Abstract
The timing of breeding in birds has important consequences if food availability varies seasonally. Optimal timing of the most energetically demanding stages of breeding to coincide with peaks in food availability can increase breeding success. Early breeding may be challenging for females producing eggs, but generally birds initiating egg-laying early in the breeding season are more likely to succeed. Initiating egg-laying later may be less costly to females, but raising offspring after the peak in food availability becomes challenging. Balancing these costs and benefits is important for the cooperative breeding Southern Ground-hornbill Bucorvus leadbeateri (hereafter ‘Ground-hornbill’), which typically produces only a single clutch of two eggs and fledges a single chick per breeding season. A previous study found that Ground-hornbill groups that lay early in the breeding season are more likely to fledge a chick than later-breeding groups. We investigated factors associated with the timing of breeding such as group size, the type of nest used, rainfall and temperature in a study site supplemented with nest boxes. Ground-hornbills laid eggs from 9 September to 14 November and the average number of days between the first and the last laying date in a season was 48 ± 32 days (range 10–101 days). Larger groups, particularly those using natural nests compared to those using nest boxes, laid later than smaller groups. Egg-laying was delayed under hot, dry spring conditions and laying dates were earlier following above-average spring rainfall.
Zusammenfassung
Die Einflüsse von Wetter, Gruppengröße und Nesttyp auf den Zeitpunkt der Eiablage beim Kaffernhornraben Bucorvus leadbeateri
Wann Vögel brüten, ist wichtig, wenn die Nahrungsverfügbarkeit im Verlauf der Brutsaison schwankt. Die energieaufwendigsten Brutstadien mit der höchsten Nahrungsverfügbarkeit zu synchronisieren, kann den Bruterfolg erhöhen. Eine frühe Brut kann für die eierlegenden Weibchen eine Herausforderung bedeuten, doch generell sind Vögel, die früh in der Saison mit der Eiablage beginnen, mit höherer Wahrscheinlichkeit erfolgreich. Ein späterer Legebeginn kann für die Weibchen mit geringeren Kosten verbunden sein, aber die Küken erst aufzuziehen, wenn die Nahrungsverfügbarkeit bereits abnimmt, ist schwierig. Die richtige Kosten-Nutzen Balance zu finden, ist wichtig für den kooperativ brütenden Kaffernhornraben Bucorvus leadbeateri, welcher pro Brutsaison normalerweise nur ein einziges aus zwei Eiern bestehendes Gelege und ein flügges Küken produziert. Eine vorherige Studie fand, dass früh in der Brutsaison legende Kaffernhornraben-Gruppen mit höherer Wahrscheinlichkeit ein flügges Küken produzierten als später legende Gruppen. Wir haben mit dem Brutzeitpunkt assoziierte Faktoren wie Gruppengröße, Nesttyp, Regen und Temperatur in einem mit Nistkästen ausgestatteten Studiengebiet untersucht. Kaffernhornraben legten vom 9. September bis zum 14. November, und der durchschnittliche Unterschied zwischen erstem und letztem Legedatum betrug 48 ± 32 Tage (Variationsbreite 10-101 Tage). Größere Gruppen legten später als kleinere, besonders wenn sie in natürlichen Nisthöhlen und nicht in den Nistkästen brüteten. Die Eiablage war bei heißem, trockenem Frühlingswetter verzögert und erfolgte früher nach überdurchschnittlichen Frühlingsregenfällen.
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Acknowledgements
We thank the landowners and wardens of the Associated Private Nature Reserves for access to properties, and assistance with fieldwork. This research was funded by the Technology and Human Resources for Industry Programme of the Department of Trade and Industry; Dow Southern Africa; the Hans Hoheisen Charitable Trust; and Senelala Estates.
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Carstens, K.F., Kassanjee, R., Little, R.M. et al. The effects of weather, group size and type of nest on the timing of egg-laying in the Southern Ground-hornbill Bucorvus leadbeateri. J Ornithol 160, 633–640 (2019). https://doi.org/10.1007/s10336-019-01654-x
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DOI: https://doi.org/10.1007/s10336-019-01654-x