(note: this question was moved to this board at Mouse's request)
TPC - the mention of Ivy's small size brought to mind a question that I have meaning to ask for the longest time (and I apologize if this has been answered previously and I hadn't noticed). What are the biological advantages to having a small male and a large female? I would imagine mating is easier and a large female would be able to produce larger eggs but are there others? And I remember in the past that one female was thought to be male because she was smaller than normal. If a male had a choice, would he be more likely to choose the larger female? In other words, would a small female be less likely to find a mate?
I'm going to answer these out of order ... it doesn't seem that the smaller females have any problems finding mates or being reproductively successful - Hurricane is a good case in point - and there have been lots of smaller females right across the Prairies/Midwest so its not just Hurricane. Smaller males don't seem to have a problem either - T-Rex for example. Larger females I'm not sure, have had only one and she injured herself and went into a breeding program. "Alice" is a big female and perhaps she doesn't have a mate yet because of that but that is purely speculative. As for it being easier to mate, there are raptor species that don't have a size difference who do just fine, so not an issue for them, though it probably looks easier to us
One suspected evolutionary advantage for sexual dimorphism like size is that it allows the species to exploit a broader range of prey species - harder for the larger females to catch fast small birds whereas the males can't carry larger prey like some ducks. If you can't carry it you have to eat/dismember it quick so it isn't stolen by other competing species. If you lose your kill you can't feed your young and your genetics don't survive.
Since the males tend to most of the hunting during incubation and brooding and there is almost always more small, fast prey (pigeons for example), better to have the hunter be the smaller and quicker of the two genders.
Another thought is that females need accumulate more reserves for producing young and (generally for the species) do more of the incubating and to sustain themselves. Perhaps in cliff-nesting environments this is more of a concern, in urban situations like ours, food doesn't seem to be difficult to acquire for healthy and/or experienced adults. None of our females have seemed deprived of food while on the nest and I can't think of a time when we had light-weight chicks at banding or had a chick in the nest needing to be rescued or have died due to malnutrition. We have had fledglings that have been malnurished who have recovered but that months after they were hunting on their own, so more a case of them not being good hunters or perhaps having suffered an injury and not being able to hunt well enough.
Does that help?
