We have large scale aircraft with long endurance and much higher speed than birds. But it seems that aircraft of comparable size to birds (i.e. drones) have much lower endurance, top speed and flight range. What technologies are currently limiting us from achieving this?
Somebody needs to design a chocolate-powered drone. Yes, seriously.
The total energy stored in a 40AH 12V battery is about the same as the calorific value of five 100g chocolate bars from your nearest supermarket.
Source: from https://www.tesco.com/groceries/product/details/?id=254381873, "Tesco Everyday Value Milk Chocolate" provides 3840 kJ/kg, and the fully charged battery holds 40x12x3600/1000 = 1728 kJ
Find a way to pack that much energy density into drone (and also consider that unlike LIPO batteries, chocolate bars don't spontaneously combust if they are mishandled!) and the comparison between drones and birds would come out rather differently.
Of course, soaring birds (and sailplanes) can stooge around all day and night, getting their energy from thermals and the wave airflows over mountain ridges - but they still need some external power to reposition themselves to take advantage of those "free" energy sources.