Parental investment and parent-offspring conflicts during the postfledging period in Montagu's harriers

We studied factors influencing the length of the postfledging period in the Montagu's harrier,Circus pygargus . Fledging date influenced the date of departure, for both wild birds and captive-reared birds (for which food supply was constant throughout the postfledging period), suggesting that departure time was constrained by migration. However, this relationship was absent in good food years, when some birds had short periods of dependency even if fledged early in the season. The end of the postfledging period was associated with a decrease in the food provided by the parents, coinciding with an increase in the fledglings' flying and hunting abilities and in aggressive food solicitation. The postfledging period was shortest when food was scarcest (when controlling for fledging date), and was also shorter for younger siblings (when controlling for brood effects), even though the parents provided less food in poor food years, and for nestlings in larger broods, particularly the younger ones. These results suggest that departure time is an adult's choice. However, we also predicted that if fledglings control parental investment, they should attempt to prolong the period of dependency when they receive little food. Length of the postfledging period was affected by food abundance, with shorter periods in better food conditions for equivalent hatching dates. In addition, the postfledging period was longer for birds from larger broods (when controlling for hatching date and food abundance), and for wild birds than captive-reared birds except in peak vole years. These results suggest that fledglings try to compensate for a deficiency in the food supply with longer periods of dependence, provided there is enough time before migration.