken into account to estimate the daily yolk-deposition rate. Moreover, the number of follicles that undergo yolk deposition commonly exceeds number of eggs actually laid (Meijer et al. 1989), which suggests that unlaid yolks become atretic (Erpino 1973, Gilbert et al. 1983). Thus, it is important to take into account the actual number of eggs laid, all eggs resorbed before oviposition, and all the follicles that undergo atresia, when assessing the energy-nutrient requirements for an entire reproductive attempt.

Among raptors, the Barn Owl (Tyto alba) has an exceptionally high reproductive potential, which is rather similar to that of passerine birds. This owl species commonly lays clutches of four to seven eggs (Taylor 1994), where clutch size varies widely according to food availability (Mikkola 1983, Taylor 1994). No accurate data are available on the laying interval, ovary structure, and timing of yolk development in Barn Owls. We predicted that the high reproductive potential in this altricial species, associated with a low daily energy requirement of egg synthesis (Durant et al. 2000, Vezina and Williams 2002), is due to a long RYD period and a possibility of recruiting a larger number of follicles for RYD than eggs actually laid.

The aim of our study was to characterize the phases and timing of egg formation in captive Barn Owls to understand the high potentiality of laying in this species. Likewise, we also conducted an ovary analysis and determined the number of developing follicles.

METHODS

The study was performed on captive European Barn Owls that were part of a breeding program conducted since 1985 in our laboratory. Each pair of owls was housed separately in an outside aviary (5 � 4 � 2.5 m) where a nest box was available. They were fed daily with laboratory mice (Mus musculus). Laying takes place from March to October. The average egg laying interval was 2.4 � 0.5 days (n = 96) (i.e. laying occurred at different times of the day for successive eggs).